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Crop Science 43:247-257 (2003)
© 2003 Crop Science Society of America

TURFGRASS SCIENCE

Soil Inorganic Nitrogen under Fertilized Bermudagrass Turf

David J. Lee, Daniel C. Bowman, D. Keith Cassel, Charles H. Peacock and Thomas W. Rufty, Jr.*

Dep. of Soil Science, 100 Derieux St., North Carolina State Univ., Raleigh, NC 27695-7620

* Corresponding author (tom_rufty{at}ncsu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Managed turfgrass acreage in the southeastern USA is steadily increasing. There is public concern that fertilization of turfgrass systems, particularly additions of N on golf courses, might be adversely affecting groundwater quality due to nitrate leaching. This study was conducted to measure soil nitrate levels in situ under continuously managed bermudagrass (Cynodon spp.) and to evaluate influences from fertilization and mineralization. Two experimental sites were established on 50- and 75-yr-old golf course fairways in the Neuse and Cape Fear River basins in eastern North Carolina. Soil sampling was done seasonally. Results indicate that nitrate-N levels were consistently low (1 to 4 mg kg-1 soil) and similar to adjacent natural areas throughout the 120-cm sampling depths during the 2-yr experiment at both sites. Levels were relatively uniform with depth and across several landscape positions. The soil nitrate levels under fertilized fairways were similar to those in adjacent nonfertilized natural areas, indicating minimal influence from turf management practices. From laboratory mineralization studies and soil temperature data, it was estimated that 60 to 154 kg N ha-1 would be released from organic N pools during the bermudagrass growing season (May to October). Because of similar temperature responses, it appeared that N release from mineralization would be synchronized with bermudagrass growth. Substantial bermudagrass growth in nonfertilized plots provided direct evidence that mineralization was a significant contributor to turf nutrition. There was no evidence that N fertilization or the ecology of the bermudagrass system posed inherent risks to water quality and the environment.

Abbreviations: OM, organic matter • PGR, plant growth regulator


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
MANAGED TURFGRASS systems are increasing in acreage throughout the southeastern USA. The increases are linked to population growth and the associated home lawns and recreational areas. Fertilizers are necessary to provide turfgrasses with adequate nutrients for growth. Yet, turf fertilization is thought to contribute to groundwater pollution (Morton et al., 1988). The greatest concern is with golf courses, which are perceived to receive excessive amounts of fertilizer N.

Nitrogen loss via leaching from mature stands of turf involves complex interactions between soil physical and biological factors. As an anion, NO3–N is not held tightly by negatively charged clay minerals or organic matter, and it is susceptible to movement in the soil solution via diffusion, dispersion, and convection (Cameron and Haynes, 1986). Convection, that is, mass flow, of water passing through the macropores of a soil directly influences NO3 movement and leaching through the soil profile. Diffusion and dispersion effects on solute movement are influenced by pore size and solution concentration differences within the soil (Groover et al., 1997), making solute movement in the field difficult to monitor and quantify.

Biological factors influencing soil NO3–N levels under turf systems include the plant species and its rooting depth, and soil organic N. The relationships among growth, N uptake, and soil solution NO3–N concentrations have been investigated with the cool-season Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), and tall fescue (Festuca arundinacea Schreb.) (Liu et al., 1997). Experiments were conducted on 3- to 5-yr-old experimental turf systems that received three annual fertilizations of 50 kg N ha-1. Results indicated soil NO3–N levels were different among the turfgrass species, but NO3–N levels and the leaching potentials associated with them were minimal for each species when environmental conditions favored optimal plant growth.

Similar conclusions about growth and soil NO3–N levels were reached in research with warm-season bermudagrass [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy] by Snyder et al. (1984). The work was conducted on experimental plots maintained similar to golf course fairways that had been in place for several years, and the plots received monthly N applications of 50 kg N ha-1 during the experiment. Soil solution NO3–N levels were found to be lowest during summer months when environmental conditions were more favorable for plant growth and levels were highest during winter months. In general, mean NO3–N concentrations in soil solution were extremely variable, ranging from <1 mg L-1 to >100 mg L-1. Nitrate-N concentrations and leaching potential were influenced by N source, irrigation method, and rainfall.

