HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Dubeux, J. C. B. Articles by Santos, H. Q. Search for Related Content PubMed Articles by Dubeux, J. C. B., Jr. Articles by Santos, H. Q. Agricola Articles by Dubeux, J. C. B. Articles by Santos, H. Q.

REVIEW & INTERPRETATION

Nutrient Cycling in Warm-Climate Grasslands

^a Dep. de Zootecnia/UFRPE, Av. Dom Manoel de Medeiros, S/N, Dois Irmãos, 52171-900, Recife-PE, Brazil
^b Agronomy Dep., Univ. of Florida, Gainesville, FL 32611-0300
^c College of Agriculture, Forestry, and Natural Resource Management, Univ. of Hawaii at Hilo, Hilo, HI 96720-4091
^d Dep. Agronômico–Cargill Fertilizantes, R. Bahia, 299, Área Verde, Alto Garças, MT, Brazil, 78770-000

^* Corresponding author (lesollen{at}ufl.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 
Nutrients cycle among pools within an ecosystem, and losses of nutrients to the environment accompany each transfer from pool to pool. Efficient recapture of nutrients by plants is critical in extensively managed grasslands if these swards are to persist. In intensively managed systems, the greatest contribution of efficient recapture of nutrients may be minimizing loss of nutrients to the environment and associated negative impacts. Regardless of management intensity, grassland management decisions should be informed by an understanding of the dynamics of nutrient cycling. A significant body of literature has emerged in recent years describing nutrient dynamics in warm-climate grasslands. In warm climates globally, grasslands are most often low-input production systems dominated by C₄ grasses. These characteristics affect nutrient cycling, resulting in very different management challenges and opportunities than in higher input, C₃–grass or legume-dominated, grasslands. This paper will focus on warm-climate grasslands. Within that context its objectives are (i) to describe the most prominent pools of C, N, P, and K, (ii) to discuss fluxes among nutrient pools, with emphasis on plant litter and animal excreta, iii) to describe the importance, management, and dynamics of soil organic matter, and (iv) to review the impact of grazing systems on nutrient cycling.

Abbreviations: DM, dry matter • OM, organic matter • Pi, inorganic P • Po, organic P • SOM, soil organic matter.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 
NUTRIENTS ARE DEFINED in this paper as the organic and inorganic elements (e.g., C, N, P, and K) that are cycling among various reservoirs or "pools" in an ecosystem. Nutrients reside temporarily in the pools. In a grassland ecosystem these pools include the plants and their residues, grazing livestock, soil fauna and flora, and inorganic and organic compounds other than living tissue (Rotz et al., 2005). A grassland is defined as a plant community in which grasses and other herbaceous plants form the dominant vegetation, while a warm-climate grassland refers to those that are dominated by C₄ grasses and occur within the approximate range of 30°N to 30°S latitude. Examples include the neotropical savannas that cover approximately 210 million ha in Bolivia, Brazil, Colombia, Guyana, and Venezuela (Vera, 2001) and the planted and native grasslands of peninsular Florida that occupy approximately 5 million ha.

Interest in nutrient dynamics in managed grasslands has intensified in recent years (Mathews et al., 2004). Reasons include planted pasture degradation in extensive systems due to inadequate soil N (Boddey et al., 2004; Miles et al., 2004) and negative environmental impact in intensive systems associated with excessive application of inorganic fertilizers or animal excreta (Woodard et al., 2003). Although these situations represent extremes in nutrient availability and management challenges, a common thread is the need for greater understanding of nutrient cycling within grassland agroecosystems.

Warm-climate grasslands differ in significant ways from most temperate grasslands. There are notable exceptions in the southern USA and elsewhere, but many warm-climate grasslands receive low rates of inorganic fertilizer because of unfavorable economics of fertilization or limited availability of fertilizer materials. Less forage is conserved as hay or silage in warm-climate than in temperate-climate grasslands, increasing the reliance on standing forage (Sollenberger et al., 2004). Conservative stocking decisions are required to allow standing forage to accumulate for use during prolonged dry seasons, meaning that many warm-climate grasslands are underutilized during the period of maximum growth. Under these conditions, senescing plant material, or litter, accumulates and constitutes an important nutrient pool (Dubeux et al., 2006a). Litter of C₄ grasses often is low in quality and may result in nutrient immobilization (Thomas and Asakawa, 1993), which aggravates the existing shortage of available nutrients in low-input systems (Rezende et al., 1999).

Limited mechanical harvesting in warm-climate grasslands implies that grazing predominates as the method of utilization (Sollenberger et al., 2004). Herbivores accelerate nutrient cycling through decomposition and excretion of plant nutrients and through the effects of grazing and excreta on soil biota (Bardgett and Wardle, 2003). Retention of consumed nutrients in ruminant animal products is low, ranging from 5 to 30%. Thus, nutrient return to the grassland via animal excreta is a major component of the nutrient cycle (Rotz et al., 2005). Recovery of these nutrients by plants varies greatly due to the heterogeneous distribution of excreta (White et al., 2001) and the potentially wide ranges in nutrient losses due to leaching, volatilization, and immobilization (Haynes and Williams, 1993). Because utilization of nutrients by livestock converts them to forms that are more susceptible to loss, presence (vs. absence) of cattle in a grassland and higher (vs. lower) stocking rates have been associated with accelerated rates of pasture degradation (Castilla et al., 1995; Boddey et al., 2004) in some environments.

To generalize, many warm-climate grasslands are characterized by low soil fertility and limited opportunity to manipulate soil nutrient status using fertilizers. Thus, increasing efficiency of nutrient cycling is critical to long-term persistence of the grassland resource. Also within this context, the roles of plant litter and animal excreta in nutrient cycling are extremely important (Boddey et al., 2004).

A comprehensive review of all issues related to nutrient cycling in grasslands is beyond the scope of this paper. Instead, within the context of warm-climate grasslands, we will (i) describe C, N, P, and K pools in grassland ecosystems, (ii) discuss nutrient dynamics in grasslands, emphasizing senescing plant biomass and animal excreta, (iii) describe the importance, management, and dynamics of soil organic matter (SOM), and (iv) review the impact of grazing systems on nutrient cycling.

	NUTRIENT POOLS IN GRASSLAND ECOSYSTEMS

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 
Carbon
Although photosynthesis is the key pathway for C accumulation in plant biomass, the major C pool in grassland ecosystems is SOM. Thomas and Asakawa (1993) and Stevenson and Cole (1999) reported that the organic matter (OM) of terrestrial soils contains 30 to 50 x 10¹¹ Mg C compared with 7 x 10¹¹ and 4.8 x 10¹¹ Mg in the atmosphere and in plant–animal biomass, respectively. Tropical grasslands and savannas contain only 6% of global terrestrial biomass (Schlesinger, 1997) but about 15% of global soil organic C (Jobbagy and Jackson, 2000). Although vegetation and grazing animal pools contain less C than the SOM, they play an important role in the cycling of C within pasture systems through surface litter deposition and decomposition, excreta return, and methane emission. The extent of pasture utilization (C consumption) by herbivores determines whether litter or excreta is the main source of aboveground C (Thomas, 1992). Dubeux et al. (2006a) reported plant litter deposition of 4540 kg OM ha^–1 during a 168-d grazing season of ‘Pensacola’ bahiagrass (Paspalum notatum Flügge). Thomas and Asakawa (1993) measured from 2830 to 11 800 kg dry matter (DM) ha^–1 of litter deposited from May to December in creeping signalgrass [Brachiaria humidicola (Rendle) Schweick.] and gambagrass (Andropogon gayanus Kunth) pastures. Castilla (1992) estimated fecal C return of 3900 kg ha^–1 yr^–1 in a creeping signalgrass-desmodium [Desmodium heterocarpon (L.) DC. subsp. ovalifolium (Prain) Ohashi] pasture, and, compared to leaf litter, it was the main source of aboveground C.

The potential of the soil as a CO₂ sink has been explored as C sequestration has emerged as a critical issue (Fisher et al., 1994; Lal et al., 1995; Rao, 1998; Silva et al., 2000). Fisher et al. (1994) suggested that introduced deep-rooted tropical grasses like gambagrass and creeping signalgrass could store greater amounts of C in the soil profile than the native savanna grasses. Tropical grasses cause storage of C mainly by producing large amounts of very poor quality belowground litter (Gijsman et al., 1997; Rao, 1998; Urquiaga et al., 1998). Estimates of C storage must be interpreted cautiously, however, because the low inputs of fertilizer and high stocking rates used in the South America savanna region have left many pastures in a process of degradation, likely decreasing their ability to act as a future sink of atmospheric CO₂ (Fisher et al., 1994; Silva et al., 2000). The process of degradation occurs when pasture productivity decreases, the pasture is invaded by nonpalatable weed species, and areas of bare soil appear. A degraded pasture ecosystem has a diminished capacity to provide economic benefits. Causes of pasture degradation, or "decline," are linked to poor management, especially lack of maintenance fertilization and overgrazing (Boddey et al., 2004).

While introduced grasses increased C storage over native savanna grasses, legume–grass associations enhanced C storage even more (Table 1; Fisher et al., 1994). Greater C accumulation in legume–grass than grass pastures was also observed by Tarré et al. (2001). In addition to increased biomass production from legume–grass associations (vs. grass monocultures receiving no N fertilizer), several other mechanisms have been proposed to explain some of the increase in C storage. Changes in belowground biota may alter soil aggregation or the biomass and enzyme activity of humus decomposers (Decaëns et al., 1994; Garcia-Montiel and Binkley, 1998; Oberson et al., 1999; Carreiro et al., 2000). Alternatively, while N additions from the legume may stimulate initial litter decomposition rates, they may also enhance soil humus concentration simply because N is a substrate for humification and may inhibit decomposition of humified soil C through formation of decay-resistant, humic-N polymers (Fog, 1988; Resh et al., 2002).

