38

CHAPTER 3

DIRECT MEASUREMENT OF DENITRIFICTION USING 15N-LABELED

FERTILIZER APPLIED TO TURFGRASS

ABSTRACT

Denitrification losses are a possibility from turfgrass because of frequent

irrigation, multiple applications ofN fertilizers, and an abundance of readily

decomposable organic C in thatch and verdure. Field experiments were conducted to

directly measure N2 and N20 evolved from a Flanagan silt loam soil under Kentucky

bluegrass (Poa pratensis L.) or creeping bentgrass (Agrostis palustris Huds.). Mass

spectrometric procedures were used to analyze atmospheric samples collected from

replicated 15N fertilized turf (49 kg ha-1). Data showed that labeled fertilizer N (LFN)

losses ranged from 2.1 to 7.3% for N2 and from 0.4 to 3.9% for N20; that large N2 and

N20 fluxes occurred after heavy rainfall events; and that more N2 was evolved than N20.

Emission of gas was detected while standing water was visible within cylinders,

suggesting the transfer of gases from the flooded soil to the atmosphere through the

turfgrass plants. Evolution ofN2 and N20 was greater from creeping bentgrass treated

with KN03 than urea through the fIrst 3 wk of the experiment, whereas N2 emission was

greater for urea during the last 2 wk of the experiment, presumably because ofN03

production through nitrification. Nitrous oxide was detected on the day of fertilization

with the KN03 treatment, and the mole fraction ofN20 decreased with each weekly

application ofN from 0.44 to 0.11.

39

INTRODUCTION

Denitrification is an important process in the soil, plant, and atmosphere

continuum (SPAC) because it is the primary mechanism for return ofN2 to the

atmosphere (Stevenson and Cole, 1999). With plant productivity frequently limited by N

supply, removal of inorganic N by denitrifying microorganisms can adversely affect plant

growth and development. Moreover, one of the gaseous products of denitrification is

N20, which contributes to the destruction of stratospheric 03 (Prather et aI., 1995).

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 N03 is present due to recent fertilization and/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 aI., 1982;

Mancino et aI., 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

(Sextone et aI., 1985), while multiple applications ofN 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

40

the emission ofN2 into ambient air, gaseous N loss by denitrification and/or volatilization

have usually been estimated from the deficits in 15Nbalance studies. Using 15N-Iabeled

41

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 aI., 1987, Sen and Chalk, 1994) or because of a direct

effect on microbial activity (Bollag et aI., 1970). Maggiotto et al. (2000) found that

sulfur coated urea, when compared to urea, suppressed N20 emissions; however, with the

slow-release fertilizer, the suppression ofN20 emission was short-lived.

Plant-based systems are more biologically active as compared to a bare soil, in

that 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 test the hypothesis that significant gaseous N loss can occur from turfgrass,

by directly measuring fluxes ofN2 and N20. A secondary objective was to evaluate the

effects of fertilizer source on the rate of denitrification.

MATERIALS AND METHODS

Soil

Field studies were conducted in 1999 and 2000 at the University of Illinois

Landscape and Horticulture Research Farm in Urbana, IL. The study site was maintained

under Kentucky bluegrass or creeping bentgrass on a Flanagan soil (fine, smectitic,

mesic, Aquertic, Argiudoll). Analyses of the soil as described by Mulvaney and Kurtz

(1982) gave the following results: pH, 6.8; total N 2.55 g kg-I; organic C, 30.3 g kg-I; a

42

sand content of 125 g kg-I, a silt content of 588 g kg-I, and a clay content of287 g kg-I.

All analyses reported were performed in triplicate.

Field Experiments

Two separate experiments were initiated in 1999 by inserting eight PVC cylinders

into Kentucky bluegrass turf to a depth of approximately 25 cm using a tractor-mounted

hydraulic press. A full description of the materials and methods used for constructing

and inserting the modified PVC cylinders is provided in chapter 2. Six PVC cylinders

were selected after verifying that infiltration rates inside and outside the cylinder did not

differ. On 5 May and 9 August 1999 at 0600, KN03 containing 98.5 atom % 15N

(obtained from Isotec, Miamisburg, OR and the enrichment was determined

experimentally) was applied in solution to each plot at a rate of 4.88 g N m-2 (equivalent

to 49 kg N ha-I) 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 approximately 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,

1400 to 1700) with two replications.