Few studies have examined soil NO-3–N concentrations under bermudagrass golf course fairways in situ. Concentrations could be quite different from those in recently constructed experimental plots. Certainly, one would expect that active growth of bermudagrass would still be a major factor controlling root-zone NO3–N levels (Snyder et al., 1984; Liu et al., 1997). However, if a fairway bermudagrass system were in place for many years and soil organic matter and associated organic-N levels were approaching equilibrium (cf. Porter et al., 1980), net accumulation of organic N would not occur and the potential for NO3–N leaching could be increased.

The main objective of this study was to measure soil NO3–N levels in situ in mature bermudagrass systems. Experimental sites were established on two golf course fairways in eastern North Carolina that have been maintained with bermudagrass for 50 and 75 yr and can be assumed to be in equilibrium with regard to net organic matter and organic-N accumulation. The results will indicate NO3–N levels in the soil profile and the potential for seasonal NO3–N loss via leaching during bermudagrass growth and dormancy cycles.

Of particular interest in this study was the role of organic-N mineralization. On golf courses in the Southeast, fairway bermudagrass clippings most often are not removed. Sustained decomposition of organic matter could exert a dominant influence on soil NO3–N levels. Thus, a second objective of the research was to begin evaluating the importance of mineralization and the release of NO3–N from organic-N pools in the bermudagrass system.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Site Description
The study was conducted on two golf courses in the Cape Fear and Neuse River watersheds of eastern North Carolina. Site 1 was in Wilmington and Site 2 was in Kinston, NC. Golf course selection was based on age, location in the river basins, landscape characteristics, and soil type. Landscapes on the sites sloped 0 to 6% from a nonfertilized natural area across managed ‘Tifway’ bermudagrass [C. dactylon (L.) Pers. x C. transvaalensis Burtt-Davy] at Site 1 and common bermudagrass (C. dactylon) at Site 2 to a water body. Site 1 was mapped as Rimini (sandy, siliceous, thermic, Entic Grossarenic Alorthods) and Site 2 as Norfolk (fine-loamy, kaolinitic, thermic Typic Kandiudults).

At each site, four transects were established perpendicular to the fairway, along natural surface drainage patterns (Fig. 1). Each transect had eight sampling locations which extended from the natural area at the highest elevation to a stream at the lowest elevation. Two soil sampling locations were in unfertilized tree-covered areas, two locations were in the bermudagrass rough mowed at a height of 3.8 cm, and four locations were in the bermudagrass fairway mowed at 1.3 cm. Four plots measuring 9 by 9 m were established in the fairways close to the transects for periodic collection of bermudagrass clippings. The two sites were managed by experienced golf course superintendents, without interference from our research staff. Management practices varied between sites and years at both locations (Table 1). At Site 1, fertilizers were applied in three equal applications of 44 kg N ha-1 in April, June, and August in 1999. The year 2000 fertility program consisted of applications of 44 kg N ha-1 in April and June followed by 73 kg N ha-1 in August. Site 1 was not seeded with a cool-season turfgrass in the winter months and did not receive plant growth regulator (PGR) treatments either year. At Site 2, fairways were overseeded with perennial ryegrass at a rate of 488 kg ha-1 in October 1998 and 1999. Fertilizer additions and amounts varied from as little as 10 kg N ha-1 to as much as 73 kg N ha-1. A PGR (trinexapac-ethyl) was applied periodically from July to September at a rate of 0.1 kg a.i. ha-1 to limit bermudagrass topgrowth. At both experimental sites, N sources generally consisted of a combination of 70% coated slow-release materials and 30% water-soluble urea, and sites were irrigated after each application to minimize volatilization.



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  		Fig. 1. Stylized depiction of sampling positions at both research sites.

 

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  		Table 1. Soil core sampling dates and fertilization details for the experimental sites during 1999 and 2000.

