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a Dep. of Hortic. Sci., 305 Alderman Hall, 1970 Folwell Ave., Univ. of Minnesota, St. Paul, MN 55108
b Dep. of Natural Resources and Environmental Sci., 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801
* Corresponding author (bphorgan{at}umn.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: DAF, days after fertilization • LFN, labeled fertilizer N • PET, potential evapotranspiration • PVC, polyvinyl chloride
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The potential for extensive denitrification losses from turf cannot be ignored, as turf represents an unusual cropping system. With traditional row crops, denitrification typically occurs in the spring or fall when NO3 is present due to recent fertilization or reduced plant uptake, evapotranspiration is minimal, rainfall is high, and readily decomposable organic C is available as a source of energy (Paul and Clark, 1989). This combination provides the substrate and anoxic conditions that are necessary for gaseous-N loss via denitrification. However, soil temperatures during these times are often low, and since the rate of denitrification is temperature dependent (Blackmer et al., 1982; Mancino et al., 1988), gaseous N loss is usually limited (Schnabel and Stout, 1994). In contrast, highly managed turfgrass represents a system where extensive denitrification losses could occur from warm soils. These losses would be promoted because irrigation keeps the soil profile near field capacity and may lead to temporary short-term anoxia (Sexstone et al., 1985), while multiple applications of N fertilizer are common, and large amounts of readily decomposable organic C are present in the thatch and verdure.
Direct measurements to characterize and quantify denitrification losses from fertilized turfgrass are limited. Because of the inherent difficulties involved in measuring the emission of N2 into ambient air, gaseous N loss by denitrification or volatilization have usually been estimated from the deficits in 15N balance studies. Using 15N-labeled (NH4)2SO4 to calculate a mass balance, Starr and DeRoo (1981) concluded that between 24 and 36% of the fertilizer N was lost to the atmosphere by NH3 volatilization or denitrification. Similarly, Miltner et al. (1996) recovered only 64 to 81% of the LFN applied as urea, suggesting the occurrence of gaseous N loss.
Other researchers have used C2H2 inhibition to measure N2O emissions from permanent grasslands or turfgrass under field conditions (Denmead et al., 1979; Mancino et al., 1988; Van Cleemput et al., 1994; Schwarz et al., 1994; Tenuta and Beauchamp, 1995; Velthof et al., 1996; Chen et al., 1999). Methods based on the C2H2 inhibition technique (Yoshinari et al., 1977) have been widely used in field studies of denitrification. Using C2H2 inhibition, Mancino et al. (1988) studied the effects of soil moisture content, soil texture, and soil temperature on denitrification from a Kentucky bluegrass sod. For silt and silt loam soil types, only 0.1 and 0.4% of N applied as KNO3 was recovered as N2O when the soil moisture content was 80% of saturation at 22°C for 10 d. Above 80% saturation, N losses accounted for 5.4 and 2.2% of the N applied to a silt and silt loam soil, respectively. When the soils were at 100% saturation and the temperature was 30°C, maximal losses from the silt and silt loam soil were 94 and 46% of applied N, respectively. Thus, during periods of soil saturation, substantial loss of N could occur by denitrification.
The form of N fertilizer can also affect denitrification losses. Mulvaney et al. (1997) found that NH4 and NH4-producing fertilizers promote denitrification in waterlogged soils. Emissions of N2 and N2O were greater with an alkaline-producing fertilizer, like urea, than with acidic fertilizers. Alkaline-producing fertilizers may promote denitrification under waterlogged conditions, either because of an increase in the supply of oxidizable C (Norman et al., 1987, Sen and Chalk, 1994) or because of a direct effect on microbial activity (Bollag et al., 1970). Maggiotto et al. (2000) found that sulfur coated urea, when compared to urea, suppressed N2O emissions; however, with the slow-release fertilizer, the suppression of N2O emission was short-lived.
