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TURFGRASS SCIENCE

The Fate of Nitrogen-15 Ammonium Sulfate Applied to Kentucky Bluegrass and Perennial Ryegrass Turfs

^a The Norwegian Crop Research Institute, Plant Protection Centre, Høgskoleveien 7, 1432 Ås, Norway
^b Department of Natural Resources and Environmental Sciences, University of Illinois, W-421 Turner Hall, 1102 S. Goodwin Dr., Urbana, IL 61801
^c Department of Horticulture, 305 Alderman Hall, 1970 Folwell Ave., University of Minnesota, St. Paul, MN 55108

^* Corresponding author (bbranham{at}uiuc.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Turfgrasses that possess a spreading growth habit typically produce an organic layer, termed thatch, that has been shown to sequester significant quantities of applied nitrogen. This study was conducted to compare the fate of nitrogen in a thatchy Kentucky bluegrass (Poa pratensis L.) and a non-thatch producing perennial ryegrass (Lolium perenne L.) turf. The effects of sampling time and sampling depths on the distribution of labeled fertilizer nitrogen (LFN) were investigated. Ammonium sulfate was applied to 36 microplots installed in a Marlette sandy loam soil (fine-loamy, mixed, mesic Glossoboric Hapluadalfs) at a rate of 293 kg ha^–1 yr^–1 in six equal applications of 48.8 kg N ha^–1, with the initial application containing ¹⁵N-labeled (NH₄)₂SO₄. Four microplots from each turf species were excavated on 17 June, 6 July, and 10 Aug. 1994 and on 15 June 1995 for nitrogen analysis. Two days after treatment, the Kentucky bluegrass LFN recoveries in shoot tissue, thatch, and soil were 34, 38, and 20% of the total LFN applied. These values changed to 47, 20, and 9% at 365 DAT. For perennial ryegrass, the corresponding values were 25, 28, and 27% at 2 DAT and 43, 10, and 14% at 365 DAT. Thatch was a better N sink than mat, but mat also sequestered significant quantities of LFN. Some downward movement of LFN was observed, particularly in perennial ryegrass. The very small LFN concentrations found in the 20- to 40-cm soil layer confirms that leaching is not a major pathway for N loss from turf. Soil LFN was primarily found in the 0- to 5-cm soil layer and as organic nitrogen. These results indicate that while thatch is important in reducing N leaching in turf, the absence of thatch does not dramatically alter nitrogen dynamics in turf.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
MAINTAINING A HEALTHY, resilient turf playing surface with high levels of wear tolerance, aesthetic quality, and an acceptable rate of growth requires efficient use of fertilizers. Application of fertilizer to athletic fields, especially those constructed with sandy rootzone mixtures, could result in nitrate leaching. Leaching of nitrates and contamination of groundwater have been studied carefully and are discussed in reviews by Petrovic (1990) and Walker and Branham (1992). Besides leaching, fertilizer nitrogen can be lost in gaseous forms or immobilized through plant uptake or long-term storage in soil organic matter. A number of studies have examined each of these N pools (Mancino et al., 1988; Horgan et al., 2002b; Brown et al., 1982; Porter et al., 1980). Only a few published works, however, have attempted to construct a mass balance of fertilizer N in the entire turfgrass system using labeled carriers (Horgan et al., 2002a; Miltner et al., 1996; Starr and DeRoo, 1981). These studies reported a total recovery of labeled fertilizer nitrogen (LFN) between 57 and 80%, and unrecovered amounts were attributed to gaseous loss. These studies also indicated that thatch was a significant sink for fertilizer N. Thatch, defined as a tightly intermingled layer of dead and living stems and roots that develops between the zone of green vegetation and the soil surface (Beard, 1973), contained 20 to 35% of LFN at the end of these studies. Miltner et al. (1996) recovered 31% of LFN in thatch 18 d after a spring application to Kentucky bluegrass turf, and nearly 20% 2 yr later. Bowman et al. (1989) determined that N uptake by thatch was very rapid, and that applied N was quickly immobilized. Mineralization of N in thatch organic matter to plant available N forms depends upon microbial transformation and is therefore temperature dependent.