An experiment was conducted in the field from 18 July to 21 August 2000 to

compare the effects of different N fertilizers on emission ofN2 and N20 during

denitrification ofN03 from creeping bentgrass turf. Six cylinders were inserted as

43

previously described, from which four were selected after verifying that infiltration rates

inside and outside the cylinders id not differ. On 18 July 2000 at 0800, KN03 containing

49.47 atom % 15Nwas applied to two cylinders at a rate of 976.44 mg N m-2 (equivalent

to 9.8 kg N ha-1). Two other cylinders were treated with the same amount ofN 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. Plots were irrigated as needed so as to maintain a healthy turf sward. At

biweekly intervals, the turfgrass was clipped to a height of approximately 1.3 em 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

(cat. 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 on 24 May 2000 with 4.88 g N m-2 (equivalent to 49 kg N ha-1) as

KN03 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 until 13 June 2000. Irrigation was applied with a polyethylene wash

44

bottle at least once a week to maintain adequate turfgrass health. Plants were maintained

under 14-h days (185 mmol sec-1m-2plus ambient sunlight) at 22:i: 2°C and 10-h nights

at 18 :i:2°C for 4 wk. Turfgrass was maintained biweekly at approximately 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 in

chapter 2. 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 ofNe.

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-IabeledN2 and N20 as described by Mulvaney

and Kurtz (1982) and for Ne as described in chapter 2 using a dual-inlet ratio mass

spectrometer (Nuclide ModeI3-60-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 15Nin the N pool from which the N2 or N20 was derived

(lSXN) and the micrograms ofN as labeled N2 or N20. A value was also obtained for the

percentage ofN2 or N20-N derived from LFN, using the isotope dilution expression, 100

x eSXN

- 0.003663)/(F - 0.003663), where F is the experimentally obtained 15Nfertilizer

45

enrichment. The total emission of labeled N2 or N20 was estimated on the assumption

that the N03 undergoing denitrification existed in a single pool that is isotopically

uniform. Emission rates were calculated from the micrograms ofN2 or N20-N

determined by taking into account the atmospheric volume, temperature, and barometric

pressure at the time each plot was sampled, and are expressed as Ilg ofN2 or N20

evolved per m2 of surface soil S.I. 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.

RESULTS AND DISCUSSION

Sampling Strategy

If, by inserting the PVC cylinders into the soil, infiltration rates inside the

cylinders differ from the surrounding area, then hydraulic conductivity may have been

reduced by compaction, potentially prolonging anaerobicity and promoting

denitrification. To minimize this potential problem, additional cylinders were inserted

into the soil beyond the number needed for each experiment, so that if infiltration rates

inside the cylinders differed from the surrounding area, these cylinders would not be used

for experimental purposes.

A turfgrass system is inherently complex compared to soil, in that roots are

constantly aerating the soil surface; organic C is readily available as a microbial energy

source due to plant senescence; evapotranspiration dries the soil; nutrients are removed

46

from the soil via plant uptake or immobilization and replenished by mineralization; and,

especially for high maintenance turfgrass, irrigation is typically applied daily. Of

concern in our work was internal heating inside the closed cylinders during a 3-h period

of enclosure, as denitrification is a temperature dependant process and elevated

temperatures inside the closed cylinder may lead to larger atmospheric emission rates of

N2 and/or N20. Preliminary work showed no difference during a 3-h period of enclosure

between the air temperature inside and outside the closed cylinder when the brass lids

were painted white to reflect sunlight and shade cloth was tented 0.6 m above the plant

surface. Moreover, no evidence of plant stress was observed following a 3-h enclosure

in any of the experiments conducted.