Plant-based systems are more biologically active as compared with a bare soil. This is because roots are constantly aerating the soil surface, plant senescence supplies microorganisms with readily available organic C as an energy source, evapotranspiration is occurring, nutrients are removed from the soil via plant uptake, and, especially for high maintenance turfgrass, irrigation is typically applied daily. The primary objective of this research was to test the hypothesis that significant gaseous N loss can occur from turfgrass, and to determine this loss by directly measuring fluxes of N2 and N2O. A secondary objective was to evaluate the effects of fertilizer source on the rate of denitrification.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Field Experiments
Two separate experiments were initiated in 1999 by inserting eight polyvinyl chloride (PVC) cylinders for each experiment into Kentucky bluegrass turf to a depth of 
25 cm using a tractor-mounted hydraulic press. Sampling cylinders were constructed of 20-cm diam. polyvinyl chloride pipe cut to 30-cm lengths and equipped with a plastic flange to permit atmospheric sampling. A full description of the materials and methods used for constructing and inserting the modified PVC cylinders is provided in Horgan et al. (2001). Six cylinders were selected for each experiment after verifying that infiltration rates inside and outside the cylinder did not differ. On 5 May (Exp. 1) and 9 August (Exp. 2) 1999 at 0600 h, KNO3 containing 98.5 atom % 15N (obtained from Isotec, Miamisburg, OH; enrichment was determined experimentally) was applied in solution to each PVC cylinder at a rate of 4.88 g N m-2 (equivalent to 49 kg N ha-1) using a polyethylene wash bottle. To ensure a complete transfer of the fertilizer solution, the wash bottle was rinsed three times with a total of 165 mL of water.
Plots were irrigated twice a week to replace 80% of the potential evapotranspiration (PET) when rainfall totals did not exceed the PET value (obtained from the Illinois State Water Survey). The turfgrass was maintained at 5 cm using a pair of manual hand clippers to cut the grass while holding a hand-held vacuum against the clippers. Clippings were collected biweekly. The experimental design involved atmospheric sampling three times a day (0800 to 1100, 1100 to 1400, and 1400 to 1700 h) with two replications for the duration of each experiment.
An experiment was conducted in the field from 18 July to 21 August 2000 to compare the effects of different N fertilizers on emission of N2 and N2O from creeping bentgrass turf. Six cylinders were inserted as previously described, from which four were selected after verifying that infiltration rates inside and outside the cylinders did not differ. On 18 July 2000 at 0800 h, KNO3 containing 49.47 atom % 15N was applied to two cylinders at a rate of 976 mg N m-2 (equivalent to 9.8 kg N ha-1). Two other cylinders were treated with the same amount of N as urea containing 46.8 atom % 15N. Weekly fertilizer applications were made throughout the experiment as specified previously and atmospheric sampling occurred daily from 1100 to 1400 h. Plots were irrigated as needed to maintain a healthy turf. At biweekly intervals, the turfgrass was clipped to a height of 1.3 cm using manual hand clippers, and clippings were removed.
Greenhouse Experiment
Six PVC cylinders were inserted into the soil in an area adjacent to the location of the 1999 experiments, of which three were inserted into bare soil and three into a soil under Kentucky bluegrass turf. Four of these cylinders were selected (two bare soil and two turfgrass) after verifying that infiltration rates inside and outside the cylinders did not differ. The intact cylinders were removed from the soil, and the bottoms were sealed by inserting modified PVC end caps equipped with a stainless steel male-hose connector (no. 6-HC-1-4, Swagelok Co., Solon, OH) to permit leachate collection. The sealed cylinders were transported to the greenhouse, and the plants and soil inside the cylinders were treated at 0800 h on 24 May 2000 with 4.88 g N m-2 (equivalent to 49 kg N ha-1) as KNO3 enriched with 98.5 atom % 15N, which was applied as previously described for the field studies.
Atmospheric sampling commenced following fertilization and occurred daily from 1100 to 1400 h until 13 June 2000. Irrigation was applied with a polyethylene wash bottle at least once a week to maintain adequate turfgrass health. Plants were maintained under 14-h days (185 mmol sec-1 m-2 plus ambient sunlight) at 22 ± 2°C and 10-h nights at 18 ± 2°C for 4 wk. Turfgrass was maintained biweekly at 5 cm using manual hand clippers and clippings were removed.
