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Published online 1 July 2008
Published in Crop Sci 48:1586-1594 (2008)
© 2008 Crop Science Society of America
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Does a Mixed-Species Landscape Reduce Inorganic-Nitrogen Leaching Compared to a Conventional St. Augustinegrass Lawn?

J. E. Ericksona,*, J. L. Cisarb, G.H. Snyderc, D. M. Parkd and K. E. Williamsb

a Agronomy, Univ. of Florida, P.O. Box 110500, Gainesville, FL 32611
b Environmental Horticulture, Univ. of Florida, Fort Lauderdale Research and Education Center, 3205 College Ave., Fort Lauderdale, FL 33314
c Soil and Water Science, Univ. of Florida, Everglades Research and Education Center, 3200 East Palm Beach Rd., Belle Glade, FL 33430
d Horticulture, Clemson Univ., Pee Dee Research and Education Center, 2200 Pocket Rd., Florence, SC 29506

* Corresponding author (jerickson{at}ufl.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Low maintenance vegetation may reduce N leaching following establishment compared to routinely fertilized conventional turfgrass lawns. Therefore, using a field-scale facility we examined N leaching from contrasting residential landscape models established on a sandy soil. Four replications each of a St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] monoculture (SA) and a mixed-species (MS) landscape were randomly assigned to 47.5-m2 plots. Fertilizer N was applied to the SA landscape bimonthly at a rate of 50 kg ha–1 (total of 900 kg N ha–1), while the MS landscape was fertilized bimonthly at a rate of 40 kg N ha–1 only during establishment (total of 480 kg ha–1). Data were collected for 3 yr (16 mo to 52 mo after planting). Cumulative mean inorganic-N leached was 4.1 kg ha–1 and 7.4 kg ha–1 for the SA and MS landscapes, respectively. Relatively long establishment requirements for the MS landscape led to significantly greater inorganic-N leaching (5.2 kg ha–1) in year 1 of the study compared to the SA landscape (1.3 kg ha–1). After year 1, inorganic-N leaching was comparable on both landscapes, although it was significantly less on the MS landscape in year 3 when no fertilizer was applied. Overall, inorganic-N leaching was low (<2% of applied N) on both landscapes following establishment, indicating the importance of management practices rather than species composition for reducing N leaching from residential land use.

Abbreviations: MS, mixed-species • SA, St. Augustinegrass


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NITROGEN IS A KEY macronutrient required by all terrestrial plants for proper growth and function. Accordingly, N fertilizers are frequently used to maintain vigorous and aesthetic residential landscapes. The quantity of fertilizers applied to home lawns continues to increase concomitant with expanding residential land use and urban populations. For example, in 1996 approximately 0.4 million tons of fertilizer were applied for nonagricultural land use in Florida alone (USEPA, 1999). In addition to the routine application of fertilizer N to lawns, residential soils in southern Florida are generally coarse textured and receive abundant precipitation. Therefore, environmental conditions are often conducive to the rapid leaching of applied fertilizer N from the soil vadose zone (Reike and Ellis, 1974; Snyder et al., 1984; Barton et al., 2006). This leaching of applied fertilizer N represents not only an economic loss to the homeowner but is also recognized as a significant environmental threat by both scientists and policymakers (Carpenter et al., 1998; Tilman et al., 2001, 2002).