Thus, thatch plays a significant role in the fate and cycling of applied fertilizer nitrogen in turfs that develop a thatch layer (Miltner et al., 1996, Starr and DeRoo, 1981). However, not all turfgrass species develop a thatch layer. Bunch-type grasses such as perennial ryegrass more often have a mat layer, which is partially decayed organic matter from leaves, stems, etc. that has become part of the soil profile (Beard, 1973). The difference in the extent and development of thatch or mat layers suggests that the distribution of fertilizer in a perennial ryegrass turf may be significantly different from a Kentucky bluegrass turf, yielding differences in both total N efficiency and in the downward mobility of fertilizer N. The objectives of this study were to compare the uptake and distribution of N in thatch and non-thatch-forming turfgrass species.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
In June 1994, 18 microplots were installed at the Hancock Turfgrass Research Center on the campus of Michigan State University into a thatchy ‘Kenblue’ Kentucky bluegrass turf and an additional 18 microplots were installed into a ‘Palmer’ perennial ryegrass turf. The bluegrass thatch layer was approximately 20 to 25 mm thick, containing a heterogenous medium of dead and living plant tissue, microorganisms and some soil particles. The ryegrass mat was 5 to10 mm thick and consisted of partly decayed organic matter mixed with some soil material. Both turfgrasses were grown on a Marlette sandy loam soil (fine-loamy, mixed, mesic Glossoboric Hapluadalfs). The particle size distributions were 590 g kg^–1 sand, 270 g kg^–1 silt and 140 g kg^–1 clay for the Kentucky bluegrass site, and 530 g kg^–1 sand, 270 g kg^–1 silt and 200 g kg^–1 clay for the perennial ryegrass site.

Microplot construction, sample collection, and methods of analysis have been described in detail by Miltner et al. (1996), but some procedural modifications were made in the present study. The microplots were constructed from 20 cm diameter PVC pipe with a total length of 40 cm. To limit soil resistance during installation, each microplot was beveled to 45° at the leading edge. A tractor-mounted hydraulic ram was used to push the microplots directly and smoothly into the soil, keeping an undisturbed soil structure in both microplots and surrounding areas.

All treatments received a total of 293 kg N ha^–1 as (NH₄)₂SO₄ over the course of the study, applied in six applications of 48.8 kg N ha^–1 on 15 June, 20 July, 19 Aug., 24 Sep., and 22 Nov. 1994, and on 1 June 1995. The first application was made with ¹⁵N-labeled ammonium sulfate (24.7498 atom-% excess). The ¹⁵N-labeled fertilizer was applied as a solution in 50 mL of water to the microplots. To ensure that all of the fertilizer solution was applied to the microplot, an additional 100 mL of water was applied from the same container immediately after fertilization. After all microplots were fertilized and irrigated, each plot was covered with a bucket and the surrounding area was fertilized with non-labeled fertilizer. This border application and all other applications of non-labeled ammonium sulfate, were made with a rotary spreader and followed immediately by 5 mm overhead irrigation.

On the basis of soil analysis, P and K levels were adequate and no supplemental applications were made during the study. No pesticides were applied during the course of the study. Irrigation was applied when necessary to prevent drought stress. Clippings were collected every 7 to 10 d during the growing season using a manual, hand-held clipper and a vacuum collector. The mowing height was 40 mm for all sampling dates, and all the clippings were dried at 65°C for 72 h and stored for later analysis. Before analysis, the clippings were ground with a Cyclone Sample Mill (UDY Corporation, Fort Collins, CO).

To examine the distribution of fertilizer N in the soil, four microplots from each turf-type were excavated on 17 June, 6 July, and 10 Aug. 1994 and on 15 June 1995 for N analysis. These excavation dates corresponded to 2, 21, 56, and 365 d after application of the ¹⁵N fertilizer. Each PVC microplot was split longitudinally with a circular saw to expose the soil core within. The microplot cores were then sectioned into verdure and four different soil depths: 0 to 5 (including thatch and/or mat layer), 5 to 10, 10 to 20, and 20 to 40 cm. Verdure was removed from the soil core with scissors, dried and processed like clippings before N and ¹⁵N analysis. After air-drying, the 0- to 5-cm layer, thatch or mat was separated from the soil component by hand massaging. The thatch or mat-sample was ground with a Wiley Mill to pass a 60-mesh screen. Soil from the 0- to 5-cm layer, as well as subsamples from each of the three remaining depths, were prepared for analysis by pulverizing into fine powder.