Atmospheric Sampling and Gas Analysis
The technique employed for atmospheric sampling is described in detail by Horgan et al. (2001). Briefly, a brass lid, equipped with two shut-off valves, was secured to the plastic flange on the PVC cylinder, thus creating a gas tight seal to collect the gases evolved from the soil and plants. After 3 h, a closed-loop circulating system was created by attaching the valves on the lid to a circulating pump and a 60-mL gas sampling tube equipped with two high vacuum stopcocks, which contained a known amount of Ne. Both valves and both stopcocks were then opened, and the atmosphere inside the circulation system was thoroughly mixed by pumping for 20 min. Following pumping, the stopcocks on the sampling tube were closed, the tube and the pump were disconnected from the brass lid, and the lid was removed from the PVC cylinder.
Samples were analyzed for 15N-labeled N2 and N2O as described by Mulvaney and Kurtz (1982) and for Ne as described by Horgan et al. (2001) using a dual-inlet ratio mass spectrometer (Nuclide Model 3-60-RMS; Spectromedix Corp., State College, PA). Ratio data were processed using equations derived by Mulvaney and Boast (1986) to obtain values for the mole fraction of 15N in the N pool from which the N2 or N2O was derived (15XN) and the micrograms of N as labeled N2 or N2O. A value was also obtained for the percentage of N2 or N2O-N derived from LFN, using the isotope dilution expression, 100 x (15XN - 0.003663)/(F - 0.003663), where F is the experimentally obtained 15N fertilizer enrichment. The total emission of labeled N2 or N2O was estimated on the assumption that the NO3 undergoing denitrification existed in a single pool that is isotopically uniform. Emission rates were calculated from the micrograms of N2 or N2O-N determined by taking into account the atmospheric volume, temperature, and barometric pressure at the time each plot was sampled, and are expressed as µg of N2 or N2O evolved m-2 surface soil s-1. For the spring and summer Kentucky bluegrass experiments, triplicate 3-h emission measurements from within each replication were summed and means and standard errors were calculated. For all other experiments, emission measurements were based on a single 3-h enclosure period per replication, from which means and standard errors were calculated.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Another concern was how to measure the atmospheric volume confined within the closed chamber, because plants preclude the use of a ruler to determine the headspace volume above the soil surface, whereby volume is used to calculate N2 flux based on the ideal gas law (PV = nRT, with P = pressure, V = volume, n = number of moles, R = gas constant, and T = absolute temperature). The technique described by Horgan et al. (2001) was developed to measure atmospheric volume, including the soil-air volume within a complex plant and soil matrix. This technique involves a standard addition of an inert gas (Ne) into the closed chamber prior to circulating the air, so that the atmospheric volume confined within the chamber can be estimated by measuring the decrease in Ne concentration. The concentration of Ne in the atmospheric sample collected is proportional to the atmospheric volume confined within the closed chamber. This technique allows determination of atmospheric volume in conjunction with mass spectrometric analyses for 15N-labeled N2 and N2O, and in effect provides a capability for real-time volume determinations.
The Ne technique requires circulation of the air inside the closed cylinder in order to facilitate diffusion of Ne. There are reports that slight pressures or pressure deficits generated when air is circulated through chambers placed over the soil surface can have marked effects on gaseous emission (Denmead, 1979; Hutchinson and Mosier, 1981). If desired, the chamber lid can be equipped with a low-conductance vent (e.g., 1.4-mm i.d. tubing) to avoid pressure fluctuations. This was not done in the present project, as previous work to evaluate a similar sampling system showed that venting did not reduce short-term variability in emission of N2 or N2O (Mulvaney and Kurtz, 1984).
Field Experiments
Figures 1 and 2
show the results of daily measurements of LFN and total N evolved as N2 and N2O from 15N-fertilized Kentucky bluegrass cores during a 6-wk experiment starting in May and a 4-wk experiment starting in August. In addition, Fig. 1 and 2 show the amounts of water supplied through irrigation or rainfall and atmospheric volume data collected by the Ne technique.