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). The technique described in chapter 2 was developed to

measure atmospheric volume, including the soil-air volume within a complex plant/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 ofNe in the atmospheric sample collected is proportional to the

atmospheric volume confined with in the closed chamber. This technique allows

determination of atmospheric volume in conjunction with mass spectrometric analyses

for lsN-labeled N2 and N20, and is in effect providing a capability for real-time volume

determinations.

47

The Ne technique requires circulation of the air inside the closed cylinder in order

to facilitate diffusion ofNe. 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 ofN2 or N20 (Mulvaney and Kurtz, 1984).

Field Experiments

Figures 3 and 4 show the results of daily measurements of LFN and total N

evolved as N2 and N20 from 15N-fertilized Kentucky bluegrass cores during a six-wk

experiment in the spring and a four-wk experiment in the summer. In addition, Fig. 3 and

4 show the amounts of water supplied through irrigation or rainfall and the atmospheric

volume data collected by the Ne technique.

Water inputs and soil texture influence infiltration rates and the soils ability to

drain soil water, thus directly affecting the length of time a soil remains anaerobic. Smith

and Tiedje (1979) found that a major part of gaseous N loss from soils occurs within a

few hours after wetting. In our work, we observed an initial flux ofN2 and N20 two h

following fertilization (Fig. 3 and 4), which is consistent with the finding that microbial

production ofN20 has been detected within 30 min following wetting of a dry soil

(Rudaz et aI., 1991). A large flux ofN2 and N20 occurred three d after fertilization

(DAF) in the spring experiment (Fig. 3) following a major rainfall event, although Freney

et aI. (1979), Rice and Smith (1982) and J0rgensen et aI. (1998) have observed that N20

48

fluxes following rainfall could be caused by the release of soil-adsorbed N20 due to

water penetration. Nitrous oxide is fairly soluble in water (1.0 L L-1 water at 5°C), and

drying of the soil surface may release previously dissolved N20 from soil water (Dowdell

et aI., 1979; Minami and Ohsawa, 1990); however, this would not be the case with N2 as

this gas is not as water soluble (0.015 L L-1 water at 25°C). As the measured atmospheric

volume increased beginning four DAF in the spring experiment (Fig. 3), we observed a

rapid decrease in denitrification that was likely due to soil drainage, permitting the

diffusion of O2 into soil pores.

Nitrogen losses through denitrification vary greatly and are highly variable over

relatively small areas, depending on N03 levels, moisture status of the soil, available

organic matter, microbial distribution, and temperature (Engler et aI., 1976; Robertson

and Tiedje, 1987; Saad and Conrad, 1993). Spatial and temporal variability ofN20 and

N2 emission from field soils and grasslands has been well documented by several

investigators (e.g., Rolston et at, 1978; Ryden et at, 1978; Robbins et aI., 1979; Bremner

et aI., 1980; Bremner et aI., 1981; Mosier et aI., 1981; Blackmer et aI., 1982; Parkin,

1993; Velthof et aI., 1996) and greatly complicates quantification ofN20 and N2

emissions in the field. Large differences in the emission rate ofN2 and N20 between Fig.

3 and 4 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 four DAF in the summer

experiment. Approximately 8.5% ofLFN was lost as N2 or N20 during and three d

following this rainfall event while emission was increased by 70% when soil-derived N

was included. By comparison, only 2.7% ofLFN was lost as N2 or N20 for the entire

six-wk spring experiment. Average daily soil temperatures (Fig. 5) can also help explain

49

the large differences in emission ofN2 and N20 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 ofN03 from the

applied fertilizer, conditions were ideal for denitrification.

During both the spring and summer experiments, plots were irrigated to replace

80% of the PET; moreover, the intensity of the rainfall event four DAF in the summer

effectively sealed the soil surface (standing water present) causing a lag in N2 and N20

emission with the largest emission rate occurring one d following the rainfall event (Fig.

4). Letey et al. (1980) cautioned that very slow diffusion ofN2 and N20 in flooded soil

might restrict the evolution of 15N-Iabeled gases formed in the soil by denitrification.