View this table: [in this window] [in a new window]   Table 1. Soil C under planted pastures compared to native tropical savanna.

Nitrogen
The major N pools in grazed grasslands are the soil, vegetation, grazing animals, and atmosphere. Fluxes among them are complex and a function of interactive factors, including climate, soil microbiota, forage species, and herbivores (Myers et al., 1986). The atmospheric N pool is the largest; however, it is available to plants only through biological N fixation, mediated by free-living or plant-associated bacteria, requiring about 960 kJ per mole of N₂ fixed (Marschner, 1995). Thus, the energy requirement for N₂ reduction, either by biological or industrial fixation, is the major reason N is considered the most limiting nutrient in many agricultural ecosystems (Wedin, 1996).

Considering all terrestrial ecosystems, the atmospheric N pool is 16 000 times greater than the sum of the soil and biotic N pools (Russelle, 1996). In grasslands, the soil (not including the earth's mantle and crust) is the second-largest N reservoir (after atmospheric N) and is affected by soil OM, soil microbial biomass, fixed NH₄⁺, and to a lesser extent, the plant-available inorganic N (Stevenson and Cole, 1999). The belowground soil mesofauna are also important components of the soil pool, and the rhizosphere may contain from 4500 to 24 000 kg N ha^–1 (Henzell and Ross, 1973). These amounts are far greater than the range of 20 to 300 kg N ha^–1 estimated in live herbage of tropical forages (Henzell and Ross, 1973). In signalgrass (B. decumbens Stapf), palisade grass [B. brizantha (A. Rich.) Stapf], gambagrass, and ‘Tanzânia’ and ‘Tobiatã’ guineagrass (Panicum maximum Jacq.) pastures, roots comprise the majority (53–76%) of plant biomass (Kanno et al., 1999) and contain relatively little N. Thus, the sum of live herbage and belowground total plant N is much lower than that for soil.

Litter is another important N pool because, along with the soil microbiota, it constitutes the link between N in metabolically active plant tissues and N available for plant uptake (Thomas and Asakawa, 1993). Excluding soil and atmospheric N, Robertson et al. (1993a) estimated that in green panic (Panicum maximum Jacq. var. trichoglume Robyns) pastures, 30 to 50% of all N in the ecosystem was in plant litter and senesced tissues, that is, currently unavailable for plant uptake.

Phosphorus
Highly weathered tropical soils (Oxisols, Ultisols, and Andisols in the Udand suborder) often used for grassland agriculture in warm climates are characterized by low available P concentration and often by a very high P sorption capacity. The P cycle is even more complex than the N cycle because availability of P depends not only on biologically mediated turnover processes of organic P (Po) but also on the chemistry of inorganic P (Pi) (Novais and Smyth, 1999; Oberson et al., 1999; Rao et al., 1999).

Low P:N ratios in both plant and animal tissues combined with high soil P sorption capacity result in soil being the largest and most important P pool in grassland ecosystems (Haynes and Williams, 1993; Rao et al., 1999). Some Oxisols of the Brazilian Cerrado region can sorb more than 4000 kg P ha^–1 within the 0- to 20-cm soil layer (Novais and Smyth, 1999). Efficient cycling of Pi is not expected because competition between the soil and the plant for orthophosphate (H₂PO₄^– and HPO₄^2–) in solution is such that soil P retention by amorphous and microcrystalline Fe and Al oxides (McLaughlin et al., 1990; Novais and Smyth, 1999; Mathews et al., 2005) coupled with microbial P demand (Olander and Vitousek, 2005) makes most of the Pi unavailable. Regardless, significant stratification of more soluble P forms usually develops in the surface 4 cm of long-term pasture soils due to litter and dung returns, and this may be enhanced by fertilizer P applications (McLaughlin et al., 1990; Mathews et al., 2005). Some heavily grazed grasslands may be more susceptible to P deficiencies if the surface few centimeters of relatively unshaded soil dry out (McLaughlin et al., 1990).

Soil Po compounds differ in their availability to plants and in their turnover rates. For tropical soils receiving little or no P fertilizer, Po is considered to be the most important P source for plants, and this P pool may be used to increase efficiency of P recycling (Beck and Sánchez, 1994; Guerra et al., 1995; Friesen et al., 1997; Novais and Smyth, 1999; Oberson et al., 1999). In recent years, management strategies aimed at minimizing the interaction of organic residue with the soil were developed to reduce Pi sorption and to increase Po pools, thereby rendering P more available to plant roots and/or mycorrhizae (Friesen et al., 1997; Gijsman et al., 1997; Oberson et al., 1999). Reviews by Rao et al. (1999) and Miyasaka and Habte (2001) provide further information on this important area of research.

Potassium
Potassium is not an integral part of organic compounds, and the chemistry of K in tropical soils is almost solely based on cation exchange reactions. The soil is the greatest reservoir of K in tropical grassland ecosystems. Most of it is in non- or slowly exchangeable forms, as orthoclase feldspar in the silt fraction and certain 2:1 minerals in the silt and clay fractions (e.g., micas, illite, and interlayered vermiculite) (Bower, 1977; Ayarza, 1988; Melo, 1998). In soils with low cation exchange capacity, exchangeable K is prone to leaching, whereas certain weathered soils (e.g., those rich in illites and interlayered vermiculites) have high K retention but low subsequent K release rates (Bower, 1977; Ayarza, 1988; Melo, 1998). In tropical grasslands in the Peruvian Amazon, K losses only occurred at high K application rates (300 kg K ha^–1) despite leaching rainfall. Via K sorption by 2:1 minerals, K supply was sustained, and high-yielding forage systems enhanced K recycling from plant residues (Ayarza, 1988).

Animals represent a relatively small K pool, yet they have a critical role in recycling because of the large amount of K ingested and excreted. In New Zealand, Williams et al. (1990) estimated that animals were directly or indirectly responsible for 74 to 92% of all K losses in pastures grazed by dairy cows over a 30-yr period. In creeping signalgrass–desmodium pastures in the Amazon region, animals disrupted K cycling, and losses were 30 to 95 kg K ha^–1 yr^–1, whereas K losses were negligible without animals (Castilla et al., 1995). The direct losses through animal products are much lower (0.12–0.18 kg K per 100 kg of animal product) than the indirect losses associated with the highly heterogeneous deposition patterns of urine and dung (Wilkinson et al., 1989; Williams et al., 1990; Mathews et al., 1994b). A comprehensive review about specific aspects of K cycling in grassland systems was published by Kayser and Isselstein (2005).

Other Nutrients
Other essential nutrients like Ca, Mg, S, and the micronutrients are also distributed in below- and aboveground pools and play important roles in plant and animal nutrition. Calcium, Mg, and micronutrients are returned to the pasture mainly in feces, whereas S and N have similar patterns of return that are dependent on diet concentrations (Haynes and Williams, 1993). Calcium and Mg are commonly added to tropical pastures through liming, and S is a component of some commonly used fertilizers and soil amendments, including ammonium sulfate, ordinary super phosphate, and gypsum. Awareness of the need for, and use of, micronutrient fertilizers in grasslands is increasing. Mineral supplements are another source of these nutrients and in most cases are more economical than pasture fertilization for overcoming mineral deficiencies in grazing animals (McDowell, 1996).

	NUTRIENT DYNAMICS IN GRASSLAND ECOSYSTEMS

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 
Unlike agroecosystems focused on crop production, grazing animals are present on grasslands, and they affect nutrient dynamics by modifying nutrient spatial distribution (Dubeux et al., 2006c) and altering nutrient availability (Thomas, 1992). Nutrient release may be estimated by determining pool size and its turnover rate. Large variability associated with the measure of these components of nutrient release make its estimation on grazed grasslands difficult. In this section, nutrient release from the SOM, plant root and rhizome, aboveground plant litter, and animal excreta pools will be discussed.

Soil Organic Matter
The SOM is the largest "nutrient reservoir" in a grassland ecosystem. Dubeux et al. (2004) estimated that in grazed bahiagrass pastures, approximately 60, 89, and 87% of the total C, N, and P pools, respectively, were accounted for by SOM, with the remainder allocated to above- and belowground vegetation, aboveground litter, and animal excreta. The SOM, however, may not be the major supplier of these nutrients for plant growth due to its low mineralization rate (20–50 g kg^–1 yr^–1). In contrast, annual turnover rates for dung, roots, and aboveground plant residue OM are on the order of 750, 670, and 850 g kg^–1, respectively (Brady and Weil, 2002).

Roots and Rhizomes
Grasses typically allocate large amounts of C to roots and rhizomes (Blue, 1988; Urquiaga et al., 1998). Kanno et al. (1999) reported that roots of five tropical grasses (‘Basilisk’ signalgrass, ‘Marandu’ palisade grass, Tanzânia and Tobiatã guineagrass, and ‘Baeti’ gambagrass) accounted for 53 to 76% of their total biomass. Dubeux et al. (2004) estimated that bahiagrass roots and rhizomes represented 29% of total C in the soil–plant–animal continuum and were the greatest contributors to N and P in the plant pool. Root and rhizome residues, however, are more persistent due to their chemical composition (Thomas and Asakawa, 1993; Gijsman et al., 1997); thus, tropical grasses with deep root systems (e.g., gambagrass and creeping signalgrass) have excellent potential for soil C sequestration (Fisher et al., 1994).