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Nitrogen losses through denitrification vary greatly and are highly variable across relatively small areas depending on NO3 levels, moisture status of the soil, available organic matter, microbial distribution, and temperature (Engler et al., 1976; Robertson and Tiedje, 1987; Saad and Conrad, 1993). Spatial and temporal variability of N2O and N2 emission from field soils and grasslands has been well documented by several investigators (Rolston et al., 1978; Ryden et al., 1978; Robbins et al., 1979; Bremner et al., 1980, 1981; Mosier et al., 1981; Blackmer et al., 1982; Parkin, 1993; Velthof et al., 1996) and greatly complicates quantification of N2O and N2 emissions in the field.
Large differences in the emission rate of N2 and N2O between Fig. 1 and 2 were observed and can be attributed, at least in part, to two factors: higher soil temperatures in the summer months and an 8.9-cm rainfall event 4 DAF in the summer experiment. Approximately 8.5% of LFN was lost as N2 or N2O during and 3 d following this rainfall event, while emission was increased by 70% when soil-derived N was included. By comparison, only 2.7% of LFN was lost as N2 or N2O for the entire 6-wk spring experiment. Average daily soil temperatures (Fig. 3) can also help explain the large differences in emission of N2 and N2O when comparing the two experiments, in that soil temperatures increased throughout the month of July presumably leading to more active microbial populations. Therefore, with anaerobic conditions from the heavy rainfall event, higher soil temperatures, and a readily available supply of NO3 from the applied fertilizer, conditions were ideal for denitrification.
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There is evidence that plants affect the flux of N2 and N2O (Reddy and Patrick, 1986; Haider et al., 1990; Mosier et al., 1990; Chang et al., 1998; Chen et al., 1999). In a field study, Mosier et al. (1990) found greater recovery of N2 and N2O from 15N-labeled urea when atmospheric samples were collected by placing chambers over, rather than between, rice plants in flooded soil. This suggests that the plants acted as a conduit for gas exchange or that air channels along leaf blades provided an avenue for gas transport. In our work, a lag in gas flux was observed (Day 5 in Fig. 2), but emission of N2 and N2O was detected while standing water was visible within the chambers, suggesting that N2 and N2O formed in flooded soil by denitrification may have been transported from the soil to the atmosphere through Kentucky bluegrass plants. Kentucky bluegrass may contain aerenchyma that are generally found in root systems of wetland (Waisel and Eshel, 1991) or flood-tolerant (Drew and Stolzy, 1991) plants to conduct gases between the atmosphere and soil root zone. Moreover, Chen et al. (1999) found that perennial ryegrass (Lolium perenne L.) significantly increased N2O emission rates from a saturated soil and concluded that perennial ryegrass can serve as a conduit for N2O release from saturated soil through the transpiration stream of the plants. The same process may account for emission of N2 and N2O observed in our work during periods of standing water.
Greenhouse Experiment
To determine if the presence of plants promote gas exchange from soil, emission rates of N2 and N2O were compared for soils with and without Kentucky bluegrass. Figure 4
shows the results of daily measurements of LFN and total N evolved as N2 or N2O from 15N- fertilized Kentucky bluegrass cores during a 3-wk experiment in the greenhouse. Turfgrass consistently led to larger fluxes of N2 and N2O with LFN emission totaling 2.37% from turfgrass and 0.91% from bare soil (Fig. 4). This may be due to plant senescence and the thatch layer which supplies microorganisms with readily available supply of organic C that is used as an energy source. These results are in accordance with Larsson et al. (1998), where emission of N2O from a grass sward (6 kg N2O-N ha-1) greatly exceeded the emission from bare soil (0.2 kg N2O-N ha-1). As with the field studies, emission of N2 and N2O were most extensive 1 or 2 d following fertilization and decreased as the soil drained. Aerobic and anaerobic microsites can develop within the same soil aggregate (Højberg et al., 1994) and NO3 reduction can occur as soils drain (Smith, 1980; Renault and Stengel, 1994), which may account for the fact that in our work, emission of N2 and N2O slowed but did not diminish immediately after irrigation.
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0.5 cm of water, and since irrigation of highly maintained turfgrass keeps the soil profile near field capacity, anaerobic microsites may have been formed leading to short-term anoxia (Sexstone et al., 1985; Højberg et al., 1994). Marked decreases in atmospheric volume coincided with emission of N2 and N2O (Fig. 1, 2, and 4). For example, three rainfall events during the spring experiment occurred on Days 23 through 25, which created conditions conducive for denitrification of soil NO3 (detectable because unlabeled soil N pooled with LFN) and the atmospheric volume during this time decreased from 2.4 to 1.6 L (Fig. 1). Similar events took place during the summer experiment (Fig. 2) when 3 to 5 DAF, atmospheric volume decreased from 2.4 to 2.0 L.