Similarly, Mulvaney and Kurtz (1984) reported a lag period between application of water

and evolution ofN2 and N20 with maximal evolution occurring 2 to 9 days after water

was applied. This long lag period observed by Mulvaney and Kurtz (1984) can be

attributed to their experimental design, in that the soil cores from which N2 and N20

fluxes were measured were sealed at the bottom, preventing drainage and prolonging

saturation.

There is evidence that plants affect the flux ofN2 and N20 (e.g., Reddy and

Patrick, 1986; Haider et aI., 1990; Mosier et aI., 1990; Chang, et aI., 1998; Chen et aI.,

1999). In a field study, Mosier et aI. (1990) found greater recovery ofN2 and N20 from

15N-Iabeled 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. In our work, a lag in gas flux was observed (day 5 in Fig. 4),

50

but emission ofN2 and N20 was detected while standing water was visible within the

chambers, suggesting that N2 and N20 formed in flooded soil by denitrification may have

been transported from the soil to the atmosphere through Kentucky bluegrass plants.

Kentucky bluegrass, apparently, does not contain aerenchyma that are generally found in

root systems of wetland (y.Iaisel and Eshel, 1991) or flood-tolerant (Drew and Stolzy,

1991) plants to conduct gases between the atmosphere and soil root zone. However,

Chen et al. (1999) found that perennial ryegrass (Lolium perenne L.) significantly

increased N20 emission rates from a saturated soil and concluded that perennial ryegrass

can serv as a conduit for N20 release from saturated soil through the transpiration stream

of the plants. The same process may account for emission ofN2 and N20 observed in out

work during periods of standing water.

Greenhouse Experiment

As previously described, a turfgrass system is much more complex than a bare

soil system, owing to the presence of roots, thatch, and aboveground biomass. To

determine if the presence of plants promote gas exchange from soil, emission rates ofN2

and N20 were compared for soils with and without Kentucky bluegrass. Figure 6 shows

the results of daily measurements ofLFN and total N evolved as N2 or N20 from 15N_

fertilized Kentucky bluegrass cores during a 3-wk experiment in the greenhouse.

Turfgrass consistently led to larger fluxes ofN2 and N20 with LFN emission totaling

2.37% from turfgrass and 0.91% from bare soil (Fig. 6). These results are in accordance

with Larsson et al. (1998) where emission ofN20 from a grass sward (6 kg N20-N ha-1)

greatly exceeded the emission from bare soil (0.2 kg N20-N ha-1). As with the field

studies, emission ofN2 and N20 were most extensive one or two d following fertilization

51

and decreased as the soil drained. Aerobic and anaerobic microsites can develop within

the same soil aggregate (Hejberg et aI., 1994) and N03 reduction can occur as soils drain

(Smith, 1980; Renault and Stengel, 1994), which may account for the fact that in our

work, emission ofN2 and N20 slowed but did not diminish immediately after irrigation.

With a turfgrass system, roots are able to extract water from deeper in the soil profile and

may lead to less rapid drying of the soil surface. In contrast to turf, bare soil dries faster

from the soil surface downward and with less frequent irrigation in the present study, the

soil surface was visibly drier therefore more aerobic which resulted in lower rates of

denitrification.

The Ne technique employed to measure the atmospheric volume confined within

a closed cylinder (see chapter 2) was developed to improve the accuracy achieved in

direct measurements of denitrification by not only measuring the volume of air above the

soil surface, but also measuring the soil-air volume. Therefore, soil moisture content can

be monitored by measuring the atmospheric volume confined within the closed chamber,

whereby, as the soil water content increases, the soil-air will be displaced and

correspondingly, the atmospheric volume will decrease. During the time immediately

following fertilization with KN03, N03 is readily available for loss if anaerobic

conditions exist. Moreover, fertilizers were applied in solution with approximately 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 aI., 1985; H0jberg et aI., 1994). Marked decreases in atmospheric volume

coincided with emission ofN2 and N20 (Fig. 3 to 5). For example, three rainfall events

during the spring experiment occurred on days 23 through 25, that created conditions