Given environmental problems associated with loss of fertilizer nutrients from managed systems, there is great interest in research to evaluate residential landscape models for their potential to reduce N leaching. As a result, recent studies have demonstrated the potential for residential landscape models (i.e., vegetation type and associated maintenance regimes) to influence N leaching (Hipp et al., 1993; Broschat, 1995; Erickson et al., 2001; Bowman et al., 2002; Easton and Petrovic, 2004; Frank et al., 2005; Amador et al., 2007). Currently, the predominant residential landscape model in Florida consists of a monoculture of St. Augustinegrass [SA; Stenotaphrum secundatum (Walt.) Kuntze], a moderate fertility warm-season turfgrass. Nitrogen is the nutrient required in the greatest quantity for SA growth and vigor (Cisar et al., 1991). As an alternative to extensive turfgrass lawns, the Florida Yards and Neighborhoods Program (Best, 1994) has promulgated the use of other landscape materials, such as native woody perennials that might conceivably reduce N leaching in addition to other potential environmental benefits like wildlife habitat and reduced irrigation inputs (Garner et al., 1996). In particular, it has been hypothesized that reduced nutrient inputs after establishment on alternative mixed-species (MS) landscapes will result in less N leaching.

Data on fertilizer N leaching from turfgrass is quite extensive (Snyder et al., 1981; Starr and DeRoo, 1981; Petrovic, 1990; Bowman et al., 2002; Easton and Petrovic 2004; Frank et al., 2005; Barton et al., 2006). Although substantial N leaching has been shown to occur under certain management and/or environmental conditions (Snyder et al., 1984), these studies generally show minimal N leaching from judiciously managed turfgrass. However, a majority of these studies used cool-season turfgrasses, relatively short-term studies (1 yr or less), and/or were conducted on recently established plants, often under greenhouse conditions (see review by Barton and Colmer, 2006). Moreover, there is a paucity of data comparing turfgrass landscapes with alternative ornamental landscapes under identical climatic conditions (Hipp et al., 1993; Amador et al., 2007). In fact, prior studies by the authors are the only published data to date to have compared hydrology and nutrient leaching from a conventional warm-season turfgrass landscape and an alternative ornamental landscape on a coarse-textured soil under subtropical climate conditions (Erickson et al., 2001, 2005; Park et al., 2005).

Previous reports published by the authors, however, have focused on N leaching during 12 mo of establishing conditions (Erickson et al., 2001), water use by the contrasting landscapes (Park et al., 2005), and phosphorus and potassium leaching dynamics (Erickson et al., 2005). Here we report 3 yr of inorganic-N leaching data after 16 mo of landscape establishment, where it has been hypothesized that an alternative MS ornamental landscape would offer its greatest potential to reduce N leaching due to reduced inputs and its functional diversity (Hipp et al., 1993; Garner et al., 1996). Therefore, the objectives of this comparative study were to collect data on soil water drainage and inorganic-N leaching losses at the lawn-scale level from contrasting landscape models grown on a sandy substrate. Given appropriate management practices for each landscape model, we tested the hypothesis that an MS landscape consisting of mostly native plants would reduce N leaching compared to a conventional turfgrass monoculture. To address this hypothesis, we designed and implemented a field experiment whereby we collected leaching data spanning 3 yr (i.e., from 16 to 52 mo after planting) from two contrasting residential landscape models.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site and Design
The study was conducted at the University of Florida's Research and Education Center in Fort Lauderdale, FL (26°03' N, 80°13' W). The climate in southern Florida is subtropical, characterized by abundant rainfall and a distinct wet season (approximately May–October) and dry season (approximately November–April). National Climatic Data Center reported normal annual rainfall (1971–2000) measured at Fort Lauderdale-Hollywood Airport (approximately 10 km from the research facility) as 1631 mm with more than two-thirds of annual precipitation occurring during the wet season. The normal mean temperature is 26.8°C in the wet season and 21.9°C in the dry season (http://www.ncdc.noaa.gov/).

In 1998 eight 9.5- by 5.0-m research plots were constructed using crushed limestone as a foundation that provided a 10% slope for the plots. A 6-mm polyvinyl plastic was laid on the foundation and along the sides of each of the plots, providing hydrological isolation for each plot. This allowed for the collection of all surface runoff and all subsurface drainage from our large-scale field plots (~50 m2). A more detailed description of the research facility can be found in Erickson et al. (1999, 2001). The 0.75 m deep root-zone mix placed in each of the plots was a mined siliceous sand (Boynton Sand and Gravel, Palm Beach County, Florida) used in landscape development. The medium-fine–textured sand had a bulk density of 1.62 Mg m–3 and a relatively high infiltration rate, similar to many residential sandy soils in southern Florida (see Table 1 in Erickson et al. [2005] for a complete physical description of the root-zone mix used in the study).


