69

CHAPTER 4

MASS BALANCE OF 15N APPLIED TO KENTUCKY BLUEGRASS INCLUDING

DIRECT MEASUREMENT OF DENITRIFICATION

ABSTRACT

Although the fate of fertilizer applied to turfgrass has been studied in the past,

recovery of applied fertilizer N is typically low and denitrification has been cited as the

reason. The objectives of this research were twofold: (1) to examine the fate of 15N

applied to Kentucky bluegrass (Poa pratensis L.) turf as KNO3, including direct

measurement of denitrification; (2) to determine whether and how plants affect fertilizer-

N recovery. Polyvinylchloride (PVC) cylinders, modified to permit atmospheric

sampling, were used throughout field experiments during the spring and summer 1999

and a greenhouse experiment in 2000 . Potassium nitrate (98.5 atom % 15N) was applied

in solution at 49 kg N ha"1 to replicated plots, and atmospheric samples were collected

three times a day from 0800 to 1100, 1100 to 1400, and 1400 to 1700 during a 6-wk

period in the spring and a 4-wk period during the summer of 1999. Emission of N2 or

N20 ranged from 3.3 to 21.3% and from 0.3 to 5.9% of labeled fertilizer N (LFN),

respectively. Recovery of LFN in the soil or plant, plus that emitted as N2 or N20,

ranged from 57.4 to 73.2%. Complete recovery was observed from bare soil, 100.2 ±

1.9%, and for turf, 98.7 ± 9.6%, during an experiment to verify the methods employed

could completely recover LFN 24 h after application. A 4-wk greenhouse experiment

comparing LFN recovery for bare soil and turf, including gas emission and leachate, was

70

initiated in the summer, 2000. Total emission of LFN as N2 or N20 was 19.0% for the

turfgrass, as compared to 7.3% for the bare soil. The corresponding values for total

recovery of LFN were 70.6% and 84.2%, respectively.

INTRODUCTION

The fate of N has been studied in turfgrass systems during the past decade,

specifically NO3-N leaching. These studies indicate that N fertilization of turf poses little

risk to the environment (Starr and DeRoo, 1981; Brown et al., 1982; Mosdell and

Schmidt, 1985; Gold and Groffinan, 1993; Miltner et ah, 1996). However, it remains

difficult to establish an accurate N balance for a given soil-plant system in the field.

Miltner et al. (1996) studied the fate of 15N-labeled urea applied to Kentucky

bluegrass turf and found that only 0.23% of the LFN was collected in the drainage water

of lysimeters 1.2 m below the soil surface. The majority of the applied N was taken up

by the plant or immobilized in the thatch, but recovery of LFN totaled 64 to 81%,

suggesting volatile losses.

Starr and DeRoo (1981) also observed very little N leaching in a study involving

application of 15N-labeled (NH^SCU to turfgrass in the northeastern USA. Throughout

their experiments, 15N-labeled NO3 was detected in leachate on only one occasion, and

total recovery amounted to 64 to 76% of the LFN applied. These authors attributed the

LFN loss to denitrification and NH3 volatilization.

Non-field and field studies to measure NH3 volatilization from turf show

extremely variable results depending on the source of N, application rate, temperature,

thatch thickness, irrigation/rainfall following application, and soil moisture (e.g., Volk,

71

1959; Nelson et al., 1980; Torello et al., 1983; Bowman et al., 1987; Titko et al, 1987).

Bowman et al. (1987) applied 58 kg N ha"1 as urea to a Yolo loam soil (pH 7.3) under

Kentucky bluegrass turf and measured volatilization following different irrigation

treatments. Without irrigation, 36% of the applied N volatilized in the form of NH3,

whereas volatilization was only 3% when 4.0 cm of irrigation was applied. One aspect of

a turfgrass system that will dramatically affect volatilization is the presence of thatch.

Significant urease activity, which hydrolyzes urea to NH3, occurs in the thatch layer

(Bowman et al., 1987). Nelson et al. (1980) observed that within eight days following

urea application to a Flanagan silt loam, 39% of the applied N volatilized as NH3 from

cores of Kentucky bluegrass containing approximately 5 cm of thatch, as opposed to only

5% volatilized from cores having 5 cm of soil and no thatch below the sod.