52

conducive for denitrification of soil N03 (detectable because unlabeled soil N pooled

with LFN) and the atmospheric volume during this time decreased from approximately

2.4 to 1.6 L (Fig. 3). Similar events took place during the summer experiment (Fig. 4)

when three to five DAF, atmospheric volume decreased from approximately 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 ofN available for plant uptake. A field study was

initiated to study emission rates ofN2 and N20 and the effect of weekly fertilization with

two soluble sources of fertilizer on creeping bentgrass turf Figure 7 shows the rates of

LFN and total N2 and N20 emission from plots fertilized weekly with 9.8 kg N ha-1 as

KN03 or urea.

For the first three applications of fertilizer, emission ofLFN and total N as N2 was

consistently greater for plots treated with KN03 than with urea (Fig. 7), because in order

for an ~-based fertilizer to be denitrified, nitrification must occur to convert Nl4 to

N03 or N02. With the large readily available supply of organic C in the thatch layer, it is

likely that following hydrolysis of the urea, considerable immobilization ofNHt occurred

leading to less substrate available for nitrification. Bowman et al. (1989) reported that

turfgrass fertilized at 50 kg N ha-1, supplied as N03 or NH4, can deplete the applied N

within 48 h after application. In the present study, the creeping bentgrass turf had not

been fertilized for over 4 wk, so in addition to immobilization and nitrification ofN14,

plant uptake ofNlI4 and N03 could account for low emission rates ofN2 during

denitrification. During the latter 2 wk of the experiment, the urea-treatment evolved

more LFN and total N as N2 which can be attributed to the accumulation ofN03 through

53

nitrification of~, which provided substrate for denitrification. With three weekly

applications of fertilizer, the N deficiency observed prior to the experiment was probably

corrected, which would result in lower rates of plant uptake a larger quantity ofN03 that

could have denitrified.

The rise in soil pH that accompanies hydrolysis of urea is only temporary due to

the acidity generated from nitrification of~ and because of the buffering capacity of

soil. It is generally accepted that the increased concentration ofN03 from nitrification of

~, and the short-term acidity produced by nitrification should favor production ofN20

relative to N2 (Blackmer and Bremner, 1978; Koskinen and Keeney, 1982; Ottow et aI.,

1985; Breitenbeck and Bremner, 1986; Weier et aI., 1993). However, in our work, no

N20 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 ofN20 (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 aI. (1980) and Breitenbeck and Bremner (1986), who report that N20

emissions occur from urea fertilized soils. No clear explanation can be offered to account

for this difference, but further work is clearly warranted before defmite conclusions can

be reached regarding the impact of turf grass N fertilization on N20 emission.

In contrast to urea, N20 evolution did occur on turf receiving applications of

KN03, but was only detected the day of fertilization (Fig. 7). The increased

concentration ofN03 immediately following fertilization would have promoted

production ofN20 relative to N2 during denitrification (Firestone et aI., 1979), as a high

N03

concentration inhibits the conversion ofN20 to N2 (Weier et aI., 1993). The mole

54

fraction ofN20 [calculated as N20-N/(N2+N20)-N] decreased with each successive

application ofKN03 fertilizer from 0.41 to 0.11. This latter fmding can be attributed to

the increase in irrigation frequency the last two wk of the experiment, since N2 emission

during denitrification is favored by an increase in the degree of anaerobicity (Weier et aI.,

1993).

The results presented in this study represent the fIrst attempt to measure

denitrification from turfgrass using 15N-Iabeledfertilizer. Field measurements suggest

that N2 losses can affect N-fertilization practices, in that gaseous N loss occurs regularly

throughout the summer with large fluxes ofN2 and N20 after heavy rainfalVirrigation

events. Nitrate-based fertilizers are more susceptible to denitrification than ammonium-

based fertilizers if irrigation is over-applied or if a large rainfall event occurs soon after

application. Inaddition, we demonstrated that even with standing water, N2 and N20

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 N20

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

We thank the United States Golf Association for partial support for this project.