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  		Table 1. Analysis of variance summary comparing the effect of the mixed-species (MS) versus St. Augustinegrass (SA) landscape models on NH4–N, NO3–N, and total inorganic N leached (n = 4). Data represent treatment means coinciding with annual wet (WS) and dry (DS) seasons followed by P values indicating whether the two previous numbers within the column differ significantly.

 
A completely randomized design with four replications was used for the study. Landscape model was the factor and contained two treatment levels. One treatment consisted of a SA (‘Floratam’) monoculture that was routinely fertilized and irrigated when needed to avoid wilt, while the other treatment was an MS arrangement of ornamental ground covers, woody shrubs, and trees that were fertilized during establishment and irrigated when needed to avoid or recover from wilt. The SA was installed as a sod and maintained at a height of 7.5 cm. The clippings were removed for the duration of the reported study period as may occur on SA lawns in south Florida. The alternative MS landscape model consisted of 12 commercially available species: 25 Liriope muscari (Dcne.) Bailey ‘Evergreen Giant’, three Lantana montevidensis (K. Spreng) Briq., three Tripsacum floridana Porter ex Vasey ‘Dwarf’, 15 Zamia pumila L., seven Ilex vomitoria Ait. ‘Schellings Dwarf’, five Hamelia patens Jacq. ‘Compacta’, four Galphimia glauca Cav., three Myrcianthes fragrans (Sw.) McVaugh, three Podocarpus macrophyllus (Thunb.) Sweet, one Morella cerifera (L.) Small, one Tabebuia heterophylla (DC.) Britt., and one Acoelorraphe wrightii (Griseb. and H. Wendl.) H. Wendl. ex Becc. Species were planted together in clumps with smaller vegetation types interspersed among the larger trees and shrubs. Over 50% of the species used were considered native to Florida. The plants were installed from pots with the soil and intact root systems. Species were selected with consideration for functional diversity, hardiness, availability, and aesthetic appeal. For more detailed information regarding the MS vegetation see Table 1 in Erickson et al. (2001) and Park et al. (2005). Eucalyptus mulch (The Bushel Stop, Pompano Beach, FL) was uniformly applied and maintained at a depth of 7.5 cm on the MS landscape. Plot establishment (planting) for both treatments occurred at the end of December in 1998.

Each landscape was fertilized according to University Extension recommendations and the fertilizer protocols for each respective landscape were common for subtropical conditions in south Florida (Cisar et al., 1991; Yeager and Gilman, 1991; Gilman and Black, 1999; Knox et al., 2002). The SA landscape was fertilized approximately bimonthly throughout the study with a blended 26–3–11 (N–P2O5–K2O) granular fertilizer (LESCO Inc., Sebring, FL) at a rate of 50 kg N ha–1 per application to each plot (approximately 900 kg N ha–1 during the course of the current study). The source of fertilizer-N was urea (58%), sulfur-coated urea (37.5%), and ammonium phosphate (4.5%). The granular material was hand-distributed and watered in with irrigation at each application. In contrast, fertilization of the MS landscape model occurred for only about 3 yr following planting (the first 2 yr of the 3-yr data period presented here) to help the plants become well established and grow into the landscape (Gilman and Black, 1999). Thus, the MS landscape was not fertilized during the final 12 mo of data collection. The fertilizer used on the MS landscape in the current study was an 8–4–12 (N–P2O5–K2O) controlled-release (polymer-coated urea, polymer-coated ammonium phosphate, and ureaform) granular fertilizer with micronutrients and was applied bimonthly for 12 applications (the first 2 yr of the current 3-yr data collection period) to the MS plots, which coincided with fertilization of the SA plots. The fertilizer was applied at a rate of 40 kg N ha–1 per application to each plot (approximately 480 kg N ha–1 during the course of the current study).