Aboveground Plant Litter
In grassland ecosystems, the aboveground deposition and decomposition of plant litter during the growing season exert a continuous influence on plant nutrient supply, whereas litter deposition in annual cropping systems occurs primarily as periodic pulses (Eilitta et al., 2003). Litter influences the net balance between mineralization and immobilization, which is especially important for N, P, and S because their availability is controlled in part by biological processes (Myers et al., 1994). Key characteristics of litter quality include physical properties and chemical composition, especially the concentrations and ratios of N, P, C, lignin, and polyphenols (Thomas and Asakawa, 1993; Myers et al., 1994). Thus, both the quantity and the quality of plant residues play a role in regulating nutrient cycling in pastures. The litter pool, particularly due to its low quality in warm-climate grasslands, acts as a sink of available soil N (Myers et al., 1986; Thomas and Asakawa, 1993). With increasing stocking rate and forage utilization rate, the relative importance of aboveground litter diminishes because a greater proportion of nutrients are returned via excreta (Thomas, 1992).

Pasture degradation is usually related to decreased soil N availability caused by an accumulation of low-quality plant litter, particularly in C₄ grasslands (Table 2) and, consequently, by an increase in net N immobilization by soil microorganisms (Robbins et al., 1989; Robertson et al., 1993a, 1993b; Cantarutti, 1996). In green panic pastures in Australia, net N mineralization did not occur until 50 to 100 d after litter deposition (Robbins et al., 1989). Even after 1 yr, only 20 to 30% of all litter N was released in the soil (Robbins et al., 1989; Robertson et al., 1993a, 1993b). Litter half-life of C₄ grasses and several legumes averaged 174 d for the grasses and 67 d for the legumes (Table 2). Reducing litter half-life leads to faster nutrient turnover and an increase in nutrient supply. Establishing grass–legume mixtures in tropical pastures may increase soil fertility and pasture sustainability through the deposition of better-quality litter (Cantarutti et al., 2002). Litter production was similar between creeping signalgrass and creeping signalgrass–desmodium pastures, but addition of the legume increased litter N concentration and the amount of N recycled. In the pure-grass system, rates of net mineralization and nitrification and the inorganic N concentration were lower than in the mixed system (Cantarutti, 1996). In contrast to its role contributing to pasture degradation in systems where N input is low, litter may serve a more positive role in highly fertilized pastures, acting as a buffering pool, immobilizing N, releasing it later, and reducing nutrient losses to the environment (Wedin, 1996). Dubeux et al. (2006b) observed that increasing N fertilization and stocking rate resulted in faster litter turnover and greater nutrient release, but nutrient release from litter was small, and significant amounts of nutrients were immobilized even under the most intensive management.

View this table: [in this window] [in a new window]   Table 2. Estimated single decomposition constant (k) and its respective half-life of some tropical forage grasses and legumes.

Excreta
Grazing animals affect nutrient cycling by removing mineral nutrients in grassland plants and returning them to the soil via excretion. The retention of ingested nutrients in body tissue and exportation in animal products are quite low, and most mineral nutrients consumed are excreted in feces and urine (Rotz et al., 2005). Number of dung and urine events per day, mass or volume of nutrients deposited, and area affected have been described by other authors (Haynes and Williams, 1993; Mathews et al., 1996; Peterson and Gerrish, 1996).

Phosphorus, Ca, Mg, and micronutrient metals (Fe, Cu, Mn, and Zn) are excreted primarily in the feces, while K and, to a lesser extent, Na are excreted primarily in the urine (Table 3). Cattle dung is an important pathway for P return to the pasture. Dubeux et al. (2004) estimated that in grazed bahiagrass pastures, the dung pool contained more P than the sum of aboveground litter, plant, and urine pools. Nitrogen and S are excreted both in feces and urine (Mathews et al., 1996), with the proportion in urine increasing with dietary N and S concentrations (Rotz et al., 2005). Braz et al. (2002) reported that 93% of the N ingested by animals grazing signalgrass pastures was excreted in dung. Nitrogen concentration was extremely low in the signalgrass DM, ranging from 3.1 to 4.7 g kg^–1 across evaluation dates. In contrast, Whitehead (1970) found that 80% of N was excreted in urine when forage N concentration was 40 g kg^–1.

View this table: [in this window] [in a new window]   Table 3. Distribution of plant macronutrients and sodium in excreta.

Excretion sites on the pasture surface are also known as "hot spots" due to the high concentration of nutrients in these areas. They become an important pathway through which nutrient losses may occur (Scholefield and Oenema, 1997). A single urination from cattle provides 400 to 500 kg N ha^–1 (Haynes and Williams, 1993; Jarvis et al., 1995) and more than 1000 kg K₂O ha^–1 (Castilla et al., 1995). This hot spot will exceed the immediate forage demand for N, and appreciable N losses by volatilization or leaching will occur. Additionally, leaching losses of K⁺ and SO₄^2– are often observed (Haynes and Williams, 1993; Mathews et al., 1996). High ammonia (NH₃) loss from urine spots are related to the localized high pH and NH₃ concentration. Urea is the source of nearly all of the NH₃ lost by volatilization, and volatilization is greatest during the first 2 d after urine deposition (Russelle, 1996). Depending on weather conditions and the amount of thatch and litter (increases urease activity), N loss may be 4 to 66% for urine-affected pasture areas, with totals of 20 to 120 kg N ha^–1 yr^–1 for whole grazed swards (Ryden, 1986). Volatile NH₃ losses predominate under dry, warm conditions, whereas NO₃^– leaching losses predominate under high rainfall conditions (Russelle, 1992). In grazed bahiagrass pastures, the urine-N pool represented only 1.3% but supplied more N (59 kg N ha^–1) than aboveground litter (42 kg N ha^–1), and less N than dung (84 kg N ha^–1) (Dubeux et al., 2004). Thus, N availability and losses through urine excretion play an important role in N cycling.

Jarvis et al. (1995) reported that denitrification in soils is thought to be the largest source of atmospheric N₂O, which is increasing at a rate of 0.2 to 0.5% yr^–1. Nitrogen (N₂) gas is the other major product, and the combined efflux of these gases translates to losses of 25 kg N ha^–1 or more in signalgrass pastures depending on the N fertilization level (Veldkamp et al., 1998). Dung beetles (Scarabeidae family) and earthworms (Lumbricidae family) reduce NH₃ volatilization and denitrification losses by incorporating feces into the soil and by elimination of anaerobic zones within dung piles (Mathews et al., 1996). Measurements of denitrification are needed in tropical grasslands to better understand the N cycle in these ecosystems; however, the availability of such data is limited. This process has been studied more extensively in temperate grasslands (Luo et al., 2000). Regardless, urine is the dominant source of gaseous N losses because urine N is not stabilized by slowly mineralized C compounds as is the case for at least half the N in dung (Jarvis et al., 1995).

Nitrate leaching may be a potential problem, particularly in shallow soils with low water-holding capacity, a large proportion of macropores, and the presence of a superficial water table (Dubeux et al., 2006e). Nitrate leaching, however, is not thought to represent a major problem in highly acid soils present in many tropical regions, such as the Oxisols in the Brazilian Cerrados. The subsoil in this case is characterized by a net positive charge (Oliveira et al., 2000), increasing the potential for nitrate adsorption. In addition, the deep root system of many tropical grasses (Fisher et al., 1994) enables absorption of N at depth in the soil profile (Martha et al., 2004).

	SOIL ORGANIC MATTER

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 
Importance
Because of the inherently low fertility of highly weathered soils and limited use of fertilizers, SOM plays a critical role in sustaining soil fertility and productivity of many warm-climate agroecosystems (Tiessen et al., 2001). Soil OM benefits include improvement of soil physical properties (soil structure, macro- and micro-aggregates, water-holding capacity), soil chemical properties (increased cation exchange capacity, reduced Al toxicity, greater nutrient supply), and soil biological properties (soil microorganism biodiversity). Because SOM is a useful indicator of agroecosystem sustainability, land management should include practices that increase SOM or at least maintain the appropriate SOM for a given land use (Greenland, 1994; Hassink, 1997). Using this criterion, well-managed grasslands are a sustainable land use because they typically are associated with stable or increasing SOM (Dubeux et al., 2006d). Intensity of management is often positively correlated with the rate of increase in SOM in grasslands (Barrow, 1969; Malhi et al., 1997; Bernoux et al., 1999; Pulleman et al., 2000; Batjes, 2004; Dubeux et al., 2006d).

Soil Organic Matter Dynamics and Management
When an ecosystem is in equilibrium, the litter deposition is equal to the litter degradation and SOM is also in equilibrium (Johnson, 1995). Changes in the vegetation or in the soil-tillage system, as observed in the substitution of native vegetation by planted pasture, are often accompanied by a decrease in SOM (Johnson, 1995). This occurs because of greater SOM decomposition rates associated with greater microbial activity and soil perturbation and lesser residue deposition. After an initial reduction in the SOM levels (disturbance phase), a new equilibrium is established between litter production and decomposition rates (Johnson, 1995; Batjes and Sombroek, 1997). Depending on the new soil management, SOM will stabilize at lower, the same, or higher levels than the original ones. Untilled, fertilized, and well-managed perennial grasslands tend to have higher deposition than decomposition rates leading to SOM accumulation.

A considerable amount of N from fertilizer can be incorporated into SOM provided that sufficient residual herbage is left following each grazing (Mathews et al., 2004). In addition to fertilization, forage use and management may also alter C accumulation. Grazed pastures have greater C accumulation than hayfields or even ungrazed, unharvested areas (Franzluebbers et al., 2001; Franzluebbers and Stuedemann, 2003). Upon clearing of native rainforest vegetation, planted pastures showed an increase of 0.33 and 0.89 kg soil C m^–2 (0–30 cm) after 4 and 15 yr, respectively (Bernoux et al., 1999). These authors concluded that 10 yr after pasture establishment, the SOM reached the same level found in the soil under native vegetation, mainly due to contribution from above- and belowground litter deposition. Maximum SOM levels depend on physical stabilization and/or protection of SOM via formation of soil microaggregates, complexation of SOM with silt and clay particles, and/or formation of recalcitrant biochemical compounds (Feller and Beare, 1997; Hassink, 1997; Hassink et al., 1997; Six et al., 2002).