For soil, mat, and thatch samples, total N and inorganic N were measured. Inorganic soil nitrogen was determined by extracting duplicate samples of dry soil with 1 M KCl (5:1 v/w). Nitrate-N and ammonium-N were determined by flow injection analysis on a Lachat QuikChem Autoanalyzer (Lachat Instruments, Milwaukee, WI). Following this analysis, inorganic N was converted for ¹⁵N analysis by the diffusion method of Brooks et al. (1989). Because of low nitrate and ammonium concentrations, a single diffusion for both N forms was performed and ¹⁵N analysis was for inorganic N. For thatch, the same procedure was used; however, because of higher N concentrations in thatch, less material was used for the extraction (25:1 v/w). The total N content and the ¹⁵N enrichment of clippings, verdure, thatch/mat, soil, and all diffusion samples were determined with a Europa Scientific Roboprep C-N Biological Sample Converter and Tracermass mass spectrometer (Europa Scientific USA, Cincinnati, OH). Each sample was analyzed twice. The statistical design was a split-split plot with species as the main plot, sampling time the subplot, and sampling depths the sub-subplot (Table 1). The data were analyzed statistically by the GLM procedure of SAS (SAS Institute Inc., 1987). The Ryan-Einot-Gabriel-Welsch Multiple range test (REGWQ) was used with a significance level of p = 0.05.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

Clipping Yield and Nitrogen in Above-Ground Biomass
Both clipping yield and N uptake followed a similar pattern for each species throughout the study (Fig. 1) . Kentucky bluegrass produced the greater yield in the summer season, while perennial ryegrass grew better in October and November. The total yearly clipping yield did not differ between the two species with Kentucky bluegrass yielding 7680 kg ha^–1 that contained 258 kg N ha^–1, while perennial ryegrass yielded 7850 kg ha^–1 dry matter containing 241 kg N ha^–1. Nitrogen content in the clippings of Kentucky bluegrass and perennial ryegrass averaged 3.4 and 3.1%, respectively. Corresponding N values in clippings at 21 and 56 DAT were 3.4 and 3.5% for Kentucky bluegrass and 2.7 and 3.1% for perennial ryegrass, indicating a relatively stable N concentration in the leaves at different sampling times.

View larger version (18K): [in this window] [in a new window]   Fig. 1. Cumulative clipping yield (upper) and total N (lower) harvested in clippings from Kentucky bluegrass and perennial ryegrass microplots from 15 June 1994 to 15 June 1995.

In the present study, both turfgrass species showed very high fertilizer N uptake following application (Fig. 2) . Note that we measured leaf N allocation as a proxy for total N uptake. Leaf N allocation is a reasonable estimate of N uptake in turfgrasses due to the relatively low level of root growth exhibited by frequently mowed turfgrasses (Mehall et al., 1984). From 15 June through 6 July, mean daily LFN uptake rates of 0.572 and 0.483 kg LFN ha^–1 d^–1 were measured for the bluegrass and ryegrass, respectively. The mean uptake rates dropped to 0.185 for Kentucky bluegrass and 0.069 kg LFN ha^–1 d^–1 for perennial ryegrass from 6 July through 10 August. Total N uptake rates for Kentucky bluegrass for the periods of 15 June through 6 July and 7 July through 10 August were 2.267 and 1.986 kg N ha^–1 d^–1, and for perennial ryegrass 1.465 and 0.978 kg N ha^–1 d^–1. Three weeks after the start of fertilization, LFN accounted for 25% of the total N recovered in Kentucky bluegrass clippings. This percentage dropped to 16% at 56 DAT and 9% at 365 DAT. Corresponding values for perennial ryegrass were 33, 19, and 8%. While the LFN uptake dropped markedly throughout the study, total N recovery in the clippings indicated a fairly constant N uptake for both turfgrasses (data not shown), due to frequent applications of non-labeled ammonium sulfate in the growing season and to mineralized N from soil organic matter. A period of increased N uptake occurred in late August for both species (Fig. 1).

View larger version (17K): [in this window] [in a new window]   Fig. 2. Cumulative labeled fertilizer nitrogen (LFN) harvested in Kentucky bluegrass (KB) and perennial ryegrass (PR) clippings from 15 June 1994 to 15 June 1995.

Nitrogen in Thatch–Mat
The amounts of LFN in thatch and/or mat were different between the two turfgrass systems at each sampling date, except at 56 DAT (Table 2), mainly because of differences in the thatch–mat organic matter content in the upper 0- to 5-cm layer. The Kentucky bluegrass thatch averaged 8% of the dry weight of the 0- to 5-cm layer, while the corresponding value for perennial ryegrass mat was 2% (data not shown). Even though the mean total N content in Kentucky bluegrass thatch (237 kg N ha^–1) was twice the amount measured in perennial ryegrass mat (119 kg N ha^–1), the latter was found to be an important sink for fertilizer N as well.