We thank Drs. Khan and Gardner for their laboratory and statistical guidance and Joe

Meyer, James Abel, Cindy Dembs, and Yoko Haneda deCaussin for their technical

support. In addition, we thank the Illinois State Water Survey for providing potential

evapotranspiration data.

FIGURE CAPTION

Fig 3. Daily measurements of LFN and total N evolved as N2 and N20 from Kentucky

bluegrass cores fertilized with 15N-labeled KN03 during the spring experiment in

the field. Mass spectrometric results from the three, 3-h flux measurements from

62

within each replication were summed, and values are reported as a mean of two

replications. Daily atmospheric volume measurements were performed using the

Ne technique during collection ofN2 and N20. Volume measurements are

reported in mL as means of the two replications. Standard errors are reported for

each calculated mean.

Fig 4. Daily measurements ofLFN and total N evolved as N2 and N20 from Kentucky

bluegrass cores fertilized with 15N-labeledKN03 during the summer experiment

in the field. Mass spectrometric results from the three, 3-h flux measurements

from within each replication were summed, and values are reported as a mean of

two replications. Daily atmospheric volume measurements were performed using

the Ne technique during collection ofN2 and N20. Volume measurements are

reported in mL as means of the two replications. Standard errors are reported for

each calculated mean.

Fig. 5. Average daily soil temperature readings reported from May through September

1999.

Fig. 6. Daily measurements ofLFN and total N evolved as N2 and N20 from Kentucky

bluegrass cores and bare soil cores fertilized with IsN-labeled KN03 in the

greenhouse. Values are reported as a mean of two replications. Daily

atmospheric volume measurements were performed using the Ne technique

during collection ofN2 and N20. Volume measurements are reported in mL as

means of the two replications. Standard errors are reported for each calculated

mean.

63

Fig. 7. Daily measurements ofLFN and total N evolved as N2 and N20 from creeping

bentgrass cores fertilized with 15N-IabeledKN03 or urea in the field. Weekly

fertilizer applications were made from 18 July through 21 August 2000. Values

reported are means of the two replications. Standard errors are reported for each

calculated mean.

64

y 3h Z g <

< L L

I l l H

CO CO

LU

1 _E3_ JL 1.

N20

LFN Loss Total Loss

14 21 28

DAYS AFTER FERTILIZATION

Fig. 3

65

~ LFNLoss~ Total Loss

7 14

N2~ LFNLoss-0- Total Loss

21 28

DAYS AFTER FERTiliZATION

Fig. 4

- 3200 Spring Summer-W experiment experiment0:::J 28I-

~WD. 24~WI-.J0 20UJ>.J

67

21

-+- Bare Soil-0- Turf

LFN~ N2 Turf-0- N2 Bare Soil~ N20Turf-9- N20 Bare Soil

Total Loss__ N

2Turf

-0- N2 Bare Soil-y- N20 Turf-v- N20 Bare Soil

147

DAYS AFTER FERTILIZATION

Fig. 6

oo

2

3600

0 - 3200Q: -IW E:t:- 2800a. wen ~0 :J~ -I 2400I- 0

2000

0.8

-E 0.6(.)-Z0 0.4~(!)

.....

~ 0.2a:-0.0

6

68

.:0:

1111

--- KN03-0- Urea

--- KN03-0- Urea

--- KN03-0- Urea

--- KN03-0- Urea

- 1.6E(,)- 1.2

:J~ 0.8..J-«I-LL« 0.4zC>-0:=~~ 0.0

0.75

0.50

0.25

0.00

2.0

1.5

-~ 1.0.en~E 0.5ZC') 0.0:J.-W 0.75I-~Z 0.500UJ~ 0.25~W

0.00

1.25

1.00

0.75

0.50

0.25

0.00a 7

DAYS AFTER INITIAL FERTiliZATIONFig. 7