Recalcitrant compounds are hard-to-decompose substances and originate either from compounds found in plants (e.g., tannins, lignin, polyphenols), or are formed during the decomposition process (Six et al., 2002). Lignin, C, N, P, and polyphenols and their ratios are often used as indicators of litter quality (Thomas and Asakawa, 1993). As a general rule, legumes have better-quality residue than grasses, and aboveground residues have better quality than roots and rhizomes. However, large variability exists among species. Thus, the use of plants with low-quality residues and higher allocation of biomass to the root system could be proposed as an alternative to increasing SOM. Fisher et al. (1994) suggested that tropical grasses (e.g., gambagrass, Brachiaria spp.) can increase C storage in the soil not only due to their large root system but also due to the low-quality residue originating from this root system. It is important to keep in mind, however, that large C:N and C:P ratios may lead to net immobilization of nutrients that otherwise could be available for the plants. Fertilization or use of mixed grass–legume pastures reduces immobilization.

A gradual increase of SOM by increasing the primary productivity of the grassland with consequent increases in residue deposition may occur (Dubeux et al., 2006d), but SOM physical protection (aggregate formation and complexing with silt and clay) is limited (Six et al., 2002). The OM deposited beyond this limit may still undergo biochemical protection by forming recalcitrant compounds; however, this quantity is not well established for particular soils (Six et al., 2002). Unprotected OM with higher turnover rates (light fraction) will also increase with greater primary productivity, providing more nutrients to the grassland ecosystem.

	THE ROLE OF THE GRAZING SYSTEM

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 
Grazing system is an overarching term that includes the environment, soil, plant and animal resources, grazing management, and the range of other management practices applied within the system (Sollenberger et al., 2002). A detailed consideration of each of these factors is beyond the scope of this paper, but examples of their impact and importance are developed below.

Environment
Environmental conditions may be the most important noncanopy factor affecting herbivore grazing behavior (Hancock, 1950), and grazing behavior can have a major impact on nutrient redistribution in grasslands. Nutrient accumulation near shade was studied for pastures in which Holstein heifers (Bos taurus) grazed bahiagrass in humid southwestern Japan (Sugimoto et al., 1987). On warm, summer days when temperatures exceeded 27°C, 44 to 53% of urinations and 26 to 29% of defecations occurred in shaded areas. In autumn, when maximum air temperature did not exceed 23.5°C, only 11% of urine and dung deposits occurred in shade areas. In North Carolina, spatial density of feces and urine deposition within 30 m of the water source was greater in July through September than in December through April (White et al., 2001). In humid subtropical Florida during summer, lactating Holstein dairy cows spent more than twice as much time under shade, primarily at the expense of time spent grazing, when photosynthetic photon flux increased from 400 to 950 µmol m^–2 s^–1 (Macoon, 1999). Average summer daytime temperature and photosynthetic photon flux explained 60 and 98%, respectively, of the variation in time spent under shade during the day. Time spent in a given area of the grassland has been found to be closely related (r = 0.99) to the number of excreta events in that area (White et al., 2001). These data clearly show that temperature and solar radiation affect animal behavior and likely the degree of nutrient redistribution in pastures. They also suggest that achieving homogenous distribution of excreta in warm-climate grasslands will be more difficult than in temperate areas because of the greater dependence of animals on shade in warm climates.

There also are likely to be differential effects of the environment on grazing behavior across a range of animal types. Lactating Holstein cows with predominantly black coats spent 20 min d^–1 more time under shade in Florida compared with predominantly white-coated cows (Macoon, 1999), and non-Brahman cattle in Australia spent more time under shade than Brahman cattle (Blackshaw and Blackshaw, 1994). Thus, breed and even coat-color differences within breed may interact with environmental conditions and thereby affect pasture utilization and nutrient redistribution patterns.

The environment will also affect nutrient release or loss from excreta, soil processes, and litter decomposition. Weather after dung deposition is a major factor affecting degradation and release of nutrients (Haynes and Williams, 1993). Dry weather results in formation of a crust on the dung patch, reducing erosion by raindrops, slowing breakdown and nutrient release. Urine-N volatilization increases linearly with soil temperature, and hot and dry conditions increase N losses (Russelle, 1996). Nitrification and denitrification rates generally also increase with temperature (Haynes and Williams, 1993). Fisher et al. (1997) indicated that litter decomposition effectively ceases during the dry season in tropical grasslands. Environment therefore affects the extent of nutrient redistribution and many other critical processes involved in nutrient cycling.

Grazing Management
Stocking Rate
Greater stocking rate increases the proportion of herbage consumed by livestock, increases the importance of excreta relative to litter in nutrient return to the soil, and, because of the greater availability to plants of nutrients in dung and urine relative to litter, increases the rates of nutrient flows between pools (Haynes and Williams, 1993; Castilla et al., 1995). Because of greater losses through the excreta pathway, Boddey et al. (2004) concluded that grassland decline is hastened by increasing stocking rates. Forage utilization could also affect soil nutrient dynamics and depth distribution, potentially changing long-term productivity and environmental quality (Franzluebbers et al., 2004). In Florida, in pasture zones closest to shade and water, Mehlich I extractable P concentrations increased in high and low stocking rate treatments by 25 and 4 mg kg^–1 (0–15 cm soil depth), respectively, following 2 yr of grazing (Sollenberger et al., 2002). In zones that were 8 m or more from shade or water, P concentrations increased by 7 mg kg^–1 for the high stocking rate and decreased by 10 mg kg^–1 for the low stocking rate. Thus, the higher stocking rate increased P redistribution across the pasture, but the size of the increase in P concentration was greatest in lounging areas. Excessive increases in soil P concentration in some areas of the pasture can occur due to high stocking rates, and in general, stocking rate can be considered a key grazing management variable affecting nutrient redistribution.

Grazing Method
Grazing methods can be used to reduce nonuniform excreta distribution. Rotational stocking with short grazing periods and high stocking rates may minimize the variation in amount of time that cattle spend in different pasture zones and increase efficiency of nutrient recycling compared to rotational stocking with long grazing periods or continuous stocking (Haynes and Williams, 1993; Dubeux et al., 2006c). The rationale for this response is that subdividing a pasture into more, smaller paddocks decreases the opportunity and tendency of animals to congregate around lounging areas by intensifying competition for feed and shortening residency periods.

In temperate environments, rotational stocking at high stocking densities enhanced uniformity of excreta return and reduced nutrient gradients in the pasture (Peterson and Gerrish, 1996). In Hawaii, there was no effect on nutrient redistribution for rotationally stocked pastures with grazing periods of 3 or 21 d (Mathews et al., 1999). They suggested that greater stocking density, achieved with more paddocks and shorter grazing periods, may not improve nutrient redistribution in warm climates compared with conventional methods because of greater dependence of livestock on areas of shade. During the summer in Florida, grazing method had no effect on nutrient redistribution in grazed bermudagrass [Cynodon dactylon (L.) Pers.] pastures (Mathews et al., 1994a). In that study, portable shade and water basins were moved daily in pastures, and animals likely spent similar amounts of time under shade among treatments due to hot conditions. In subsequent work in Florida, shade and watering points remained fixed throughout the grazing season for continuously stocked pastures and were moved only when cattle were moved to a new paddock under rotational stocking (Dubeux, 2005). This approach more nearly represents producer practice and resulted in more time spent, greater number of dung and urine deposits (proportional to area), and greater soil nutrient concentrations in zones closest to shade and water for continuously stocked pastures and rotationally stocked pastures with a 21-d grazing period. Pastures that were rotationally stocked with short grazing periods (1–7 d) had greater spatial uniformity in time spent by cattle, excreta deposition, and soil nutrient concentration (Dubeux, 2005).

Other Management Practices
Fertilization
If no fertilizer is applied, and the ability of the soil to supply nutrients is limited, then nutrient recycling is crucial for grassland persistence (Fisher et al., 1997). Often, recycling alone is insufficient to maintain productive grasslands for livestock. Maintenance fertilizer applications are usually required following successful establishment to sustain pasture productivity and survival. In low-fertility acidic soils, Fisher et al. (1997) recommended biannual fertilizer applications to compensate for nutrient losses due to grazing and soil losses. In the case of tropical grass swards, N fertilization is likely needed to minimize pasture degradation associated with production of low-quality litter and subsequent N immobilization by microbes. Fertilization enhances nutrient cycling, acting as a catalyst of key recycling processes, particularly in low soil fertility environments. Fertilization increases the overall biomass production resulting in an increase in (i) stocking rate and excreta deposition (Mathews et al., 2001), (ii) litter production and its respective decomposition rate, and (iii) SOM mineralization rate (Dubeux et al., 2006d). Fertilization with P promoted plant growth (Novais and Smyth, 1999) and accelerated plant residue decomposition, thereby increasing nutrient availability from residues (Cadisch et al., 1994; Gijsman et al., 1997).

Excreta distribution from grazing animals is usually described by a negative binomial function, which is characterized by clustered and overlapped areas of excreta in some pasture areas (Braz et al., 2003). This results in higher soil fertility close to shade, water, and lounging areas, and these patterns must be linked to nutrient management strategies (Mathews et al., 1996; Franzluebbers et al., 2000; Dubeux et al., 2006c). Lounging areas should be avoided in plant and soil sampling for routine fertilizer recommendations and when applying maintenance fertilizer.

Supplementation
Supplements are a source of additional nutrients to the system, and their use may affect nutrient concentrations, pool sizes, and stocking-rate decisions. In most cases feeding concentrate will reduce individual animal forage intake (Moore, 1980; Vendramini et al., 2006). This allows for an increase in stocking rate and thereby enhances the flow of nutrients through the system by increasing the amount of excreta deposited.