 
In addition to the normal site design, four separate 9 by 9 m plots were established in the fairways at Site 1 for a low fertilizer treatment. Those plots did not receive any fertilizer after an initial April application of 22 kg N ha-1 and were sampled the same as fertilized plots (see below). Because the plots were not fertilized during the bermudagrass growing season, they are referred to as unfertilized in the text.

Sampling and Analysis
Soil NO3–N concentrations were measured seven times during a 2-yr period. Eight soil cores were collected at the landscape positions along each of four transects prior to spring green-up in early March, during early season growth in May, during maximum growth in August, and post dormancy in early November (Table 1). A 1.9-cm diam., manually operated soil probe was used. Plant material and the thatch layer were removed, and the soil cores were separated into the following depth increments: 0 to 15, 15 to 30, 30 to 60, 60 to 90, and 90 to 120 cm. Following established protocols (Keeney and Nelson, 1982), the soil samples were air dried for 96 h and ground using a mechanical soil grinder.

Soil inorganic N was extracted using a 1 M KCl solution (Keeney and Nelson, 1982). Samples were filtered using Whatman No. 2 filter paper to remove soil particles, and the clear extract was stored at 4°C. Nitrate-N and NH4–N in the extract were determined by the rapid diffusion process (Carlson, 1986). Additionally, organic matter content was determined as the weight loss of ground samples following overnight combustion at 550°C.

Bermudagrass tissue production in the fertilized (both sites) and unfertilized plots (Site 1 only) was measured beginning in May and continued until growth slowed and mowing heights were raised in mid-September in preparation for winter dormancy. Clippings were collected from randomly selected 55-cm lanes within the fairway plots at one mowing during the week and normalized for a 7-d period. The harvested clippings were oven-dried at 70°C for 72 h and weighed. Seasonal rainfall and mean daily temperatures were obtained from the National Weather Service and a North Carolina agricultural research station (Kinston, NC), which were within several miles of the experimental sites. Irrigation was used to supplement rainfall during periods when evapotranspiration exceeded precipitation. Twelve temperature probes (Stowaway Tidbit XT, Onset Co., Pocasset, MA) were placed in two locations on fairways at each site. Beginning in January 2000 and continuing for 1 yr, soil temperatures were recorded at 5, 10, and 15 cm every 15 min.

Measurement of Mineralization
Mineralization rates were estimated using a combination of two methods. A temperature gradient block that provided 19 discrete temperatures ranging from 10 to 46°C (Lab-Line Instruments, Inc., Melrose Park, IL) was used to establish temperature effects on net release of NH4–N in the presence of microbial populations extracted from soils from the two sites (Lee et al., 2001). Ammonium-N release is considered the rate-limiting process in mineralization. Actual mineralization rates in field samples were estimated using bulk samples collected from the top 15 cm of soils in the area of the transects. At Site 1, separate samples were collected from upper and lower portions of the fairway. Preliminary analyses had indicated that soils lower on the transect and closer to the stream had higher organic matter (OM) contents. The bulk samples were mixed thoroughly, with randomly selected subsamples sealed in plastic bags and incubated at 33°C for 5 wk. Samples were removed from the incubator each week and analyzed for inorganic N release using established protocols (Keeney and Nelson, 1982; Carlson, 1886).

Modeling and Statistics
A model to estimate inorganic N mineralization rates in the field in situ (Y) was developed based on data obtained from temperature gradient experiments, inorganic N release rates from field soil in incubation experiments, and soil temperature data (T) collected with the field temperature probes (Eq. [1]; Lee et al., 2001).

[1]

More information will be given in the RESULTS section. Briefly, minimal mineralization occurred below 18°C, a linear equation fit the change in reaction rates between 18°C and 34°C, and each 1°C increase resulted in a 6.7% increase in inorganic-N release rate (Lee et al., 2001). Inorganic-N release at 33°C provided a baseline for mineralization from the field soils. Data from the field temperature probes provided the hourly measurements needed to establish the total number of hours (t) at each temperature during the year.