Precipitation, Irrigation, and Drainage
Precipitation records from the National Climatic Data Center for nearby Fort Lauderdale-Hollywood Airport (26°06' N, 80°12' W) were used for the current study. Irrigation volume applied to each treatment was recorded by a flow meter installed in the irrigation system. The SA landscape was irrigated by a rectangular perimeter irrigation system comprised of six inward-facing spray nozzles. On the MS landscape, a microjet irrigation system that delivered water directly to the plants was used during the course of the study.

Drainage was collected from a slotted drainage pipe installed below the root zone mix at the lower edge of each plot. Drainage data were collected from each plot by routinely calibrated tipping bucket flow gauges (Unidata America-Model 6406H, Lake Oswego, OR) that continuously monitored drainage volume. Drainage was summed hourly and recorded by a datalogger (CR10X, Campbell Scientific, Logan, UT). Periodically, drainage data were lost as a result of tipping bucket malfunctions associated with debris or wire damage. When such malfunctions occurred drainage was then interpolated based on data values before and after the down time or on random point measurements obtained during the interruption. During one 2-wk period in July 2001, the datalogger was down and drainage data were lost on all plots, so we assumed that the root-zone mix was saturated and drainage for all plots was set equal to received precipitation. Surface runoff data were also collected at the onset of the study; however, no significant surface runoff was observed during the first 12 mo of the study despite intense precipitation events. As a result, surface runoff was not considered for the remainder of the study (Erickson et al., 2001, 2005).

Inorganic-Nitrogen Leaching
Drainage samples were collected periodically in high-density polyethylene bottles for both NH4–N and NO3–N analyses. Sampling occurred frequently (from one to three times a day until drainage ceased) following fertilization and significant rainfall events. Sampling was less frequent (thrice weekly to biweekly) during periods of no and/or low drainage. The water samples were immediately acidified with concentrated sulfuric acid on collection and refrigerated at 4°C until analysis. The samples were analyzed for NH4–N and NO3–N by colorimetric methods (OI Analytical, College Station, TX; USEPA Methods 350.1 and 353.2). All analyses were performed at the University of Florida Analytical Research Laboratory in Gainesville, FL. Leaching losses of NH4–N and NO3–N were calculated as the product of the biweekly average nutrient concentration and the respective biweekly drainage daily for each plot. Losses of inorganic-N were estimated as the sum of NH4–N and NO3–N leaching losses.

Because of the importance of root production for nutrient uptake, estimates of root weight density were determined periodically throughout the study. Roots were extracted from soil cores (approximately 81 cm2) collected from the upper 20 cm of the root-zone mix (measured below the thatch and mulch layers) taken from each plot. One core was taken from a random location on each of the turf plots at each sampling date. To better represent the diversity of species in the MS treatment without excessive coring to damage the plants, two cores were arbitrarily taken from each plot near different species functional types (i.e., trees, shrubs, and ground covers). Roots were washed, oven-dried to a constant mass at 70°C, weighed, ashed in a muffle furnace (550°C), and reweighed. Root weights are presented as oven dry weight minus ash weight, thus minimizing any potential error associated with residual mineral soil elements on the washed roots.