Energy availability and synchrony with N compounds is considered one of the most important factors affecting microbial synthesis in the rumen (Valadares Filho and Cabral, 2002). Animals grazing forages with high concentrations of rapidly degraded N should receive supplement that provides a readily available source of energy in the rumen. Valk and Hobbelink (1992) reported increased N-use efficiency with a 50% reduction in N excreted through urine when cows had a balanced diet in terms of energy and protein. In contrast, forages with a low concentration of rumen-degradable N, a characteristic of many C₄ grasses, should be supplemented strategically with rumen-degradable N (Lima et al., 1999).

Irrigation
Supplemental irrigation may enhance pasture productivity (Müller et al., 2002; Marcelino et al., 2003) and may also affect nutrient cycling. Soil moisture is one of the abiotic variables affecting microorganism activity (Brady and Weil, 2002). Therefore, residue and SOM decomposition is also affected by irrigation. Annual N mineralization increased by 50% in irrigated vs. nonirrigated pastures (Pakrou and Dillon, 2000). This was related to higher excreta deposition (higher stocking rate due to irrigation), faster decomposition rate, and higher water availability during the summer season. Irrigation may also increase soil compaction, especially when soils are excessively wet and high stocking rates are used (Warren et al., 1986; Silva et al., 2003). Soil compaction alters nutrient availability due to changes in SOM mineralization, residue decomposition, and nutrient movement in the soil, potentially contributing to pasture degradation (Cantarutti et al., 2001).

Fire
The high N-use efficiency of C₄ grasses together with their rapid growth rates leads to accumulation of plant biomass that is relatively high in lignin and low in N. Levels of forage utilization of introduced pastures in the tropics are commonly low, so a large amount of net primary production is recycled as litter (Fisher et al., 1997). Decomposition rates for this litter are slow, leading to rapid fuel accumulation. Fire is used as a management tool to rid the grassland of this low-quality, unpalatable biomass, to stimulate new growth, and to control woody plants. The flush of growth seen in grasslands after a fire is commonly, but mistakenly, attributed to the fertilizing effect of nutrients returned in the ash (Wedin, 2004). While basic cations, P, and various micronutrients are returned to the soil surface in ash (Raison, 1979), laboratory combustion studies suggest that approximately 90% of the N in plant biomass is volatilized during complete combustion (Lobert et al., 1990). Thus, the long-term effect of fire is to reinforce the N limitation that characterizes most C₄–dominated grasslands (Blair, 1997), and fire as a management tool should be used judiciously to avoid further N depletion.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 
In many warm climates, grassland degradation is a major problem and is due primarily to depletion of soil N and inadequate grazing management. Efficiency of plant capture of nutrients cycling through animal excreta and plant litter has a major impact on warm-climate grassland persistence and sustainability. Factors that contribute to the elevated importance of nutrients from excreta and plant litter in these environments include the preponderance of C₄ grasses, limited contribution by legumes, prevalence of infertile soils, minimal input of inorganic fertilizers, and the relatively low level of forage utilization by grazing herbivores leading to large accumulations of low-quality plant litter. Of concern from the perspective of grassland sustainability is that nutrient loss to the environment from excreta can be significant, and nutrient release from litter extremely slow. Management practices that improve the uniformity of excreta distribution and minimize losses of nutrients from excreta are needed, as are strategies to improve litter quality and reduce nutrient immobilization, especially N and P.

Much of the research conducted to date has focused on components of the nutrient cycle. This is understandable considering the complexity of the overall system. Research that integrates multiple pools and the fluxes among them is needed, yet funding for such work is limited. Modeling merits consideration as an approach for integrating multiple components of the system and for predicting the impacts of varying levels of individual components. Our ability to understand nutrient dynamics and to use that understanding to develop appropriate management strategies may determine the future of grassland agroecosystems in warm climates.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication September 15, 2006.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION NUTRIENT POOLS IN GRASSLAND... NUTRIENT DYNAMICS IN GRASSLAND... SOIL ORGANIC MATTER THE ROLE OF THE... CONCLUSIONS REFERENCES

 