Another N loss mechanism is denitrification, which involves the reduction of N

oxides to N gases. This process is carried out by facultative organisms that in the

absence of 0 2 use N oxides as terminal electron acceptors (Broadbent and Clark, 1965).

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

(SPAC) because it is the primary mechanism for return of N2 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 (N20) is a

greenhouse gas and has been implicated in stratospheric 0 3 destruction (Prather et al.,

1995).

Relatively few quantitative estimates have been made of the N loss from turfgrass

or grasslands through denitrification because of the difficulties associated with measuring

72

N2 emission under field conditions (Steele and Vallis, 1987). Some researchers have

used acetylene (C2H2) inhibition to estimate denitrification losses from turfgrass under

laboratory conditions (Mancino et aI., 1988; Schwarz et aI., 1994; Tenuta and

Beauchamp, 1995). Mancino et aI. (1988), for example, studied the effects of soil

moisture content, soil texture, and soil temperature on denitrification losses from a

Kentucky bluegrass sod. For silt and silt loam soil types, only 0.1 and 0.4% of the N

applied as KN03 was recovered as N20 when the soil moisture content was 80% 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. At 75% saturation, denitrification losses from the

silt soil increased linearly with temperature between 22 and 30°C and accounted for 0.02

to 0.11 % of the applied N. 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 saturation in a soil, substantial losses ofN could

occur by denitrification.

Because recovery of applied fertilizer N to turfgrass is typically less than

quantitative, the objectives of this research were twofold: (1) to determine the fate ofN

applied to turfgrass, including direct measurement of denitrification; and (2) to determine

whether the completeness of recovery of 15N-Iabeled fertilizer applied to turfgrass is

influenced by the presence of plants. The answers to these questions will provide further

information on N-cycling dynamics in turf and on environmental impacts of fertilizer N

use in turf.

73

MATERIALS AND METHODS

Soil

Field studies were conducted in 1999 at the Landscape Horticulture Research

Center of the University of Illinois, Urbana, IL. The study site was located on a Flanagan

soil (fine, smectitic, mesic, Aquertic, Argiudoll) under Kentucky bluegrass. Analyses of

the soil as described by Mulvaney and Kurtz (1982) gave the following results: pH, 6.8;

total N 2.55 g kg"1; organic C, 30.3 g kg"1, a sand content of 125 g kg'1, a silt content of

588 g kg"1, and a clay content of 287 g kg"1. Particle density (Dp) determined by the

pycnometer method (Blake and Hartge, 1996a) was 2.57 Mg m"3 and the bulk density

(Db) was 1.3 Mg m"3 when determined by the core method described by Blake and

Hartge (1996b). 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. Sampling cylinders were constructed of 20-cm diam. PVC 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 chapter 2. Six PVC cylinders were selected after verifying

that infiltration rates inside and outside the cylinder did not differ. At 0600 on 5 May and

9 August 1999, KN03 containing 98.5 atom % 15N (obtained from Isotec, Miamisburg,

OH) was applied in solution to each plot at a rate of 4.88 gNm"2 (equivalent to 49 kg N

ha ') using a polyethylene wash bottle. To ensure complete transfer of the fertilizer

74

solution, the wash bottle was rinsed three times with a total of 165 mL of deionized

water.

Plots were irrigated twice a week to replace 80% of the potential

evapotranspirtation (PET) when rainfall totals did not exceed this value (PET values

obtained from the Illinois State Water Survey). The turf was maintained at

approximately 5 cm using a pair of manual hand clippers, and a hand-held vacuum was

held against the clippers in order to collect clippings as quantitatively as possible.

Clippings were collected at biweekly intervals and were promptly dried in a forced air

oven at 60°C for 72 h. Prior to analysis, the samples were ground to pass a 0.15-mm

screen.

To accomplish atmospheric sampling as described in chapter 2, brass lids were

secured to the plastic flange on the PVC cylinder using a silicone gasket to create a gas-

tight seal, and the lid was left in place for three h to trap the gases evolved from the soil

and plants. Following this period of enclosure, a closed-loop circulating system was

created by attaching a circulating pump and a gas sampling tube containing Ne to the

brass lid. The air inside the closed chamber was thoroughly mixed by pumping for 20

min and a representative gas sample was collected in the sampling tube originally

employed to introduce Ne. Analyses were performed for 15N-labeled N2 and N20 as

described by Mulvaney and Kurtz (1982), and for Ne as described in chapter 2, using a

Nuclide Model 3-60-RMS mass spectrometer (Spectromedix Corp., State College, PA).