Statistical Analyses
Mean treatment drainage and NH4–N and NO3–N leaching losses are presented from May 2000 to April 2003. This resulted in 36 mo of data collection across three full wet and dry seasons. For statistical comparison, drainage and nutrient leaching losses were summed on a plot-by-plot basis over 6-mo intervals (annual wet and dry seasons). Analysis of variance (ANOVA) was used on the seasonal sums to compare NH4–N and NO3–N and total inorganic-N leaching losses between the contrasting landscape models. The mixed model procedure of SAS (SAS Institute, 1999) was used to analyze data across the entire study (season and treatment as fixed effects) as well as within each season (treatment as fixed effect).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Precipitation, Irrigation, and Drainage
Annual rainfall (May through April) averaged 1542 mm throughout the study with approximately 75% of that rainfall occurring during the wet season months (http://www.ncdc.noaa.gov/). Accordingly, applied irrigation was minimal during the wet season (<5% of total inputs), whereas roughly twice the irrigation was applied to both the MS (40% of inputs) and SA (25% of inputs) landscapes during the dry season (Fig. 1 ). During the first year of the study, more irrigation was applied to the SA landscape, but during the last two years of the study less irrigation was applied to the SA landscape. This resulted in greater total water inputs to the MS landscape during the last 2 yr of the study.


Figure 1
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  		Figure 1. Stacked bar graph showing total seasonal water inputs, consisting of rainfall (gray) and irrigation (black), applied to the mixed-species (MS) and St. Augustinegrass (SA) landscapes during a 3-yr data collection period. Labels on the x axis represent wet (WS; May–October) and dry (DS; November–April) seasons beginning during the years of 2000 to 2003.

 
Abundant precipitation and a relatively coarse-textured root-zone mix resulted in 30% of water inputs lost as drainage on the MS landscape and 40% lost as drainage on the SA landscape throughout the study (Fig. 2 ). Mirroring seasonal patterns of rainfall, drainage was higher during the wet season and lower in the dry season, with the greatest relative difference between the landscapes occurring in the dry season (i.e., SA drainage was almost twice MS drainage). In other words, the MS landscape exhibited greater seasonal variability, as drainage increased over 400% on the MS and only 220% on the SA landscape from dry to wet season averaged across all 3 yr. In total, drainage was always less on the MS landscape during the six seasonal collection periods and was significantly less (P < 0.10) in four of the six seasons (data not shown) compared to the SA landscape.


Figure 2
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  		Figure 2. Biweekly mean drainage (mm) and NH4–N and NO3–N concentrations (mg L–1) in sampled drainage for 36 mo of data collection (May 2000 to April 2003) on the MS (filled circles) and SA (unfilled circles) landscape models (n = 4 plots). Note: vegetation for both landscapes was planted in December 1998. Data collected during the wet season are in the shaded region and the dry season data are not shaded. Error bars represent ± 1 SE.

 
Nitrogen Leaching
Mean biweekly NH4–N concentrations in the drainage were very similar among both landscape models, averaging 0.16 ± 0.01 mg L–1 on the MS treatment and 0.17 ± 0.01 mg L–1 on the SA treatment. Concentrations of NH4–N in drainage were relatively low on both landscapes, never exceeding a mean treatment value of 1 mg L–1 (Fig. 2). Biweekly NO3–N concentrations, however, were more variable among treatments, averaging 0.44 ± 0.12 mg L–1 (maximum of 16.3 mg L–1) on the MS treatment and 0.05 ± 0.01 mg L–1 (maximum of 1.18 mg L–1) on the SA treatment (Fig. 2). Generally, drainage and concentrations of NH4–N and NO3–N were inversely related.

Despite the large disparity between wet and dry seasons in precipitation and drainage (Fig. 1 and 2), the magnitude of inorganic-N leaching losses between the seasons was not proportional to increased drainage (Fig. 3 ). For instance, inorganic-N leaching in the wet season increased by about 120% on the MS landscape and 100% on the SA landscape relative to the dry season averaged across all 3 yr. Although NH4–N concentrations in drainage were generally similar among treatments (Fig. 2), the SA landscape leached significantly (P < 0.10) more NH4–N during four of the six seasons throughout the study (Fig. 3, Table 1). In contrast, significantly (P < 0.1) more NO3–N was leached on the MS landscape during the first 12 mo of the study. However, after fertilization ceased on the MS landscape, significantly (P < 0.01) greater NO3–N leaching was observed on the SA landscape during the final 6 mo of the study, albeit very low levels were lost from both landscapes. These patterns resulted in greater total inorganic-N leaching from the MS landscape early in the study, but significantly greater inorganic-N leaching from the SA landscape toward the end of the study.