• Ayarza, M.A. 1988. Potassium dynamics in a humid tropical pasture in the Peruvian Amazon. Ph.D. diss. North Carolina State Univ., Raleigh.
• Bardgett, R.D., and D.A. Wardle. 2003. Herbivore-mediated linkages between aboveground and belowground communities. Ecology 84:2258–2268.[CrossRef][Web of Science]
• Barrow, N.J. 1969. The accumulation of soil organic matter under pasture and its effects on soil properties. Aust. J. Exp. Agric. Anim. Husb. 9:437–445.[CrossRef]
• Batjes, N.H. 2004. Estimation of soil carbon gains upon improved management within croplands and grasslands of Africa. Environ. Dev. Sustain. 6:133–143.[CrossRef]
• Batjes, N.H., and N.H. Sombroek. 1997. Possibilities for carbon sequestration in tropical and subtropical soils. Glob. Change Biol. 3:161–173.[CrossRef]
• Beck, M.A., and P.A. Sánchez. 1994. Soil phosphorus fraction dynamics during 18 years of cultivation on a Typic Paleudult. Soil Sci. Soc. Am. J. 58:1424–1431.[Abstract/Free Full Text]
• Bernoux, M., B.J. Feigl, C.C. Cerri, A.P.A. Geraldes, and S.A.P. Fernandes. 1999. Carbono e nitrogênio em solos de uma cronosseqüência de floresta tropical: Pastagem de Paragominas. Sci. Agric. (Piracicaba) 56:777–783.
• Blackshaw, J.K., and A.W. Blackshaw. 1994. Heat stress in cattle and the effect of shade on production and behaviour: A review. Aust. J. Exp. Agric. Anim. Husb. 34:285–295.[CrossRef]
• Blair, J.M. 1997. Fire, N availability, and plant response in grasslands: A test of the transient maxima hypothesis. Ecology 78:2359–2368.[CrossRef][Web of Science]
• Blue, W.G. 1988. Response of Pensacola bahiagrass (Paspalum notatum Flügge) to fertilizer nitrogen on an Entisol and a Spodosol in north Florida. Proc. Soil Crop Sci. Soc. Fla. 47:135–139.
• Boddey, R.M., R. Macedo, R.M. Tarré, E. Ferreira, O.C. de Oliveira, C.P. de Rezende, R.B. Cantarutti, J.M. Pereira, B.J.R. Alves, and S. Urquiaga. 2004. Nitrogen cycling in Brachiaria pastures: The key to understanding the process of pasture decline. Agric. Ecosyst. Environ. 103:389–403.[CrossRef]
• Bower, C.A. 1977. Potassium in Hawaiian soils. p. 43–45. In Hawaii Fertilizer Conf., 10th. Univ. of Hawaii, Hilo.
• Brady, N.C., and R.R. Weil. 2002. The nature and properties of soil. 13th ed. Prentice Hall, New Jersey.
• Braz, S.P., D. Nascimento, Jr., R.B. Cantarutti, C.E. Martins, D.M. Fonseca, and R.A. Barbosa. 2003. Caracterização da distribuição espacial das fezes por bovinos em uma pastagem de Brachiaria decumbens. (In Portuguese, with English abstract.) Rev. Bras. Zootecnia 32:787–794.
• Braz, S.P., D. Nascimento, Jr., R.B. Cantarutti, A.J. Regazzi, C.E. Martins, D.M. Fonseca, and R.A. Barbosa. 2002. Aspectos quantitativos do processo de reciclagem de nutrientes pelas fezes de bovinos sob pastejo em pastagem de Brachiaria decumbens na Zona da Mata de Minas Gerais. (In Portuguese, with English abstract.) Rev. Bras. Zootecnia 31:858–865.
• Cadisch, G., K.E. Giller, S. Urquiaga, C.H.B. Miranda, R.M. Boddey, and R.M. Schunke. 1994. Does phosphorus supply enhance soil-N mineralization in Brazilian pastures? Eur. J. Agron. 3:339–345.
• Cantarutti, R.B. 1996. Dinâmica de nitrogênio em pastagens de Brachiaria humidicola em monocultivo e consorciada com Desmodium ovalifolium cv. Itabela no sul da Bahia. (In Portuguese, with English abstract.) D.S. diss. Federal Univ. of Viçosa, Viçosa, Minas Gerais, Brazil.
• Cantarutti, R.B., D. Nascimento, Jr., and O.V. Costa. 2001. Impacto do animal sobre o solo: Compactação e reciclagem de nutrientes. (In Portuguese, with English abstract.) CD-ROM. Reunião Anual da Sociedade Brasileira de Zootecnia. SBZ, Piracicaba, São Paulo, Brazil.
• Cantarutti, R.B., R. Tarré, R. Macedo, G. Cadisch, C.P. de Rezende, J.M. Pereira, J.M. Braga, J.A. Gomide, E. Ferreira, B.J.R. Alves, S. Urquiaga, and R.M. Boddey. 2002. The effect of grazing intensity and the presence of a forage legume on nitrogen dynamics in Brachiaria pastures in the Atlantic forest region of the south of Bahia, Brazil. Nutr. Cycling Agroecosyst. 64:257–271.[CrossRef]
• Carreiro, M.M., R.L. Sinsabaugh, D.A. Repert, and D.G. Parkhurst. 2000. Microbial enzyme shifts explain litter decay responses to simulated nitrogen deposition. Ecology 81:2359–2365.[CrossRef][Web of Science]
• Castilla, C.E. 1992. Carbon dynamics in managed tropical pastures: The effect of stocking rate on soil properties and above- and below-ground carbon inputs. Ph.D. diss. North Carolina State Univ., Raleigh.
• Castilla, C.E., M.A. Ayarza, and P.A. Sanchez. 1995. Carbon and potassium dynamics in grass/legume grazing systems in the Amazon. p. 191–210. In J. M. Powell (ed.) Livestock and sustainable nutrient cycling in mixed farming systems of sub-Saharan Africa. Vol. 2. ILCA, Addis Ababa, Ethiopia.
• Dalal, R.C. 1979. Mineralization of carbon and phosphorus from carbon-14 and phosphorus-32 labeled plant material added to soil. Soil Sci. Soc. Am. J. 43:913–916.[Abstract/Free Full Text]
• Decaëns, T., P. Lavelle, J.J. Jiminez, G. Escobar, and G. Rippstein. 1994. Impact of land management on soil macrofauna in the Oriental Llanos of Colombia. Eur. J. Soil Biol. 39:157–168.[CrossRef]
• Dubeux, J.C.B., Jr. 2005. Management strategies to improve nutrient cycling in grazed Pensacola bahiagrass pastures. Ph.D. diss. Univ. of Florida, Gainesville.
• Dubeux, J.C.B., Jr., H.Q. Santos, and L.E. Sollenberger. 2004. Ciclagem de nutrientes: Perspectivas de aumento da sustentabilidade da pastagem manejada intensivamente. p. 357–400. In C.G.S. Pedreira et al (ed.) Fertilidade do solo para pastagens produtivas. Luiz de Queiroz Agricultural Foundation, Piracicaba, São Paulo, Brazil.
• Dubeux, J.C.B., Jr., L.E. Sollenberger, J.M.B. Vendramini, R.L. Stewart, Jr., and S.M. Interrante. 2006a. Litter mass, deposition rate, and chemical composition in bahiagrass pastures managed at different intensities. Crop Sci. 46:1299–1304.[Abstract/Free Full Text]
• Dubeux, J.C.B., Jr., L.E. Sollenberger, S.M. Interrante, J.M.B. Vendramini, and R.L. Stewart, Jr. 2006b. Litter decomposition and mineralization in bahiagrass pastures managed at different intensities. Crop Sci. 46:1305–1310.[Abstract/Free Full Text]
• Dubeux, J.C.B., Jr., R.L. Stewart, Jr., L.E. Sollenberger, J.M.B. Vendramini, and S.M. Interrante. 2006c. Spatial heterogeneity of herbage response to management intensity in continuously stocked Pensacola bahiagrass pastures. Agron. J. 98:1453–1459.[Abstract/Free Full Text]
• Dubeux, J.C.B., Jr., L.E. Sollenberger, N.B. Comerford, J.M. Scholberg, A.C. Ruggieri, J.M.B. Vendramini, S.M. Interrante, and K.M. Portier. 2006d. Management intensity affects density fractions of soil organic matter from grazed bahiagrass swards. Soil Biol. Biochem. 38:2705–2711.[CrossRef]
• Dubeux, J.C.B., Jr., M.A. Lira, M.V.F. Santos, and M.V. Cunha. 2006e. Fluxo de nutrientes em ecossistemas de pastagens: Impactos no ambiente e na produtividade. p. 439–506. In C.G.S. Pedreira et al (ed.) As pastagens e o meio ambiente. Luiz de Queiroz Agricultural Foundation, Piracicaba, SP, Brazil.
• Eilitta, M., L.E. Sollenberger, R.C. Littell, and L.W. Harrington. 2003. On-farm experiments with maize–mucuna systems in southeastern Veracruz, Mexico: I. Mucuna biomass and maize grain yield. Exp. Agric. 39:5–17.[CrossRef]
• Feller, C., and M.H. Beare. 1997. Physical control of soil organic matter dynamics in the tropics. Geoderma 79:69–116.[CrossRef][Web of Science]
• Ferreira, E., C.P. Rezende, R.M. Boddey, S. Urquiaga, and B.J.R. Alves. 1997. Decomposição da liteira de diferentes espécies forrageiras avaliadas no campo em diversas condições climáticas. (Arq496; CD-ROM.) 26th Congresso brasileiro de ciência do solo. SBCS, Rio de Janeiro, Rio de Janeiro.
• Fisher, M.J., I.M. Rao, and R.J. Thomas. 1997. Nutrient cycling in tropical pastures, with special reference to the neotropical savannas. p. 371–382. In J.G. Buchanan-Smith et al (ed.) Int. Grassl. Cong., 18th, Winnipeg and Saskatoon, Canada. 8–17 June 1997. Canadian Grassl. Soc., Winnipeg.
• Fisher, M.J., I.M. Rao, M.A. Ayarza, C.E. Lascano, J.I. Sanz, R.J. Thomas, and R.R. Vera. 1994. Carbon storage by introduced deep-rooted grasses in the South American savannas. Nature 371:236–238.[CrossRef]
• Fog, K. 1988. The effect of added nitrogen on the rate of decomposition of organic matter. Biol. Rev. 63:433–462.
• Franzluebbers, A.J., and J.A. Stuedemann. 2003. Bermudagrass management in the southern Piedmont USA: III. Particulate and biologically active soil carbon. Soil Sci. Soc. Am. J. 67:132–138.[Abstract/Free Full Text]
• Franzluebbers, A.J., J.A. Stuedemann, and H.H. Schomberg. 2000. Spatial distribution of soil carbon and nitrogen pools under grazed tall fescue. Soil Sci. Soc. Am. J. 64:635–639.[Abstract/Free Full Text]
• Franzluebbers, A.J., J.A. Stuedemann, and S.R. Wilkinson. 2001. Bermudagrass management in the southern Piedmont USA: I. Soil and surface residue carbon and sulfur. Soil Sci. Soc. Am. J. 65:834–841.[Abstract/Free Full Text]
• Franzluebbers, A.J., S.R. Wilkinson, and J.A. Stuedemann. 2004. Bermudagrass management in the Southern Piedmont USA: VIII. Soil pH and nutrient cations. Agron. J. 96:1390–1399.[Abstract/Free Full Text]
• Friesen, D.K., I.M. Rao, R.J. Thomas, A. Oberson, and J.I. Sanz. 1997. Phosphorus acquisition and cycling in crop and pasture systems in low fertility tropical soils. Plant Soil 196:289–294.[CrossRef][Web of Science]