To construct a mass balance of applied fertilizer N, intact PVC cylinders were

extracted from the field upon completion of a 6-wk period in the spring and a 4-wk

period in the summer. Each PVC cylinder was split longitudinally to expose the soil core

75

within. The top segment of the core (verdure) was separated from the rest of the core at

the soil-plant interface. Verdure samples were dried in a forced-air oven at 6DOC for 72 h,

and were then ground to pass a D.15-mm screen prior to analysis. The remainder of the

soil core was sectioned by depth into the following increments; 0-5 cm, 5-1 D cm, 10-20

em, and >20 cm. Each section was weighed and then transferred to deep-freeze storage (_

20°C) until analysis.

Prior to analyses for N and 15N,soil sections were transferred to a forced-air oven

at 60°C, and drying was carried out until no change in mass could be detected for a 24-h

period. The sections were then weighed and pulverized with a hand-grinding mill,

further ground and homogenized with a ball mill for 24 h, and finally, disk-milled so the

entire sample passed through a O.15-mm screen. In addition, the weekly clippings

collected from a single PVC cylinder were combined with the corresponding verdure

sample, and the mixture was thoroughly homogenized. Analyses for total N and 15N

were performed on four 50-mg subsamples of plant tissue per PVC cylinder and on four

500-mg subsamples per soil section. These analyses involved Kjeldahl digestion by a

semimicro method using a pretreatment with Fe and KMn04 to recover (N03 + N02)-N

(Bremner, 1996), followed by diffusion of the digest after treatment with NaOH in a

Mason jar. The diffusion method employed was essentially that of Stevens et a!. (2000),

but with modifications to permit analysis of the entire digest. Following acidimetric

titrations to determine total N, samples were processed as described by Mulvaney et a!.

(1997a) for N-isotope analysis with an automated Rittenberg system (Mulvaney et a!.,

1990; Mulvaney and Liu, 1991; Mulvaney et aI., 1997b).

76

Laboratory Experiment

A laboratory experiment was initiated to determine whether complete recovery of

fertilizer ,5N could be achieved 24 h after fertilization using the soil and plant preparation

and analysis procedure previously described. Three bare soil cores and three cores

containing soil under turfgrass were extracted from the field using a 91.5-cm2 cup-cutter

approximately 12.5 cm deep. Each core was fertilized with KN03 (2.14 atom % 15N) at

44.69 mg N plot"1 (equivalent to 8.0 kg N ha"1). The fertilizer was applied by pipetting 10

mL of an aqueous solution, and the pipette was then rinsed twice with deionized water

(20 mL total) to ensure a complete transfer. After 24 h, cores were frozen at -20°C. Plant

and soil sample preparation and total N and I5N analyses followed the procedures

previously described. No atmospheric samples were collected.

Greenhouse Experiment

A greenhouse experiment was initiated by inserting six PVC cylinders into the

field at a location adjacent to the site of the experiments previously described, of which

three were inserted into bare soil and three into soil under Kentucky bluegrass turf. Two

cylinders of each type were selected after verifying that infiltration rates inside and

outside the cylinder 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 0.95 cm

ID. by 4.4 cm in length 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/or soil inside the cylinders were treated at 0800 with a

solution of KN03 containing 98.5 atom % 15N and supplying 4.88 g N m"2 (equivalent to

49 kg ha1). Fertilization was performed as previously described for the field studies.

77

Atmospheric sampling commenced following fertilization and occurred daily

from 1100 to 1400 during the period, 24 May to 13 June 2000. Irrigation was applied

with a polyethylene wash bottle at least once a week for three wk to maintain adequate

turfgrass health with 14-h days (185 mmol sec-1 m-2 plus ambient sunlight) at 22:f: 2° C

and 10-h nights at 18 :f:2° C. Turfgrass was maintained biweekly at approximately 5 cm

using manual hand clippers, and clippings were collected as previously described. In

addition, leachate was collected twice a week. Total volume of leachate was recorded ,

and approximately 100 mL was transferred to a 125-mL polypropylene screw-cap bottle

and frozen at -20°C.