Multiple factor analyses of variance were performed on soil NO3–N and NH4–N levels (SAS, 1999). Main factors were season, landscape position, soil depth, and site. Significant differences (P < 0.05) were determined using an LSD test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Bermudagrass growth in the North Carolina coastal plain occurs primarily between 1 June and 30 September, and growth is correlated with high aerial and root temperatures (Fig. 2, 3). Mean daily aerial temperatures ranged from {approx}5 to 30°C. During the June to September periods, the mean monthly maximum temperatures at both sites were 29°C to 30°C, which are well into the optimal ranges for warm-season grasses (Beard, 1982). Maximum daily temperatures frequently reached 36 to 38°C. Cumulative rainfall totals were 175 cm in 1999 and 125 cm in 2000. In normal years, the highest rainfall months are June through September. The major precipitation events during the 2-yr experiment were hurricanes Dennis and Floyd that resulted in >60 cm of rainfall in the latter part of September 1999.



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  		Fig. 2. Daily mean aerial temperatures, monthly precipitation, and turfgrass growth (100% = 24 g m-2 d-1) for two consecutive years at Site 1. The approximate times of fertilizer N applications are indicated by arrows.

 


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  		Fig. 3. Daily mean aerial temperatures, monthly precipitation, and turfgrass growth (100% = 6.8 g m-2 d-1) for two consecutive years at Site 2. The approximate times of fertilizer N applications are indicated by arrows.

 
Fertilizer applications were typical for bermudagrass fairways in this region. At Site 1, a total of 132 kg N ha-1 was applied in 1999 and 161 kg N ha-1 in 2000. At Site 2, 150 kg N ha-1 were applied during the 1999 growing season and 213 kg N ha-1 in 2000. The rationale for the higher fertilizer N rate during the 2000 growing season was an attempt to improve bermudagrass density and quality in preparation for bermudagrass dormancy, as the fairways were not to be overseeded in Fall 2000.

Measurement of clippings indicated that bermudagrass growth was maximized from July to September at Site 1. The clipping data from Site 2 were more ambiguous because of overseeding. Nonetheless, the turf system was dominated by bermudagrass after June and vigorous bermudagrass growth would be responsible for the maximal values during July and August. As noted in the figure headings, grass yields were lower at Site 2, which probably reflected negative effects of overseeding and the extensive use of PGRs throughout the summer to limit growth.

For Site 1, the pattern of NO3–N concentrations with depth for a given sampling date was generally consistent throughout the 2 yr of the study (Fig. 4). Statistical differences by site, season, depth, and landscape position existed (Table 2); however, NO3–N levels were generally <4 mg kg-1 and in some instances <2 mg kg-1 (Fig. 4). Nitrate-N concentrations were slightly higher, {approx}5 mg kg-1, at the 0- to15-cm depth in March 1999, but that was the only notable exception. The NO3–N levels at Site 2 were similar to those at Site 1, consistently remaining <4 mg kg-1 at all depths. The main exceptions were in March 1999 when levels were 5 to 6 mg kg-1 throughout the profile, and in August 2000 when levels were elevated to 8 mg kg-1 at the 0- to 15-cm depth, possibly reflecting sampling 3 d after the 73 kg N ha-1 fertilization.



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  		Fig. 4. Soil NO3–N concentrations at different depths under fairways during seven sampling dates at each site in 1999 and 2000. The means at each depth were statistically different from one another when separated by 0.22 at Site 1 and 0.34 at Site 2, based on LSD0.05.

 

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  		Table 2. Mean squares from the analysis of variance conducted on soil NO3–N and NH4–N.

 
The effect of recent fertilizer applications on bermudagrass growth and root-zone NO3–N concentrations was evaluated in separate plots at Site 1 in 2000 (Fig. 5). In those plots, the first fertilization in April was reduced to 22 kg N ha-1 and no subsequent fertilizer N was added during the bermudagrass growing season. Turf quality was reduced during May and June but was not different from the fertilized plots during July and August (data not shown). Clipping weights from the unfertilized plots were noticeably lower throughout the year (Fig. 5a). Later in the summer, however, growth was closer to that occurring in the fertilized plots. Soil cores taken in August revealed that NO3–N concentrations under both the fertilized and unfertilized plots were both <2 mg kg-1 (Fig. 5b). There was one anomalous data point, a slightly higher NO3–N level in the upper 15 cm of the unfertilized plot. Still, the level was <3 mg kg-1.