Figure 3
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  		Figure 3. Biweekly mean NH4–N, NO3–N, and total inorganic-N leached (kg ha–1) for 36 mo of data collection (May 2000 to April 2003) on the MS (filled circles) and SA (unfilled circles) landscape models (n = 4 plots). Data collected during the wet season are in the shaded region and the dry season data are not shaded. Error bars represent ± 1 SE.

 
Cumulative leaching losses of NH4–N were similar between the treatments until the end of the study when greater losses from the SA landscape became evident, but cumulative leaching losses of NO3–N showed big differences between the landscapes due to large losses of NO3–N on the MS landscape early in the study (Fig. 4 ). This led to greater overall inorganic-N leaching from the MS landscape during the course of the study, although after the MS landscape was no longer fertilized very little N leaching was observed. Overall, only about 4.1 kg inorganic-N ha–1 and 7.4 kg inorganic-N ha–1 were lost via leaching from the SA and MS landscapes, respectively, throughout the current study.


Figure 4
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  		Figure 4. Cumulative mean NH4–N, NO3–N, and total inorganic-N leached (kg ha–1) during 36 mo of data collection (May 2000 to April 2003) on the MS (filled circles) and SA (unfilled circles) landscape models (n = 4 plots). Data collected during the wet season are in the shaded region and the dry season data are not shaded. Error bars represent ± 1 SE.

 
Root Biomass
The standing crop of root biomass (0–20 cm) was significantly (P < 0.01) greater on the SA landscape, averaging about 450 g m–2 throughout the study (Fig. 5 ). Moreover, root biomass was relatively invariant on the SA landscape throughout the study. However, root biomass on the MS landscape was very low at the beginning of the study (approximately 100 g m–2), but increased to about 300 g m–2 after the first year, resulting in a significant (P = 0.06) time x treatment interaction.


Figure 5
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  		Figure 5. Mean (n = 4) standing root biomass collected from the upper 20 cm of the root-zone mix versus time on the MS (filled circles) and SA (unfilled circles) landscape models. Error bars represent ± 1 SE.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
While the goal of fertilizing residential landscapes is to provide N for plant uptake to maintain vigorous and aesthetic vegetation, the actual fate of applied N is complex due to the many chemical and biological transformations involved in N cycling. Applied N may be stored in the soil, lost in gaseous forms to the atmosphere, or lost from the system via leaching (Starr and DeRoo, 1981; Petrovic, 1990). In the current study we focus on the impacts of N leaching because of the adverse health and environmental consequences associated with N leaching. For example, an anthropogenic doubling of global N inputs into terrestrial systems has greatly increased N movement, in part via leaching, to rivers, estuaries, and coastal oceans, often causing eutrophication and other environmental problems (Howarth, 1988; Vitousek et al., 1997; Carpenter et al., 1998). Our results showed that residential landscape model affected not only the total quantity of inorganic-N leached, but also the temporal dynamics of N leaching.

During the course of the study, significantly greater inorganic-N leaching was observed from the MS landscape compared to the SA landscape. The rate of N leaching depends on soil drainage (rainfall and soil texture), amount of N in soil solution, and N uptake by the vegetation (Snyder et al., 1984; Bergstrom and Brink, 1986; Frank et al., 2005). Despite generally greater inputs of water to the MS landscape during the course of the study (Fig. 1), less drainage was observed compared to the SA landscape (Fig. 2). While reduced drainage as a result of increased water use by an expanding canopy is generally a positive factor for reducing N leaching (Snyder et al., 1984), that was not clearly the case in the current study. In other words, reduced drainage on the MS landscape compared to the SA landscape was not always associated with reduced N leaching. For example, the MS landscape showed greater inorganic-N leaching losses during the first two seasons, even though more drainage occurred on the SA landscape. Whereas, during the last two seasons when no fertilizer was applied to the MS landscape, reduced inorganic-N leaching was associated with reduced drainage compared to the SA landscape.

Thus, differences in drainage between the two landscape models could not fully explain the observed differences in the quantity of inorganic-N leached, particularly NO3–N leaching losses. Toward the beginning of the study, concentrations of inorganic-N in soil drainage were considerably higher in the MS landscape. Given that fertilizer N inputs were less on the MS landscape, increased inorganic-N in soil solution could have resulted from reduced immobilization (i.e., soil storage), reduced plant uptake, and/or reduced loss of gaseous N. While gaseous losses of N were not measured in the current study, management practices, such as irrigation after fertilization (Bowman et al., 1987), were intended to minimize gaseous losses on both landscapes. Greater inorganic-N concentrations on the MS landscape at the beginning of the study, therefore, were most likely due to lower plant uptake and possibly lower soil storage, which is supported by observed dynamics in root biomass (see discussion on root biomass below).

In addition to differences in total quantity of inorganic-N leached, the contrasting landscape models showed temporal differences in N leaching. Throughout the current study, losses of inorganic-N showed no relationship with time on the SA landscape, while losses declined exponentially through time on the MS landscape (Fig. 4; Table 1). Correspondingly, root biomass increased exponentially during the study on the MS landscape, while root biomass was not related with time on the SA landscape (Fig. 5). However, less inorganic-N leaching was observed from SA landscape in the current study (approximately 0.5% of applied fertilizer-N) compared to recently planted conditions when about 1.4% of fertilizer-N was lost via leaching (Erickson et al., 2001). Similarly, Bowman et al. (2002) reported about 1.2% of applied fertilizer N lost via inorganic-N leaching on recently planted SA in pots grown under greenhouse conditions. The disparity in fertilizer-N losses following planting (32%) and the current study (1.5%) were even greater for the MS landscape. These data illustrate the importance of establishing conditions for N leaching. They further illustrate the longer establishing conditions required by an MS ornamental landscape compared to a turfgrass installed as sod, and the consequences of that longer establishing period for inorganic N leaching. Essentially, it took over 2 yr for the MS landscape to achieve comparable levels of N leaching that were accomplished by the SA landscape in a couple of months.

Differences in N species leached were also seen during the study as relatively greater NH4–N leaching was observed on the SA landscape, while greater NO3–N leaching occurred on the MS landscape (Table 1). Due to cation exchange reactions with the soil, NH4–N is generally considered to be less mobile than NO3–N, and thus less likely to leach from most soils. Interestingly, however, we observed greater NH4–N leaching compared to NO3–N leaching on the SA landscape throughout the course of the study. In contrast, we saw greater NO3–N leaching compared to NH4–N on the MS landscape when fertilized. Overall, NH4–N leaching was correlated with drainage volume, which resulted in greater NH4–N leaching on the SA landscape. This relatively abundant loss of NH4–N on both landscapes likely resulted from the low cation exchange capacity of the sandy root-zone mix used in this study (Huang and Petrovic, 1994; Bowman et al., 2002). The high rate of NO3–N leaching on the MS landscape reflected the developmental stage of the landscape and a lack of roots to remove NO3–N from soil solution, so the mobile NO3–N was readily leached. The low levels of NO3–N leaching from SA landscape showed the efficiency of NO3–N uptake by SA, as concentrations of NO3–N in the drainage were generally less than that observed for precipitation in south Florida (Grimshaw and Dolske, 2002).