The level of LFN in Kentucky bluegrass thatch and perennial ryegrass mat showed significant changes within the experimental period (Table 2). Thirty-eight percent of applied LFN was recovered in Kentucky bluegrass thatch on 17 June (2 DAT). This level dropped to about 13% at 6 July (21 DAT) and 12% at 10 August (56 DAT). The decreases in thatch LFN in July and August were related, in part, to increases in the amount recovered in clippings, indicating translocation from active roots and rhizomes within the thatch layer as well as mineralization and transport of LFN from the thatch upwards to shoot tissue. No increase in soil LFN was observed at the same time, indicating that downward transport of mineralized thatch LFN was not a significant process. While the total N content of both species followed the same trend throughout the experiment (data not shown), a major difference was observed for LFN recovery at the final sampling of Kentucky bluegrass. Approximately 20% of LFN applied was found in Kentucky bluegrass thatch at 365 DAT. A significant increase in Kentucky bluegrass thatch N content and yield (data not shown) from 10 August (56 DAT) to 15 June (365 DAT), probably because of increased root growth and rhizome development within the thatch layer during fall and spring growing seasons, may explain the increase in LFN and total N content within this period. Alternatively, not all of the LFN found in verdure at the 56 DAT sampling was recovered in clippings at the 365 DAT sampling date. Leaf and tiller senescence may have returned plant LFN to the thatch layer. The total thatch LFN recovery after one year (20%) was similar to the 21% reported by Starr and DeRoo (1981), but less than the LFN recovery (30–35%) reported by Miltner et al. (1996).

About 28% of the applied LFN was recovered in perennial ryegrass mat at 17 June (2 DAT). This LFN level dropped to 11% over the next 2 mo, and remained relatively constant through the last sampling date (Table 2). The different growth patterns of the two turfgrass species combined with a lower quantity of organic matter (and associated microbial activity) within the mat layer may explain the differences in LFN and total N immobilization.

Total N and LFN in Kentucky bluegrass thatch and perennial ryegrass mat were predominantly in organic forms (data not shown). Less than 0.2% of the total LFN, and <0.1% of the total N, were in the inorganic pool for the two turfgrass species. The separation technique of thatch and mat organic matter from the soil fraction by hand-massaging may not remove all the mineral particles, giving a small percentage of ammonium nitrogen in the samples.

Nitrogen in Soil
Total LFN recovery in soil was slightly higher in perennial ryegrass than Kentucky bluegrass (Table 2). Between 2 and 56 DAT, 94 and 87% of the total soil LFN was recovered in the 0- to 5-cm layer for Kentucky bluegrass and perennial ryegrass, respectively. Within each species, LFN recovery in the 0- to 5-cm layer was relatively constant until 365 DAT, when a significant drop in LFN recovery occurred for both species. In bluegrass, mineralization of LFN in the soil and upward transport to thatch–mat or shoot tissue accounted for about one-half of the decrease in soil LFN. The remainder of the decrease in LFN recovery can be attributed to gaseous losses or experimental error. For the perennial ryegrass plots, the reduction in soil LFN was closely related to a decrease in the total LFN recovery.

Between 21 and 27% of the total LFN applied was found in perennial ryegrass soil at 2 and 56 DAT, respectively. By 15 June (365 DAT) only 14% was recovered from the soil. For Kentucky bluegrass, 20, 14, 16, and 9% was recovered as soil LFN at 2, 21, 56, and 365 DAT, respectively. This decreasing LFN recovery over time is similar to the values presented by Power and Legg (1984), but they differ with the findings of Miltner et al. (1996) who found 8% of the LFN recovered from soil at 18 DAT, and 14% 2 yr later. The total LFN recoveries found in the soils at the end of present study were similar to the value of 14% reported by Starr and DeRoo (1981) and the 13 to 17% noted by Watson (1987) at 120 and 49 d from the last ¹⁵N treatment.

Because soil LFN did not significantly change below 5 cm, and the total LFN recovery decreased over time, gaseous loss seems the most likely explanation for these results. Some downward LFN transport may have occurred particularly for the ryegrass plots (Table 2); however, this seems to be more related to N transport in soil macropores shortly after application, than downward leaching of N throughout the study.

Soil LFN occurred primarily in the organic pool (Table 3). Two days after application only 2.8% (0.59 µg g^–1) of total LFN in Kentucky bluegrass soil was in the inorganic pool. This data reinforces the findings of Bowman et al. (1989) who observed a similarly rapid disappearance of urea from Kentucky bluegrass turf. Within a year of sampling this fraction dropped to 0.4% (0.06 µg g^–1). Almost identical results were obtained from perennial ryegrass plots, showing that 3.2 and 0.7% of the LFN concentrations was presented in the inorganic N pool at 2 and 365 DAT, respectively.