• Garcia-Montiel, D.C., and D. Binkley. 1998. Effect of Eucalyptus saligna and Albizia falcataria on soil processes and nitrogen supply in Hawaii. Oecologia 113:547–556.[CrossRef][Web of Science]
• Gerrish, J.R., J.R. Brown, and P.R. Peterson. 1993. Impact of grazing cattle on distribution of soil mineral. p. 66–70. In American Forage and Grassland Council Proc., Des Moines, IA. 29–31 March 1993. American Forage and Grassland Council, Georgetown, TX.
• Gijsman, A.J., H.F. Alarcón, and R.J. Thomas. 1997. Root decomposition in tropical grasses and legumes, as affected by soil texture and season. Soil Biol. Biochem. 29:1443–1450.[CrossRef]
• Greenland, D.J. 1994. Soil science and sustainable land management. p. 1–15. In J.K. Sypers and D.L. Rimmer (ed.) Soil science and sustainable land management in the tropics. CAB Int., Wallingford, UK.
• Guerra, J.G.M., M.C.C. Fonseca, D.L. Almeida, H. Polli, and M.S. Fernandes. 1995. Conteúdo de fósforo da biomassa microbiana de um solo cultivado com Brachiaria decumbens Stapf. (In Portuguese, with English abstract.) Pesqui. Agropecu. Bras. 30:543–551.
• Hancock, J. 1950. Grazing behaviour of cattle. Anim. Breed. Abstr. 21:1–13.
• Hassink, J. 1997. The capacity of soils to preserve organic C and N by their association with clay and silt particles. Plant Soil 191:77–87.[CrossRef][Web of Science]
• Hassink, J., A.P. Whitmore, and J. Kubát. 1997. Size and density fractionation of soil organic matter and the physical capacity of soils to protect organic matter. Eur. J. Agron. 7:189–199.[CrossRef]
• Haynes, R.J., and P.H. Williams. 1993. Nutrient cycling and soil fertility in the grazed pasture ecosystem. Adv. Agron. 49:119–199.
• Henzell, E.F., and P.J. Ross. 1973. The nitrogen cycle of pasture ecosystems. p. 227–246. In G.W. Butler and R.W. Bailey (ed.) Chemistry and biochemistry of herbage. Vol. 2. Academic Press, London.
• Jarvis, S.C., D. Scholefield, and B. Pain. 1995. Nitrogen cycling in grazing systems. p. 381–419. In P.E. Bacon (ed.) Nitrogen fertilization in the environment. Marcel Dekker, New York.
• Jobbagy, E.G., and R.B. Jackson. 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecol. Appl. 10:423–436.[CrossRef]
• Johnson, M.G. 1995. The role of soil management in sequestering soil carbon. p. 351–363. In R. Lal, J. Kimble, E. Levine, and B.A. Stewart (ed.) Soil management and greenhouse effect. Lewis, Boca Raton, FL.
• Kanno, T., M.C. Macedo, V.P.B. Euclides, J.A. Bono, J.D.G. Santos, M.C. Rocha, and L.G.R. Beretta. 1999. Root biomass of five tropical grass pastures under continuous grazing in Brazilian savannas. Grassl. Sci. 45:9–14.
• Kayser, M., and J. Isselstein. 2005. Potassium cycling and losses in grassland systems: A review. Grass Forage Sci. 60:213–224.[CrossRef]
• Lal, R., J. Kimble, E. Levine, and C. Whitman. 1995. Towards improving the global data base on soil carbon. p. 433–436. In R. Lal (ed.) Soils and global change. CRC Lewis, Boca Raton, FL.
• Lima, G.F.C., L.E. Sollenberger, W.E. Kunkle, J.E. Moore, and A.C. Hammond. 1999. Nitrogen fertilization and supplementation effects on performance of beef heifers grazing limpograss. Crop Sci. 39:1853–1858.[Abstract/Free Full Text]
• Lobert, J.M., D.H. Scharffe, W.M. Hao, and P.J. Crutzen. 1990. Importance of biomass burning in the atmospheric budgets of nitrogen-containing gases. Nature 346:552–554.[CrossRef]
• Luo, J., R.W. Tillman, and P.R. Ball. 2000. Nitrogen loss through denitrification in a soil under pasture in New Zealand. Soil Biol. Biochem. 32:497–509.[CrossRef]
• Macoon, B. 1999. Forage and animal responses in pasture-based dairy production systems for lactating cows. Ph.D. diss. Univ. of Florida, Gainesville.
• Malhi, S.S., M. Nyborg, J.T. Harapiak, K. Heier, and N.A. Flore. 1997. Increasing organic C and N in soil under bromegrass with long-term N fertilization. Nutr. Cycling Agroecosyst. 49:255–260.[CrossRef]
• Marcelino, K.R.A., L. Vilela, G.G. Leite, A.F. Guerra, and J.M.S. Diogo. 2003. Manejo da adubação nitrogenada e de tensões hídricas sobre a produção de matéria seca e índice de área foliar de Tifton 85 cultivado no cerrado. (In Portuguese, with English abstract.) Rev. Bras. Zootecnia 32:268–275.
• Marschner, H. 1995. Mineral nutrition of higher plants. 2nd ed. Academic Press, New York.
• Martha, G.B., Jr., L. Vilela, L.G. Barioni, D.M.G. Sousa, and A.O. Barcellos. 2004. Manejo da adubação nitrogenada em pastagens. p. 155–215. In C.G.S. Pedreira, J.C. Moura, and V.P. Faria (ed.) Fertilidade do solo para pastagens produtivas. Luiz de Queiroz Agricultural Foundation, Piracicaba, São Paulo, Brazil.
• Mathews, B.W., J.R. Carpenter, L.E. Sollenberger, and K.D. Hisashima. 2001. Macronutrient, soil organic carbon, and earthworm distribution in subtropical pastures on an Andisol with and without long-term fertilization. Commun. Soil Sci. Plant Anal. 32:209–230.[CrossRef][Web of Science]
• Mathews, B.W., J.R. Carpenter, L.E. Sollenberger, and S. Tsang. 2005. Phosphorus in Hawaiian kikuyugrass pastures and potential phosphorus release to water. J. Environ. Qual. 34:1214–1223.[Abstract/Free Full Text]
• Mathews, B.W., S.C. Miyasaka, and J.P. Tritschler. 2004. Mineral nutrition of C₄ forage grasses. p. 217–265. In L. E. Moser, B.L. Burson, and L.E. Sollenberger (ed.) Warm-season (C₄) grasses. Agron. Monogr. 45. ASA, CSSA, and SSSA, Madison, WI.
• Mathews, B.W., L.E. Sollenberger, V.D. Nair, and C.R. Staples. 1994a. Impact of grazing management on soil nitrogen, phosphorus, potassium, and sulfur distribution. J. Environ. Qual. 23:1006–1013.[Abstract/Free Full Text]
• Mathews, B.W., L.E. Sollenberger, P. Nkedi-Kizza, L.A. Gaston, and H.D. Hornsby. 1994b. Soil sampling procedures for monitoring potassium distribution in grazed pastures. Agron. J. 86:121–126.[Abstract/Free Full Text]
• Mathews, B.W., L.E. Sollenberger, and J.P. Tritschler, II. 1996. Grazing systems and spatial distribution of nutrients in pastures: Soil considerations. p. 213–229. In R.E. Joost and C.A. Roberts (ed.) Nutrient cycling in forage systems. PPI/FAR, Columbia, MO.
• Mathews, B.W., J.P. Tritschler, J.R. Carpenter, and L.E. Sollenberger. 1999. Soil macronutrient distribution in rotationally stocked kikuyugrass paddocks with short and long grazing periods. Commun. Soil Sci. Plant Anal. 30:557–571.[Web of Science]
• McDowell, L.R. 1996. Feeding minerals to cattle on pasture. Anim. Feed Sci. Technol. 60:247–271.[CrossRef]
• McLaughlin, M.J., and A.M. Alston. 1986. The relative contribution of plant residues and fertilizer to the phosphorus nutrition of wheat in a pasture/cereal system. Aust. J. Soil Res. 24:517–526.[CrossRef]
• McLaughlin, M.J., T.G. Baker, T.R. James, and J.A. Rundle. 1990. Distribution and forms of phosphorus and aluminum in acidic topsoils under pastures in southeastern Australia. Aust. J. Soil Res. 28:371–385.[CrossRef]
• Melo, V.F. 1998. Potássio e magnésio em minerais de solos e relação entre propriedades da caulinita com formas não-trocáveis destes nutrientes. (In Portuguese, with English abstract.) Ph.D. diss. Federal Univ. of Viçosa, Viçosa, Minas Gerais, Brazil.
• Miles, J.W., C.B. do Valle, I.M. Rao, and V.P.B. Euclides. 2004. Brachiariagrasses. p. 745–783. In L.E. Moser, B.L. Burson, and L.E. Sollenberger (ed.) Warm-season (C₄) grasses. Agron. Monogr. 45. ASA, CSSA, and SSSA, Madison, WI.
• Minson, D.J. 1990. Forage in ruminant nutrition. Academic Press, San Diego, CA.
• Miyasaka, S.C., and M. Habte. 2001. Plant mechanisms and mycorrhizal symbioses to increase phosphorus uptake efficiency. Commun. Soil Sci. Plant Anal. 32:1101–1148.[CrossRef][Web of Science]
• Moore, J.E. 1980. Forage crops. p. 61–91. In C.S. Hoveland (ed.) Crop quality, storage, and utilization. ASA and CSSA, Madison, WI.
• Müller, M.S., A.L. Fancelli, D. Dourado Neto, A.G. García, and R.F.L. Ovejero. 2002. Produtividade do Panicum maximum cv. Mombaça irrigado, sob pastejo rotacionado. (In Portuguese, with English abstract.) Sci. Agric. (Piracicaba) 59:427–433.
• Myers, R.J.K., C.A. Palm, E. Cuevas, I.U.N. Gunatillike, and M. Brossard. 1994. The synchronisation of nutrient mineralisation and plant nutrient demand. p. 81–116. In P. L. Woomer and M. J. Swift (ed.) The biological management of tropical soil fertility. John Wiley & Sons, Chichester, UK.
• Myers, R.J.K., I. Vallis, W.B. McGill, and E.F. Henzell. 1986. Nitrogen in grass-dominant, unfertilized pasture systems. p. 761–771. Int. Soc. Soil Sci. Cong., 13th. Vol. 6. 13–20 Aug. 1986. Hamburg, Germany.
• Novais, R.F., and T.J. Smyth. 1999. Fósforo em solo e planta em condições tropicais. Federal Univ. of Viçosa, Viçosa, Minas Gerais, Brazil.
• Oberson, A., D.K. Friesen, H. Tiessen, C. Morel, and W. Stahel. 1999. Phosphorus status and cycling in native savanna and improved pastures on an acid low-P Colombian Oxisol. Nutr. Cycling Agroecosyst. 55:77–88.[CrossRef]
• Olander, L.P., and P.M. Vitousek. 2005. Short-term controls over inorganic phosphorus during soil and ecosystem development. Soil Biol. Biochem. 37:651–659.[CrossRef]
• Oliveira, J.R.A., L. Vilela, and M.A. Ayarza. 2000. Adsorção de nitrato em solos de cerrado do distrito federal. (In Portuguese, with English abstract.) Pesqui. Agropecu. Bras. 35:1199–1205.
• Pakrou, N., and P. Dillon. 2000. Key processes of the nitrogen cycle in an irrigated and a non-irrigated grazed pasture. Plant Soil 224:231–250.[CrossRef][Web of Science]