At the conclusion of the experiment, the PVC cylinders were split longitudinally

to expose the soil core within. The top segment of the cores containing turfgrass

(verdure) was separated from the rest of the core at the soil-plant interface. Verdure

samples were dried in a forced air oven at 60°C for 72 h and ground to pass a O.IS-mm

screen prior to analysis. The remainder of the soil core was sectioned by depth into the

following increments: 0-5 cm, 5-10 cm, 10-20 cm, and >20 cm. The entire section was

then weighed and frozen (-20°C). Plant and soil preparation, and analyses for total N and

15N were performed by the procedures previously described. The same analyses were

performed with four replications on 10-mL aliquots of leachate samples.

Statistical Analysis

For the spring and summer field experiments, data obtained by mass

spectrometric analysis for N2 and N20 were extrapolated to a 24-h period assuming

uniform emission rates, and means for replicate plots are reported with a corresponding

standard deviation. Total N for plant, soil, and leachate was determined in the laboratory

78

with four replications followed by duplicate isotope-ratio analyses of each replicate

sample. Data for replicate plots were averaged, and are reported with a corresponding

standard deviation.

RESULTS AND DISCUSSION

Others have implicated volatilization and denitrification from turfgrass as the

reason for incomplete recovery in mass balance experiments (Starr and DeRoo, 1981;

Miltner et aI., 1996; Logan and Thomas, 1999). Inour work, we hypothesized that the

lack of recovery observed in LFN experiments with turfgrass was due to denitrification.

Emission ofN2 and N20 accounted for 3.3 to 21.3% and 0.3 to 5.9 % ofLFN,

respectively (Table 3). As expected, extensive differences were observed between N

emission rates in spring and summer, which can be attributed to wide variation in

environmental conditions. In the summer experiment, ideal conditions were created for

denitrification when an 8.9-cm rainfall event occurred 4 d after fertilization (DAF),

coupled with elevated soil temperatures. During and for three d following this event,

emission of labeled N2 and N20 was greater than during the entire six-wk experiment in

the spring (Chapter 3). Following the rainfall event during the summer experiment,

higher soil temperatures (Fig. 8) could have caused more rapid soil drying and may have

resulted in more rapid emission ofN2 and N20 (Leteyet aI., 1980; Jury et aI., 1982).

Diurnal variation in emission ofN2 and N20 was also observed. Typically, daily

soil temperatures in the spring will increase throughout the morning and reach a

maximum around midday. Since the rate of denitrification is temperature-dependent,

gaseous N loss should be highest at the latter time (Schnabel and Stout, 1994). This was

the apparent trend in our work, with the largest flux ofN2 and N20 occurring between

79

1100 and 1400 (Table 3). In contrast, during the summer experiment, emission of N2 was

greatest from 1400 to 1700, as one might expect because soil temperatures would be

greatest during this time. Similarly, Mancino et al. (1988) reported that total

denitrification losses were linearly related to soil temperatures from 22 to 30°C with the

largest flux occurring at 30°C. However, N20 emission during the summer experiment

did not follow this trend and the diurnal pattern was not as apparent, which may be

attributed to the rainfall event four d after fertilization, since N2 emission during

denitrification is favored by an increase in the degree of anaerobicity (Weier et al., 1993).

Recovery of LFN in clippings ranged from 27 to 32% for the spring and from

20.5 to 22.3% for the summer (Table 3). Differences in fertilizer N recovery in clippings

between seasons are common as plant growing conditions at the time of fertilizer

application will affect plant uptake, which may explain why lower LFN recoveries were

observed in the summer because of higher soil temperatures (Fig. 8). Plant stress during

hot summer months can have an effect on fertilizer N recovery in clippings (Petrovic,

1990) and with numerous factors influencing N uptake by a plant, comparing research

from various experiments is somewhat difficult. However, similar results were reported

by Miltner et al., (1996) and Starr and DeRoo (1981), in which case recovery in clippings

over a 2-yr period totaled 35% and 30% of LFN, respectively.

u r *™ turnover (MIT) of N occurs naturally in soil, one Mineralization-immobilization turnover (Mii; ui

• ^ »in thp 15N enrichment of mineral N following 15N result being a progressive decrease in the JN enncnn

1. 1 ^ rvM-ppntages of 15N found in the turf during the fertilization. It is possible that the lower percentages

u A,P tn MIT owing to active microbial cycling of summer experiment may have been due to Mil, owi g

, rpCoverv of LFN would have been expected for the 0-5 cm mineral N. If so, an increased recovery 01 i-r

80

soil section. This was not in fact observed, therefore, the smaller amounts ofLFN found

in turf plants during the summer experiment can probably be attributed to removal of soil

N03 through denitrification.