The distribution of LFN in plant material remained relatively constant throughout the study. Between 63 and 72% of the LFN applied to Kentucky bluegrass, and 51 to 55% of the LFN applied to perennial ryegrass plots, was recovered in shoots plus thatch or mat organic matter at each sampling date. The large decrease in Kentucky bluegrass thatch LFN between 2 and 21 DAT was not compensated with an equivalent LFN increase in the shoots or soil, but rather coincided with a decrease in total recovery. The decrease in mat LFN, however, was associated with a corresponding increase in LFN in the above-ground plant tissue.

Even though leaching losses were not measured, it is reasonable to conclude that leaching did not play significant role in mass balance constructions. Very small LFN concentrations were found in the 20- to 40-cm soil layer for both turfgrass systems, and the data presented does not implicate downward leaching as a major pathway for N loss. However, there is evidence of some downward movement of ¹⁵N, particularly in the ryegrass plots between 10 Aug. 1994 and 15 June 1995 (Table 2).

A comparison of LFN amounts in various sampling depths for Kentucky bluegrass and perennial ryegrass at different sampling dates shows significantly higher LFN recoveries in verdure and thatch of bluegrass at 2 DAT. These results indicate that thatch can tie up more fertilizer N than mat. Still more LFN was retained in thatch than in mat at 365 d after treatment, which indicates that thatch is a significantly larger N sink than mat. Despite the difference observed in this study, the two species are remarkably similar with regards to nitrogen uptake and distribution. While perennial ryegrass and other bunch-type grasses do not form a thatch layer, the organic layer that forms at the soil surface functions much the same as thatch and helps to immobilize added nitrogen. Turfgrasses rapidly take up nitrogen because of the extremely high root and shoot densities present in a well-maintained turf. This study showed that a single nitrogen application of 49 kg ha^–1 resulted in minimal movement of nitrogen below 5 cm of soil.

	ACKNOWLEDGMENTS

 
The authors wish to thank Profs. Don Christenson and Darryl Warncke for allowing the use of their laboratories and equipment for soil and plant preparations, Dave Harris for running the diffusion samples and Prof. Bal Ram Singh for useful comments on the manuscript. Technical assistance from Ron Calhoun and Prof. Antoni Szafranek is gratefully acknowledged.

Received for publication October 13, 2002.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

• Beard, J.B. 1973. Turfgrass science and culture. Prentice Hall, Englewood Cliffs, NJ.
• Bowman, D.C., J.L. Paul, W.B. Davis, and S.H. Nelson. 1989. Rapid depletion of nitrogen applied to Kentucky bluegrass turf. J. Am. Soc. Hortic. Sci. 114:229–233.
• Bristow, A.W., J.C. Ryden, and D.C. Whitehead. 1987. The fate at several time intervals of ¹⁵N-labelled ammonium nitrate applied to an established grass sward. J. Soil Sci. 38:245–254.
• Brooks, P.D., J.M. Stark, B.B. McInteer, and T. Preston. 1989. Diffusion method to prepare soil extracts for automated nitrogen-15 analysis. Soil Sci. Soc. Am. Proc. 53:1707–1711.
• Brown, K.W., J.C. Thomas, and R.L. Duble. 1982. Nitrogen source effects on nitrate and ammonium leaching and runoff losses from greens. Agron. J. 74:947–950.[Abstract/Free Full Text]
• Dowdell, R.J., and C.P. Webster. 1980. A lysimeter study using nitrogen-15 on the uptake of fertilizer nitrogen by perennial ryegrass swards and losses by leaching. J. Soil Sci. 31:65–75.
• Horgan, B.P., B.E. Branham, and R.L. Mulvaney. 2002a. Mass balance of ¹⁵N applied to Kentucky bluegrass including direct measurement of denitrification. Crop Sci. 42:1595–1601.[Abstract/Free Full Text]
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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in Crop Science Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (3) Citing Articles via Google Scholar Google Scholar Articles by Engelsjord, M. E. Articles by Horgan, B. P. Search for Related Content PubMed Articles by Engelsjord, M. E. Articles by Horgan, B. P. Agricola Articles by Engelsjord, M. E. Articles by Horgan, B. P. Related Collections Nitrogen Nutrients Turfgrass