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  		Fig. 5. Unfertilized plots: (a) weekly turfgrass growth expressed as a percentage of the fertilized plots, and (b) soil NO3–N concentrations under fertilized and nonfertilized plots on 18 Aug. 2000.

 
Soil NO3–N concentrations at different landscape positions along the transects were compared on sample dates when the largest variations were present (Fig. 6). At Site 1, natural areas in adjacent woods were not fertilized, but the roughs were. In general, NO3–N levels in the fertilized fairways were similar to those in the rough and unfertilized natural areas. Some increases were noticeable at all three landscape positions in March 2000 compared with August 1999, but levels remained in the 2.5 to 3.5 mg kg-1 range. Similar trends were present at Site 2, with levels typically <3 mg kg-1. The notable exception was the upper 30 cm of the nonoverseeded rough in March 2000, where concentrations approached 6 mg kg-1. At depths below 30 cm, the NO3–N concentrations were below 3 mg kg-1, similar to those found under other landscape positions.



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  		Fig. 6. Soil NO3–N concentrations by depth at different landscape positions. Landscape position x depth values were not significantly different (see Table 2).

 
Soil NH4–N concentrations were statistically different by season, depth, and landscape position (Table 2), but generally the concentrations were low, between 2 to 4 mg kg-1 at each site and sampling date (Fig. 7). Organic matter under the fairways ranged from 45 to <10 kg kg-1 (Fig. 8). Samples from the top 15 cm at both sites had higher amounts of OM, >35 mg kg-1, as might be expected for samples taken from a zone with intense root growth. At greater depths, organic matter was generally <22 mg kg-1.



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  		Fig. 7. Soil NH4–N concentrations with depth under fairways. The means at each depth were statistically different from one another when separated by 0.93 at Site 1 and 0.60 at Site 2, based on LSD0.05.

 


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  		Fig. 8. Soil organic matter at five depths under fairways.

 
Beginning in early May and continuing through September, soil temperatures were consistently >18°C at the 5-cm depth (Fig. 9). Previous experiments indicated that mineralization in these soils is minimal below 18°C and increases linearly from 18 to 36°C (Lee et al., 2001). Soil incubation experiments, adjusted for temperature effects, indicated that mineralization at Site 1 increased from {approx}1 to 2 mg kg-1 of soil wk-1 in May to 4.0 mg kg-1 wk-1 during July and August (high OM; Fig. 10). The Rimini high OM soil from Site 1 released higher amounts of inorganic N than the Rimini low OM soil and the Norfolk soil. A similar pattern of mineralization across time was apparent for the latter two soil types, and maximal rates for both were in the 1.5 to 2.0 mg kg-1 wk-1 range.



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  		Fig. 9. Soil temperatures at 5 cm below the soil surface.

 


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  		Fig. 10. Predicted inorganic-N release by mineralization.

 
Integrating mineralization rates across time allowed estimates of inorganic-N release from May through September (Table 3). Calculations revealed that 41 to 77 mg NO3–N kg-1 of soil would be released from mineralization during that time in the Rimini soils at Site 1 and 30 mg NO3–N kg-1 of soil in the Norfolk soils at Site 2. The estimates equate with additions of 60 kg N ha-1 and 154 kg N ha-1 yr-1 that would supplement applied N fertilizer.