Although a significant amount of LFN was recovered as N2 or N20, recovery of

LFN was far from complete in our work (Table 4) and did not exceed 73.1 %. Total

recovery ofLFN ranged from 65 to 73% for the spring experiment and from 57 to 68%

for the summer experiment. As noted by Miltner et al. (1996), sampling and mixing of

soil extracted from the field can contribute substantially to variation in recovery data. To

minimize such variation, preliminary studies were conducted to compare the precision

achieved by different methods of processing soil samples for recovery of LFN (data not

shown). The results from these studies showed that greatest precision was achieved by

extracting the entire soil core (several kilograms) from the field without subsampling,

oven-drying at 60°C, hand grinding to pulverize large soil aggregates, ball milling for 12

h to homogenize and further grind the soil, and lastly, disk milling so that the entire soil

core would pass through a 0.15-mm screen. With this technique, the coefficient of

variation (CV) did not exceed 1% when total-N and 15Nanalyses were performed on ten

replicate 500-mg samples of soil.

As expected, soil recovery ofLFN was greatest for the 0-5 cm soil section (Table

3), which contained the thatch layer, representing approximately 25% of that applied. In

the spring experiment, recovery ofLFN decreased consistently with greater depth,

whereas a larger percentage ofLFN was found in the 10-20 cm soil section than in the 5-

10 cm section during the summer experiment. The latter finding can likely be attributed

to downward movement of labeled N03, as a result of the 8.9-cm rainfall event 4 DAF.

81

The risk of such movement cannot be avoided when using an N fertilizer source that is

readily soluble, and one effect will be a decrease in plant N availability. In addition,

recovery ofLFN in the 0-5 cm soil section would have been reduced by denitrification,

which was much more extensive in the summer than in the spring. It is generally

accepted that microbial activity is higher in the surface soil than at greater depths (Speir

et aI., 1984) because C compounds released from plants serve as an energy source for

microorganisms. For this reason, denitrification losses typically decrease with depth

(e.g., Yeomans et al., 1992; Sotomayor and Rice, 1996; Luo et aI., 1998; Richards and

Webster, 1999), which helps to further explain why recovery of LFN was reduced in the

0-5 cm soil section during the summer experiment. Only 0.5 to 1.7% of the LFN was

found below 20 cm, which suggests negligible leaching of fertilizer N03, and is

consistent with similar fmdings from numerous other studies (Nelson et aI., 1980; Synder

et aI., 1981; Starr and DeRoo, 1981; Synder et aI., 1984; Petrovic et aI., 1986; Gold and

Gro ffinan, 1993; Miltner et aI., 1996; Owens et aI., 1999).

According to our original hypothesis, denitrification accounts for the lack of

complete recovery observed when LFN is applied to turfgrass. Although our research

demonstrates the potential for substantial N loss as N2 or N20, our recoveries were

disconcertingly low and are not readily explained (Table 4). These fmding suggests that

denitrification does not account for the 15Ndeficits observed in our work, or in previous

studies that did not involve direct measurement of denitrification losses from turf. To

determine whether the methods employed in our work were adequate, a laboratory

experiment was conducted to compare LFN recovery from a bare soil and a turfgrass

system 24 h after fertilizer application. In both cases, recovery of LFN was essentially

82

complete at 100.2 :f: 1.9 and 98.7 :f:9.6%, respectively. However, the relatively high

standard deviation for the plant-based system suggests the possibility that N loss may

have occurred by other processes beside denitrification.

Since our efforts to obtain a mass balance of applied fertilizer N were not

achieved in the field (Table 4), a greenhouse experiment was initiated to compare

recovery ofLFN for a bare soil and a turfgrass system. Unlike the field experiments, this

experiment was designed to measure the loss of labeled N by leaching as well as by

denitrification. Recovery ofLFN for the bare soil was 84.2:f: 16.4%, as compared to

70.6:f: 9.9% for the turf.