• Peterson, P.R., and J.R. Gerrish. 1996. Grazing systems and spatial distribution of nutrients in pastures: Livestock management considerations. p. 203–212. In R.E. Joost and C.A. Roberts (ed.) Nutrient cycling in forage systems. PPI/FAR, Columbia, MO.
• Pulleman, M.M., J. Bouma, E.A. Van Essen, and E.W. Meijles. 2000. Soil organic matter as a function of different land use history. Soil Sci. Soc. Am. J. 64:689–693.[Abstract/Free Full Text]
• Raison, R.J. 1979. Modification of the soil environment by vegetation fires, with particular reference to nitrogen transformations: A review. Plant Soil 51:73–108.[CrossRef][Web of Science]
• Rao, I.M. 1998. Root distribution and production in native and introduced pastures in the South America savannas. p. 19–41. In J.E. Box, Jr. (ed.) Root demographics and their efficiencies in sustainable agriculture, grasslands, and ecosystems. Kluwer Academic Publishers, Amsterdam, the Netherlands.
• Rao, I.M., D.K. Friesen, and M. Osaki. 1999. Plant adaptation to phosphorus-limited tropical soils. p. 61–95. In M. Pessarakli (ed.) Handbook of plant and crop stress. 2nd. ed. Marcel Dekker, New York.
• Resh, S.C., D. Binkley, and J.A. Parrotta. 2002. Greater soil carbon sequestration under nitrogen-fixing trees compared with Eucalyptus species. Ecosystems 5:217–231.[CrossRef]
• Rezende, C.P., R.B. Cantarutti, J.M. Braga, J.A. Gomide, E. Ferreira, R. Tarré, R. Macedo, B.J.R. Alves, S. Urquiaga, G. Cadisch, K.E. Giller, and R.M. Boddey. 1999. Litter deposition and disappearance in Brachiaria pastures in the Atlantic forest region of the South of Bahia, Brazil. Nutr. Cycling Agroecosyst. 54:99–112.[CrossRef]
• Robbins, G.B., J.J. Bushell, and G.M. McKeon. 1989. Nitrogen immobilization in decomposing litter contributes to productivity decline in ageing pastures of green panic (Panicum maximum var. trichoglume). J. Agric. Sci. 113:401–406.
• Robertson, F.A., R.J.K. Myers, and P.G. Saffigna. 1993a. Carbon and nitrogen mineralization in cultivated and grassland soils in subtropical Queensland. Aust. J. Soil Res. 31:611–619.[CrossRef]
• Robertson, F.A., R.J.K. Myers, and P.G. Saffigna. 1993b. Distribution of carbon and nitrogen in a long-term cropping system and permanent pasture system. Aust. J. Agric. Res. 44:1323–1336.[CrossRef][Web of Science]
• Rotz, C.A., F. Taube, M.P. Russelle, J. Oenema, M.A. Sanderson, and M. Wachendorf. 2005. Whole-farm perspectives of nutrient flows in grassland agriculture. Crop Sci. 45:2139–2159.[Abstract/Free Full Text]
• Russelle, M.P. 1992. Nitrogen cycling in pasture and range. J. Prod. Agric. 5:13–28.
• Russelle, M.P. 1996. Nitrogen cycle in pasture systems. p. 125–166. In R.E. Joost and C.A. Roberts (ed.) Nutrient cycling in forage systems. PPI/FAR, Columbia, MO.
• Ryden, J.C. 1986. Gaseous losses of nitrogen from grassland. p. 59–74. In H.G. Van Der Meer (ed.) Nitrogen fluxes in intensive grassland systems. Martinus Nijhoff Publishers, Dordrecht, the Netherlands.
• Schlesinger, W.H. 1997. Biogeochemistry: An analysis of global change. Academic Press, San Diego, CA.
• Scholefield, D., and O. Oenema. 1997. Nutrient cycling within temperate agricultural grasslands. p. 357–370. In J.G. Buchanan-Smith (ed.) Int. Grassl. Cong. Proc., 18th, Winnipeg and Saskatoon, Canada. 8–17 June 1997. Canadian Grassl. Soc., Winnipeg.
• Schunke, R.M. 1998. Qualidade, decomposição e liberação de nutrientes da "litter" de quatro cultivares de Panicum maximum Jacq. (In Portuguese, with English abstract.) Ph.D. diss. Rural Federal Univ. of Rio de Janeiro, Seropédica, Rio de Janeiro.
• Silva, A.P., S. Imhoff, and M. Corsi. 2003. Evaluation of soil compaction in an irrigated short-duration grazing system. Soil Tillage Res. 70:83–90.[CrossRef]
• Silva, J.E., D.V.S. Resck, E.J. Corazza, and L. Vivaldi. 2000. Carbon storage under cultivated pastures in a clayey Oxisol in the cerrado region. (S5 T4; CD-ROM.) 1st Int. Symp.: Aoil Functioning under Pasture in Intertropical Areas. EMBRAPA, IRD, Brazil.
• Six, J., R.T. Conant, E.A. Paul, and K. Paustian. 2002. Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils. Plant Soil 241:155–176.[CrossRef][Web of Science]
• Sollenberger, L.E., J.C.B. Dubeux, Jr., H.Q. Santos, and B.W. Mathews. 2002. Nutrient cycling in tropical pasture ecosystems. p. 151–179. In A.M.V. Batista et al. (ed.) Reunião Anual da Soc. Bras. Zootecnia. 30 July–2 Aug. 2002. SBZ, Recife, Pernambuco, Brazil.
• Sollenberger, L.E., R.A. Reis, L.G. Nussio, C.G. Chambliss, and W.E. Kunkle. 2004. Forage conservation. p. 355–387. In L.E. Moser, B.L. Burson, and L.E. Sollenberger (ed.) Warm-season (C₄) grasses. Agron. Monogr. 45. ASA, CSSA, and SSSA, Madison, WI.
• Stevenson, F.J., and M.A. Cole. 1999. Cycles of soil-carbon, nitrogen, phosphorus, sulfur, and micronutrients. 2nd ed. John Wiley & Sons, New York.
• Sugimoto, Y., M. Hirata, and M. Ueno. 1987. Energy and matter flows in bahiagrass pasture. V. Excreting behavior of Holstein heifers. J. Jpn. Soc. Grassl. Sci. 33:8–14.
• Tarré, R., R. Macedo, R.B. Cantarutti, C.P. de Rezende, J.M. Pereira, E. Ferreira, B.J.R. Alves, S. Urquiaga, and R.M. Boddey. 2001. The effect of the presence of a forage legume on nitrogen and carbon levels in soils under Brachiaria pastures in the Atlantic forest region of the south of Bahia, Brazil. Plant Soil 234:15–26.[CrossRef][Web of Science]
• Thomas, R.J. 1992. The role of the legume in the nitrogen cycle of productive and sustainable pastures. Grass Forage Sci. 47:133–142.[CrossRef]
• Thomas, R.J., and N.M. Asakawa. 1993. Decomposition of leaf litter from tropical grasses and legumes. Soil Biol. Biochem. 25:1351–1361.[CrossRef]
• Tiessen, H., E.V.S.B. Sampaio, and I.H. Salcedo. 2001. Organic matter turnover and management in low input agriculture of NE Brazil. Plant Soil 61:99–103.
• Urquiaga, S., G. Cadisch, B.J.R. Alves, R.M. Boddey, and K.E. Giller. 1998. Influence of decomposition of roots of tropical forage species on the availability of soil nitrogen. Soil Biol. Biochem. 30:2099–2106.[CrossRef]
• Valadares Filho, S.C., and L.S. Cabral. 2002. Aplicação dos princípios de nutrição de ruminantes em regiões tropicais. p. 514–543. In A.M.V. Batista et al. (ed.) Reunião Anual da Soc. Bras. Zootecnia. 30 July–2 Aug. 2002. SBZ, Recife, Pernambuco, Brazil.
• Valk, H., and M.E.J. Hobbelink. 1992. Supplementation of dairy grazing cows to reduce environmental pollution. p. 400–405. General Meeting of the European Grassland Fed., 14th, Lahti, Finland. 8–11 June. European Grassland Federation, Cirencester, UK.
• Veldkamp, E., M. Keller, and M. Nuñez. 1998. Effects of pasture management on N₂O and NO emissions from soils in the humid tropics of Costa Rica. Global Biogeochem. Cycles 12:71–79.[CrossRef][Web of Science]
• Vendramini, J.M.B., L.E. Sollenberger, J.C.B. Dubeux, Jr., S.M. Interrante, R.L. Stewart, Jr., and J.D. Arthington. 2006. Concentrate supplementation effects on forage characteristics and performance of early weaned calves grazing rye-ryegrass pastures. Crop Sci. 46:1595–1600.[Abstract/Free Full Text]
• Vera, R.R. 2001. The future for savanna and tropical grasslands: A Latin American perspective. p. 935–938. In J.A. Gomide et al (ed.) Proc. Int. Grassl. Cong., 19th, São Pedro, Brazil, 10–21 Feb. Brazilian Society of Animal Husbandry, Piracicaba, Brazil.
• Warren, S.D., M.B. Nevill, W.H. Blackburn, and N.E. Garza. 1986. Soil response to trampling under intensive rotational grazing. Soil Sci. Soc. Am. J. 50:1336–1341.[Abstract/Free Full Text]
• Wedin, D.A. 1996. Nutrient cycling in grasslands: An ecologist's perspective. p. 29–44. In R.E. Joost and C.A. Roberts (ed.) Nutrient cycling in forage systems. PPI/FAR, Columbia, MO.
• Wedin, D.A. 2004. C₄ grasses: Resource use, ecology and global change. p. 15–50. In L.E. Moser, B.L. Burson, and L.E. Sollenberger (ed.) Warm-season (C₄) grasses. Agron. Monogr. 45. ASA, CSSA, and SSSA, Madison, WI.
• White, S.L., R.E. Sheffield, S.P. Washburn, L.D. King, and J.T. Green, Jr. 2001. Spatial and time distribution of dairy cattle excreta in an intensive pasture systems. J. Environ. Qual. 30:2180–2187.[Abstract/Free Full Text]
• Whitehead, D.C. 1970. The role of nitrogen in grassland productivity. Commonwealth Agricultural Bureau, Farnham Royal, Bucks, UK.
• Wilkinson, S.R., J.A. Stuedemann, and D.P. Belesky. 1989. Soil potassium distribution in grazed K-31 tall fescue pastures as affected by fertilization and endophytic fungus infection level. Agron. J. 81:508–512.[Abstract/Free Full Text]
• Williams, P.H., P.E.H. Gregg, and M.J. Hedley. 1990. Mass balance modeling of potassium losses from grazed dairy pasture. N. Z. J. Agric. Res. 33:661–668.
• Woodard, K.R., E.C. French, L.A. Sweat, D.A. Graetz, L.E. Sollenberger, B. Macoon, K.M. Portier, S.J. Rymph, B.L. Wade, G.M. Prine, and H.H. Van Horn. 2003. Nitrogen removal and nitrate leaching for two perennial, sod-based forage systems receiving dairy effluent. J. Environ. Qual. 32:996–1007.[Abstract/Free Full Text]

This article has been cited by other articles:

				A. H. Heggenstaller, K. J. Moore, M. Liebman, and R. P. Anex Nitrogen Influences Biomass and Nutrient Partitioning by Perennial, Warm-Season Grasses Agron. J., November 1, 2009; 101(6): 1363 - 1371. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Dubeux, J. C. B. Articles by Santos, H. Q. Search for Related Content PubMed Articles by Dubeux, J. C. B., Jr. Articles by Santos, H. Q. Agricola Articles by Dubeux, J. C. B. Articles by Santos, H. Q.