Fertilizer Effects on Denitrification
Creeping bentgrass often receives weekly foliar applications of soluble fertilizer at low rates to control the amount of N available for plant uptake. A field study was initiated to study emission rates of N2 and N2O and the effect of weekly fertilization with two soluble sources of fertilizer on creeping bentgrass turf. Figure 5
shows the rates of LFN and total N2 and N2O emission from plots fertilized weekly with 9.8 kg N ha-1 as KNO3 or urea.
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The rise in soil pH that accompanies hydrolysis of urea is only temporary due to the acidity generated from nitrification of NH4 and because of the buffering capacity of soil. It is generally accepted that the increased concentration of NO3 from nitrification of NH4, and the short-term acidity produced by nitrification should favor production of N2O relative to N2 (Blackmer and Bremner, 1978; Koskinen and Keeney, 1982; Ottow et al., 1985; Breitenbeck and Bremner, 1986; Weier et al., 1993). However, in our work, no N2O emissions were detected from soil under turfgrass treated with urea. Similar results were reported by Maggiotto et al. (2000) for a turfgrass system where urea fertilization resulted in very low emission rates of N2O (0.05 to 0.33% of applied N) determined by a micrometeorological technique. The results reported in our work contrasts those reported by Breitenbeck et al. (1980) and Breitenbeck and Bremner (1986), who report that N2O emissions occur from urea fertilized soils. No clear explanation can be offered to account for this difference, but further work is clearly warranted before definite conclusions can be reached regarding the impact of turfgrass N fertilization on N2O emission.
In contrast to urea, N2O evolution did occur on turf receiving applications of KNO3, but was only detected the day of fertilization (Fig. 5). The increased concentration of NO3 immediately following fertilization would have promoted production of N2O relative to N2 during denitrification (Firestone et al., 1979), as a high NO3 concentration inhibits the conversion of N2O to N2 (Weier et al., 1993). The mole fraction of N2O [calculated as N2O - N/(N2 + N2O) - N] decreased with each successive application of KNO3 fertilizer from 0.41 to 0.11. This latter finding can be attributed to the increase in irrigation frequency the last 2 wk of the experiment, since N2 emission during denitrification is favored by an increase in the degree of anaerobicity (Weier et al., 1993).
The results presented in this study represent the first attempt to measure denitrification from turfgrass using 15N-labeled fertilizer. Field measurements suggest that N2 losses can affect N-fertilization practices, in that gaseous N loss occurs intermittently throughout the summer with large fluxes of N2 and N2O after heavy rainfall or irrigation events. Nitrate-based fertilizers are more susceptible to denitrification than ammonium-based fertilizers if irrigation is overapplied or if a large rainfall event occurs soon after application. In addition, we demonstrated that even with standing water, N2 and N2O losses occur, suggesting that plants act as a conduit for gas exchange between the soil and the atmosphere. Nitrous oxide emission rates from N fertilizer applied to turfgrass will depend largely on the source of fertilizer, and additional studies of fertilizer-induced N2O and N2 emissions from turf over a wide range of conditions are necessary to understand the dynamics of the turfgrass N cycle.
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ACKNOWLEDGMENTS |
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Received for publication April 25, 2001.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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This article has been cited by other articles:
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  M. E. Engelsjord, B. E. Branham, and B. P. Horgan The Fate of Nitrogen-15 Ammonium Sulfate Applied to Kentucky Bluegrass and Perennial Ryegrass Turfs Crop Sci., July 1, 2004; 44(4): 1341 - 1347. [Abstract] [Full Text] [PDF]
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B. P. Horgan, B. E. Branham, and R. L. Mulvaney Mass Balance of 15N Applied to Kentucky Bluegrass Including Direct Measurement of Denitrification Crop Sci., September 1, 2002; 42(5): 1595 - 1601. [Abstract] [Full Text] [PDF]
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