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  		Table 3. Soil characteristics and mineralization rates. Net mineralization is calculated using data from incubation studies, temperature block experiments, and field temperature probes (see METHODS and RESULTS for details).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The main intent of this study was to quantify NO3–N levels under established bermudagrass turf systems during growth and dormancy cycles, and to begin evaluating the potential for leaching and groundwater contamination. These are the first comprehensive measurements of temporal and spatial changes in soil NO3–N concentrations in situ for golf course fairways. No data are available on the time required for soil organic matter and organic-N to reach equilibrium under managed turfgrass in this geographical region. Nevertheless, it is reasonable to assume that soil organic-N levels had long been stabilized in the 50- and 75-yr-old fairways. Even with bermudagrass growth and repeat mowings across the seasons and years, organic matter levels beneath the shallow root-zone were below 25 mg kg-1, reflecting the intense weathering and organic matter decomposition. The results of this study therefore appear to provide a quantitative description of soil NO3–N levels in a mature, relatively stable bermudagrass system.

The stability of the turf system probably was, at least in part, the basis for statistical resolution of relatively small differences in NO3–N levels among sites and landscape positions and within the soil profile (Table 2). Nonetheless, from a larger, environmental perspective, the concentrations of NO3–N under the bermudagrass fairways were consistently low throughout the 2-yr experiment. This conclusion is based on several factors. First, the NO3–N concentrations were similar to those present under nonfertilized natural areas, designated woods in Fig. 6. The natural areas were trees and undefined underbrush that did not receive any fertilizer and were generally unmanaged. Nitrate-N in those soil profiles was consistently 1 to 3 mg kg-1 compared with the 2 to 4 mg kg-1 range in the fairways. Second, the NO3–N levels were well below those reported for agricultural fields. In studies with corn at similar fertilizer rates as used in these turf sites (100 to 200 kg N ha-1), for example, soil NO3–N levels typically averaged 60 kg N ha-1 and reached as high as 100 to 150 kg N ha-1 in the top 60 cm (Schuman et al., 1975; Kamprath, 1986; Angle et al., 1993; Wagger, 1993; Guillard et al., 1995). In comparison, our values of 2 to 4 mg kg-1 are equivalent to 9 to 36 kg N ha-1 in the upper 60 cm (assuming a bulk density of 1.5 g cm-3). Third, the NO3–N levels in our study are on the low end of those reported by Snyder et al. (1984) in the previous study with bermudagrass. They collected soil solution samples using suction lysimeters. A comparison of the results from lysimeters and soil cores requires data transformation. Assuming a bulk density of 1.5 g cm-3, 30 to 50% pore space, and 18 to 30% field capacity, we estimate that the NO3–N in our soil extracts should be multiplied by 3 to 6 for conversion to mg NO3–N L-1 of soil solution. A relatively high soil core extract value of 4 mg kg-1 (Fig. 4), would convert to a solution NO3–N level in the 12 to 24 mg L-1 range. Snyder et al. (1984) found values ranging from 14 to 75 mg L-1, with lower NO3–N levels typical of slow release fertilizers and appropriate irrigation. The turf managers at the sites examined in the current study used combinations of urea and coated slow release fertilizers and irrigated only when needed.

One of the most interesting aspects of the results from this study was the absence of obvious influence of fertilization on soil NO3–N levels. In field crops, elevated levels or fronts of NO3–N often are observed in the soil profile for extended periods (several weeks) after fertilization (Wagger, 1996), and the fronts dissipate with time as the NO3–N is taken up by plant roots or leached. In our study, fertilizer was added 3 to 6 times during the growing season (refer to Fig. 2, 3). The soil sampling schedule included cores taken in March before fertilization began, and in May and August, the period when fertilizer was being applied. Yet, soil NO3–N levels were almost always <4 mg kg-1 and concentrations were generally uniform with depth. The notable exception was the single August 2000 sample at the top of the soil profile from Site 2. In that case, fertilizer was applied 3 d before the cores were taken and no rain had occurred, so it is likely that fertilizer NO3–N was present in the analyzed sample. The lack of fertilizer effect also was apparent when comparing NO3–N concentrations under fertilized plots to those under plots that received only a low amount of fertilizer before the start of the bermudagrass growing season (Fig. 5b). Nitrate-N levels were nearly identical at all depths. The absence of a fertilizer effect in the soil profile should not be entirely unexpected. It has been shown previously with cool-season grasses that inorganic-N fertilizer applied at normal rates is efficiently taken up within 24 to 48 h, which appeared to reflect the extensive root length density and absorption surface (Bowman et al., 1989). Bermudagrass root length density appears to be at least as great as that for the cool-season grasses, especially in mature turf, and bermudagrass has been shown to take up fertilizer-N efficiently (Bowman et al., 2002).