Although significant differences in the quantity and patterns of N leaching were observed, the use of controlled-release fertilizers and irrigation on visible stress resulted in relatively low inorganic-N leaching losses from both landscapes during the study. In fact, total inorganic-N leaching losses averaged only about 1.4 and 2.5 kg inorganic-N ha–1 yr–1 from the SA and MS landscapes, respectively. By comparison, Brye et al. (2001) reported total inorganic-N leaching losses of 0.15, 50.3, and 44.8 kg N ha–1 yr–1 for prairie, no-tillage corn, and chisel-plowed corn ecosystems, respectively. Morton et al. (1988) reported N losses of 1.88 kg inorganic-N ha–1 yr–1 on an unfertilized established stand of Poa pratensis L. Under greenhouse conditions, Bowman et al. (2002) reported that SA was the most efficient out of six warm-season turfgrasses at reducing N leaching. Nevertheless, losses of N observed in the current study may pose adverse environmental consequences as leaching losses were still an order of magnitude greater than a restored prairie (Brye et al., 2001), and much of the applied fertilizer N remained unaccounted for in the current study.

At the end of the study we saw a marked improvement in the performance of the MS landscape with respect to inorganic-N leaching, in part because the landscape was more fully developed, but also in part because it no longer received fertilizer inputs. Therefore, we found some support for the hypothesis that an MS landscape model consisting of mostly native plants, if unfertilized, could reduce N leaching compared to a conventional SA lawn maintained at the maximum recommended rate for N fertility, as in the current study. While this study was relatively long-term for experimental purposes (three years of data collection on landscapes planted over 4 yr ago), it is still a relatively short period of time with respect to landscape persistence and performance through time. A key question that remains unresolved is whether the MS landscape vegetation could continue to persist without further fertilizer additions (note: visual observations indicated a marked decline in vegetation performance during the final year). Furthermore, the implications of altered management practices for either landscape are not known. Nevertheless, the findings of the current study indicated that, following respective establishing periods, relatively little inorganic-N leaching was observed from either landscape, indicating a greater emphasis and need for research on management practices rather than species composition for reducing N leaching from residential land use (Amador et al., 2007).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
In this study we tested the hypothesis that a landscape model including many native trees and shrubs could reduce N leaching from residential land use compared to a more traditional turfgrass monoculture. Given abundant and frequently intense precipitation, coarse-textured soils, and routinely fertilized landscapes, relatively little inorganic-N leaching occurred on either landscape in the study. Still, we found that landscape model impacted the quantity and patterns of inorganic-N leaching. Initially, while many of the species in the MS landscape were still rapidly expanding, the quantity of inorganic-N leached from the MS landscape was greater and due largely to losses of NO3–N. These greater losses occurred even though less fertilizer N was applied to the MS landscape. During the second year of the study, no significant differences in inorganic-N leaching were observed between the two landscape models, despite routine fertilization of both landscapes. This was likely due to expansion of growth by the MS landscape, as reflected by increased root biomass (i.e., root closure). Finally, in the last year of the study, after fertilization was no longer applied to the MS landscape, it leached significantly less inorganic-N compared to the SA landscape. Overall, after both landscapes became well established, inorganic-N leaching was relatively low with concentrations of NH4–N and NO3–N in drainage generally less than that reported for precipitation in south Florida. Thus, we believe the challenges going forward to reduce environmental impacts from residential land use are to refine establishment and management practices on both landscape models and to identify the fate of fertilizer N not lost via leaching.


   ACKNOWLEDGMENTS
 
This research was supported by the Florida Agricultural Experiment Station and a grant from the Florida Department of Environmental Protection and Sarasota Bay National Estuary Program. We also acknowledge additional funding provided by the Florida Turfgrass Association and the Turfgrass Producers International Foundation. We acknowledge Dr. T. Broschat for post-establishment fertilization and irrigation recommendations on the MS landscape. Technical assistance and recommendations provided by Allen Garner, Mike Holsinger, Mark Shelby, John Rowlands, Raymond Snyder, Kevin Wise, Eva Green, and David Rich were also greatly appreciated.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication September 18, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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