Plant-based systems differ inherently from bare soil, in that roots are constantly

aerating the soil surface, evapotranspiration is occurring, plant senescence supplies

microorganisms with organic C as an energy source, and nutrients are removed from the

soil via plant uptake. A further difference arises for high maintenance turfgrass, in that

irrigation is typically applied daily. Because of these differences between turf and bare

soil, LFN losses as N2 and N20 were greater from turfgrass than from bare soil (Table 5),

and totaled 19% for turf as compared to 7% for bare soil. These results are consistent

with previous work by Larsson et al. (1998), who found that emission ofN20 from a

grass sward (6 kg N20-N ha-1) greatly exceeded emission from a bare soil (0.2 kg N20-N

ha-1). With bare soil, the lower N emission rates observed in our work are likely due to

infrequent wetting of the soil surface by irrigation, which kept the soil profile drier and

therefore more aerobic; moreover, these plots would have contained less available C, as

compared to soil under turfgrass. The experiment in the greenhouse only involved

irrigation approximately once a week to maintain adequate turfgrass growth (versus

83

replacing 80% of PET for the field experiments twice a week). Rolston et al. (1982) have

suggested that less frequent irrigation moves the fertilizer N deeper into the root zone

resulting in less N03 in the upper part of the soil profile where high C and high water

contents may occur simultaneously.

As previously observed in the field experiments (Table 3), soil LFN decreased

with soil depth (Table 5). For the turfgrass plots, a total of 40% ofLFN was recovered in

plant tissue and the surface 5 cm of soil. Without plants, very nearly the same percentage

ofLFN was found in the 0-5 cm soil section of bare soil plots (41%), but the leachate

contained four times as much LFN. Irrigation frequency and amounts were the same for

both systems studied, but a larger volume of leachate was recovered for bare soil because

of the lack of plant water uptake and transpiration. Baker and Timmons (1994) reported

losses of LFN from no-till com in lysimeter drainage ranging from 4 to 19%, which are

similar to our results observed for bare soil. As previously described, turfgrass, even

under artificial conditions in the greenhouse, seems to be an excellent system for

impeding downward movement ofN03 by plant uptake.

Recovery of LFN from turfgrass for the field and greenhouse experiments

reported here ranged from 57 to 73% (Table 4), even when taking into account N losses

due to denitrification and leaching. Results from our work suggest that turfgrass, or

plants in general, lead to large deficits in recovery ofLFN; however, a deficit was also

obtained for bare soil, which may reflect errors associated with the sampling procedure

for N2 and N20. Data extrapolation from 3-h measurements ofN2 and N20 emission to a

24-h period is a possible source of error in our recovery estimates (VeIthof et aI., 2000).

However, because denitrification is temperature-dependent, emissions ofN2 and N20

84

would tend to be greatest during the day when the sun provides radiative heating of the

soil surface (one possible exception to this theory would be during the summer

experiment where soil temperatures may be greatest following the last atmospheric

sample collection period). Although data extrapolation no doubt led to some error in

estimating these emissions, the effect was probably to slightly overestimate gaseous N

loss because atmospheric sampling occurred from 0800 to 1100, from 1100 to 1400, and

from 1400 to 1700 h. The methods employed in our work would not have detected

gaseous loss OfNH3, but such loss is unlikely to have been appreciable, given the fact

that N was applied as N03 to a fine-textured acidic soil. Some N loss as NH3 may have

occurred from senescing plant tissue or from sod as previously noted by many

investigators (e.g., Yolk, 1959; Meyer et aI., 1961; Watkins et aI., 1972; Torello et aI.,

1983; Bowman et aI., 1987; Titko et aI., 1987; Gooding and Davies, 1992). Moreover,

the possibility cannot be excluded that other'N compounds may have volatilized from

leaves, such as amines, oxides ofN, HeN, oximes, and some alkaloids (Wetselaar and

Farquhar, 1980).

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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 James

Abel 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. 8. Average daily temperature readings reported from May through September, 1999.

91

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Table 4. Recovery of labeled fertilizer N with different

periods of atmospheric sampling to estimate

emission as N2 or NiO.f

Period of atmospheric sampling

Season 0800-1100 1100-1400 1400-1700

% Recovery

Spring 66.8(5.0) 73.1(6.8) 65.2(5.0)

Summer 57.4(11.0) 59.2(9.1) 68.5(3.4)

f Percentage recovery of labeled fertilizer N was calculated

from cumulative measurements of N2 or N2O emission

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of plant and soil, relative to the total amount of fertilizer

N applied (4.88 g N m"2). Values in parentheses are

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