The NO3–N concentration in the soil profile at a given time will reflect interactions among several processes. Without a noticeable fertilizer effect, the dominant factors in the turf systems examined here would appear to be N uptake by the bermudagrass and mineralization. As argued previously, the relatively low organic matter content of soil under the fairways implies intense mineralization, and the uniformity of NO3–N within the profile would be consistent with slow and steady release of NO3–N from decomposition of clippings and roots. The importance of mineralization and the release of N for plant uptake was evident in the low fertilization plots at Site 1, where turf quality improved during July and August and substantial growth occurred (Fig. 5a). Also, estimates of cumulative NO3–N release during the bermudagrass growing season indicated amounts are roughly 30 to 100% of the amount supplied with fertilizer (Table 3). Furthermore, although not reported here, N concentrations in clippings were measured, and the cumulative amount of N recovered in the tissue for the growing season exceeded by 2 to 3 times the amount of fertilizer-N applied. Since inorganic-N pools in the soil did not vary greatly, the additional N must have been associated with N cycling through organic N pools.

For N use efficiency, a beneficial aspect of the bermudagrass system in this climatic zone is that mineralization rates apparently are closely aligned with bermudagrass growth potential. Microbial populations and their activities are highly temperature dependent (Stanford et al., 1973; Lee et al., 2001), reaching optima around 30 to 35°C. That response curve closely resembles the temperature growth response curve for bermudagrass (Satorre et al., 1996; Fagerness et al., 2002). Thus, as optimal growth conditions occur, the release of NO3–N is synchronized with plant uptake. The seasonal alignment of the processes can be seen by comparing graphs of growth (Fig. 2, 3) and mineralization rate (Fig. 10).

Implications for Groundwater Quality
Care must be taken in interpreting these results, because the study was done at only two sites from a single geographical zone across 2 yr. Nonetheless, based strictly on the concentrations of NO3–N present in the soil profile, it would seem that the potential for groundwater contamination from bermudagrass systems like those in this study is relatively low. The most likely time for NO3–N loss, logically, would be in winter months. Nitrate leaching potentials are greatest when total water influx into the soil system is greater than water loss from evapotranspiration (Smith and Cassel, 1991). This was shown with bermudagrass in the work by Snyder et al. (1984), where the greatest NO3–N losses were associated with excess irrigation. Historical environmental data from southeastern North Carolina indicate that water inputs typically are greater than losses from November to March (data not shown), and that certainly would be true in a system with dormant bermudagrass, where it would also coincide with minimal N uptake. The temperature dependence of the mineralization process, however, would seem to limit continual NO3–N production in winter. Indeed, relatively low NO3 –N concentrations were present in the soil profile in winter months in this study. If fairways are overseeded and a cool-season grass is growing during winter months, plant uptake of NO3 could ameliorate the potential for leaching losses.

As implied from previous reasoning, measurements of NO3–N in the soil profile (along with downward water movement) have been the primary basis for estimating losses from turf systems (Snyder et al., 1984; Liu et al., 1997) and the potential for groundwater contamination. In reality, though, it provides information only about events in the upper zone of the hydrologic cycle. Nitrate that is leached through the soil profile then moves with subsurface water flows to streams and lakes (Gilliam et al., 1985). It is conceivable that denitrification could occur during the hydrologic process, further diminishing the negative impact from a bermudagrass system. Indeed, emerging research results from related projects appear to indicate that substantial denitrification does occur (Adams, 2001; B.B. Thapa, D.C. Bowman, D.K. Cassel, and T.W. Rufty, unpublished data, 2001).

Received for publication August 23, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome