Nitrogen loss through denitri®cation in a soil under pasture in
New Zealand
J. Luo*, R.W. Tillman, P.R. Ball
The Institute of Natural Resources, Massey University, Palmerston North, New Zealand
Accepted 24 September 1999
Abstract
Denitri®cation on several contrasting topographical sites in a New Zealand dairy-farm pasture was measured periodically over a year, using the acetylene inhibition technique, by incubating undisturbed soil cores in a closed system. The measured denitri®cation rates varied considerably both spatially and temporally. High coecients of variation (CV) and log-normal distributions of denitri®cation rate were often observed. The spatial variance in denitri®cation rate changed temporally and was apparently related to soil moisture content and the grazing pattern. Denitri®cation rates followed a marked seasonal pattern, with highest rates being measured during the wet winter and lowest rates during the dry summer and early autumn. Dierences in denitri®cation rates among sites were not consistent. However, slightly higher denitri®cation rates were usually detected in the ¯oor of a gully and in a gateway area than on other sites. Mean denitri®cation rates from individual dates were positively correlated to soil moisture content. However, there was a negative correlation between denitri®cation rate and soil nitrate
concentration, respiration rate and temperature. An annual nitrogen loss of 4.5 kg N haÿ1through denitri®cation was estimated
in this legume-based dairy-farm pasture. Low soil moisture content was the primary factor limiting denitri®cation during the dry
summer and early autumn. Low denitri®cation rates were also caused by lack of available soil NO3ÿ-N.72000 Elsevier Science
Ltd. All rights reserved.
Keywords:Denitri®cation; Pasture; Nitrous oxide; New Zealand; Soil
1. Introduction
Most of the previous research on nitrogen cycling in grazed pastures has demonstrated the importance of the grazing animal in returning N ingested in the herbage to the soil in the form of urine and
dung (Ball and Tillman, 1994). Although early
research indicated that grazing animals had a ben-e®cial role in nutrient cycling through the transfer of fertility in the form of excreta around the farm (Sears, 1950), this viewpoint has since been modi-®ed. Research has shown that the return of N to
the soil in the form of extremely concentrated urine spots can lead to greater losses than originally indi-cated, particularly on intensively managed, high fer-tility farms (Ball and Keeney, 1981). The fate of excretal N in pastures under New Zealand
con-ditions has been investigated by a number of
workers (e.g. Ball et al., 1979; Carran et al., 1982; Field et al., 1985; Williams et al., 1989). In several of these studies, mass balance considerations indi-cated that signi®cant amounts of N were unac-counted for. Loss of this N through denitri®cation is one possibility. Certainly, at ®rst glance the
po-tential for denitri®cation from pastures would
appear to be high due to high amounts of organic C in the surface soil and high concentrations of
NO3ÿ-N present in the soil under urine and dung
patches (Haynes and Williams, 1993). Although studies
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* Corresponding author. Present address: Land and Environmental Management, AgResearch, P.O. Box 3123, Hamilton, New Zealand. Tel.: +64-7-838-5125; fax: +64-7-838-5155.
on the potential denitri®cation in New Zealand pas-tures have been made by Limmer and Steele (1982), Luo et al. (1996) and Crush (1998), the extent of deni-tri®cation and the factors aecting it in New Zealand pastures requires study, and more information about denitri®cation is needed to assess the contribution of denitri®cation in New Zealand grasslands to regional and global N cycling.
There are a number of reports of denitri®cation in grassland soils, and most studies have been con-ducted in the northern hemisphere (e.g. Ryden, 1983; Jarvis et al., 1994; Schwarz et al., 1994). In-vestigations have included the eect of N fertilisers, slurry application and irrigation on denitri®cation rates. The measured annual denitri®cation rates in
the ®eld vary from 0 to >100 kg N haÿ1
(Jordan, 1989; Aulakh et al., 1992; Groman and Turner, 1995; Ledgard et al., 1997).
An issue complicating the study of denitri®cation in the ®eld is the marked spatial and temporal variability (Ryden, 1983; Folorunso and Rolston, 1984). Spatial variability results from the pathy dis-tribution of denitrifying `hot spots' in the soil. `Hot spots' are caused by non-homogeneous distribution of available C (Parkin, 1987) and other factors that
regulate denitri®cation, such as NO3ÿ and soil
aera-tion (Smith, 1980; Groman and Tiedje, 1989a). The temporal variations can be explained mainly by corre-sponding variations in soil temperature and other deni-tri®cation regulating factors (Groman and Tiedje, 1989b). The general tendency is for highest denitri®ca-tion rates to occur when soils are warm, wet and soil
NO3ÿand C are available. Denitri®cation rates are
gen-erally highest in spring, summer and autumn in north-ern temperate agricultural soils (e.g. Ryden, 1983; Parsons et al., 1991; Estavillo et al., 1994; De Klein and Van Logtestijn, 1994; Schnabel and Stout, 1994), and in winter in New Zealand pasture soils (Ruz-Jerez et al., 1994). Variations in N fertiliser application, irri-gation and rainfall are also reasons for temporal vari-ation in denitri®cvari-ation rate (Jarvis et al., 1991; Aulakh et al., 1992; Schnabel and Stout, 1994). Animal grazing in pastures also in¯uences the temporal variation of denitri®cation rates (Carran et al., 1995; Luo et al., 1999a).
We investigated the extent of denitri®cation in a New Zealand pasture. The study site was located on an intensive dairy-farm on a poorly-drained soil. The combination of poor drainage and compaction from high stocking rates was expected to restrict aeration. The return of dung and urine should have ensured a
ready supply of NO3ÿ-N and soluble C. It was felt that
such a site might provide a useful insight into the `upper boundary' to denitri®cation N losses from pas-ture.
2. Materials and methods
2.1. Site description
The research was carried out using soil samples col-lected from a paddock of the Massey University No. 4 dairy-farm, Palmerston North, New Zealand. The size of the paddock was about 2.5 ha. The paddock was under ryegrass±white clover pasture (i.e. legume-based pasture) and was periodically grazed by cows at
stock-ing density of about 100 cows haÿ1. Throughout the
study, the paddock received no N fertilisers. The soil at this site, Tokomaru silt loam (Cowie, 1974), is classi®ed as a yellow grey earth (Taylor and Pohlen, 1968) or a pallic soil (Hewitt, 1992). It is a poorly-drained soil with wet conditions in winter, and rela-tively dry conditions in summer. The paddock was pre-dominantly ¯at with a small gully (about 3 m deep) running through it. Five contrasting sites were located within the paddock. These were a ¯at land area, north- and south-facing gully slopes, the gully bottom and a fertile and compacted gateway area. Soil proper-ties of the upper 7.5 cm of the pro®le are given in Table 1.
2.2. Field denitri®cation measurement
2.2.1. Collection of samples
One sampling area (9 9 m) was usually selected
within each of the gateway, south-facing and north-facing slope sites and two sampling areas selected within each of the ¯at land and gully bottom sites. Denitri®cation measurements were made regularly at all the sites from July 1992 to July 1993, with more frequent sampling in the late summer and early autumn of 1993. On measurement occasions, 16 soil cores were taken from randomly-selected areas at each site. The sampling points in each sampling were
arranged at 3-m intervals over the 99 m area.
2.2.2. Measurement of denitri®cation rate
The rate of denitri®cation was measured using the acetylene-inhibition technique (Yoshinari et al., 1977), using the individual soil core incubation system under ®eld conditions as described by Ryden et al. (1987). The technique involved incubation of soil samples in
the presence of C2H2to prevent conversion of N2O to
N2 (Yoshinari et al., 1977). N2O is the sole gaseous
product of denitri®cation in soils incubated in
atmos-phere containing 0.1±10% v/v C2H2 and the moles of
N2O produced (with C2H2) are equal to moles of
N2O+N2(without C2H2) (Yoshinari et al., 1977). This
procedure simpli®es analytical procedures for denitri®-cation assays, since denitri®denitri®-cation can be estimated by
a single measurement of N2O using a gas
acetylene and core incubation method (Tiedje et al., 1989), we believed that it is an appropriate technique for measuring denitri®cation rates in grazed pasture soils, because the uneven distribution of urine and dung causes considerable spatial variability and this technique allows numerous core incubations to be quickly carried out to integrate the denitri®cation rate. On each sampling date, soil cores were collected using a soil corer. A previous study found that denitri®ca-tion activities were maximal in the surface soil (0±5 cm) and generally decreased exponentially with depth in the same pasture (Luo et al., 1998). Consequently, surface soil samples (0±7.5 cm depth) were collected. Each core was approximately 2 cm in diameter. Indi-vidual cores were transferred from corers into PVC tubes (2.5 cm diameter by 15 cm in length). The tubes were closed at both ends with rubber septa. 6 ml of air was withdrawn from the tube and the same amount of puri®ed C2H2(by passing industrial C2H2 gas through
a high concentration of H2SO4) was injected into each
tube using a syringe. The syringe was pumped several
times to mix the C2H2within the tube. Approximately
10% of the volume of the headspace was replaced with C2H2.
All tubes were incubated for 24 h on the ground in a shaded place close to the paddock. At 1 and 24 h
after the addition of C2H2, samples of the headspace
gases were collected in 5 ml `venoject' evacuated test tubes (Becton Dickinson Vacutainer Systems). A gas
chromatograph, equipped with a63Ni electron capture
detector, was used to measure the concentrations of
nitrous oxide (N2O) in the samples. The details of the
measurements and the calculations were described by Luo et al. (1999a).
2.3. Soil moisture, NO3ÿ, CO2measurements and soil
temperature
On most occasions, soil cores were brought back to the laboratory after 24 h of incubation and removed
from the tubes. The individual soil samples from each of the tubes were then bulked. Soil moisture was deter-mined from the weight loss of sub-samples dried
over-night at 1058C. Soil NO3ÿ (including NO2ÿ) were
extracted by shaking a 5 g soil sample with 20 ml of 2 M KCl for 60 min and ®ltering through Whatman No.
42 ®lter paper. NO3ÿ-N was determined
colorimetri-cally on a Technicon Autoanalyser (Downes, 1978).
CO2 concentration was determined from the same
gas samples as those used for N2O analysis and was
measured in a gas chromatograph equipped with a thermal conductivity detector.
Monthly data for soil temperature (10 cm depth), rainfall and evaporation were obtained from the nearby meteorological station of AgResearch Grass-lands. The data for soil temperature, rainfall and evap-oration over the study period are shown in Fig. 1.
2.4. Statistics
Coecients of skewness were calculated to quantify departures from normality on both untransformed and log-transformed data. The signi®cance of the dierence from zero of the coecients of skewness was evaluated as described by Zar (1974). Frequency distributions of denitri®cation rates and log-transformed rates at each site on each sampling date were calculated by Wilk± Shapiro statistics to assess whether these rates were normally distributed (SAS Institute, 1982).
The mean soil denitri®cation rate and NO3ÿ-N
con-centration were calculated using the Uniform Mini-mum Variance Unbiased Estimators (White et al., 1987; Parkin and Robinson, 1992), when data were highly skewed and log-normally distributed. Conse-quently, the comparisons of log-normally distributed
denitri®cation rates or NO3ÿ-N concentrations of the
dierent sampling sites and dates were carried out by testing the overlaps of upper and lower 95% con®-dence limits using the untransformed data (Parkin, 1993). Pearson correlation coecients were calculated
Table 1
Major characteristics of the soil at 0±7.5 cm depth
Site
gully bottom N-facing slope S-facing slope ¯at land area gateway
pH (H2O) 6.06 6.00 5.91 6.04 5.94
Texture
% Sand 23.90 22.98 21.26 22.71 22.74
% Silt 63.79 62.39 63.08 67.32 62.83
% Clay 12.31 14.63 15.66 9.97 14.55
Bulk density (Mg mÿ3
) 0.83 0.87 0.88 0.84 0.93
Total N (%) 0.47 0.36 0.37 0.42 0.48
Total P (%) 0.13 0.09 0.08 0.09 0.16
among the measured variables. The log-transformed
data for denitri®cation rate and NO3ÿ-N concentration
were used as input variables in the correlations.
3. Results and discussion
3.1. Spatial variation and frequency distribution
3.1.1. Denitri®cation
Measurements of ®eld denitri®cation rates by the
acetylene inhibition and soil core incubation technique were made on 14, 13, 13, 20 and 13 occasions on gully, south-facing slope, north-facing slope, ¯at land and gateway sites, respectively. The measured denitri®-cation rates exhibited a high degree of skewness and a large spatial variation at all the sampling sites through-out the sampling periods. The frequency distributions
were positively skewed (P< 0.01) for 63 of the total
73 data sets for individual sites during the sampling periods (Table 2). Statistical analysis con®rmed that most of ®eld denitri®cation rates measured by this
technique had a log-normal rather than a normal dis-tribution, irrespective of sites and sampling dates (Table 2). It was also observed that the values of the coecients of skewness for log-transformed rates on most occasions were not signi®cantly positive (Table 2). The coecient of variation (CV) of denitri®cation rates exceeded 100% in 45 of the total 73 data sets (Table 2). The CVs for the log-transformed rates were smaller than for the untransformed rates (Table 2).
Large CVs and skewed distributions occur when most samples have low denitri®cation rates and a few samples have very high rates. In the current study about 25% of the soil cores contributed more than 50% of the total N loss through denitri®cation from all the soil cores on each sampling occasion. Large CVs and log-normal distribution patterns of denitri®-cation rates in other ®eld studies have been often reported (Christensen et al., 1990; Parsons et al., 1991; Estavillo et al., 1994; Schnabel and Stout, 1994).
3.1.2. NO3ÿN
The concentrations of soil NO3ÿ-N were variable and
they also appeared to be highly skewed and in most cases exhibited a log-normal distribution (Table 2). This agrees with the ®ndings of White et al. (1987) and Bramley and White (1991).
3.1.3. CO2
CO2emission rates have also been found to be
log-normally distributed (Focht et al., 1979). However, in
our study soil CO2 emission rates from core
incu-bations did not often exhibit a high degree of spatial variation and were ®tted better by normal than log-normal distributions on most sampling dates at most sites (Table 2). This may indicate that a high
pro-portion of the measured CO2 originated from the
res-piration of evenly-distributed soil C or possibly grass roots in this pasture.
3.1.4. Moisture
Soil moisture content was relatively uniform and could be described by normal distributions on almost all sampling dates at all sites (Table 2). However, skewed distributions for soil moisture content were generally observed when the soil was relatively dry (Table 3). It is possible that there may be a patchy dis-tribution of moist soil under dry ®eld conditions due to urine and dung deposits from dairy cattle.
3.1.5. Temporal patterns of spatial variability
The pattern of variation in denitri®cation rates in this pasture showed a temporal dependence, that was mostly in¯uenced by grazing events and rainfall. After
an intensive grazing event in the winter, soil NO3ÿ-N
concentration and denitri®cation rates increased
(Table 4). The grazing also increased the skewness of
the soil NO3ÿ-N concentration and denitri®cation rates
(Table 4). These results suggest that the high skewness of the denitri®cation rates was probably due to uneven
distribution of the soil NO3ÿ-N from animal excreta in
the ®eld.
Our study also showed that denitri®cation rates were low and the skewness of denitri®cation rates was also low when the soil was relatively dry in the sum-mer and early autumn (Table 3). After a rainfall event, the soil moisture content and denitri®cation rates
increased and the skewness of denitri®cation rates also increased initially, but then tended to decrease again as the soil became saturated (Table 3). The coecients of skewness for soil moisture content were large in rela-tively dry soil, and decreased rapidly when the soil
became wet (Table 3). Although the NO3ÿ-N
concen-tration decreased rapidly, the skewed distribution remained after rainfall (Table 3).
It seems that following rainfall anaerobic con-ditions developed and caused the patchily-distributed
Table 3
Eect of rainfall on the variation of denitri®cation rate, soil nitrate concentration and soil moisture content
Date (1993)a Site Soil moisture Soil nitrate Denitri®cation
content (% w wÿ1
) skewness content (mg N kgÿ1
) skewness rate (mg N kgÿ1 dÿ1
) skewness
19 February gully bottom 21.9 1.85 8.5 2.4 4.6 1.8
north slope 20.9 1.25 7.9 1.9 4.8 1.7
south slope 25.6 ÿ1.753 6.5 1.3 4.7 2.4
¯at land 20.0 2.4 7.5 2.1 3.6 1.9
gateway 22.2 ÿ1.24 14.6 1.4 8.4 1.1
20 February gully bottom 37.2 1.185 5.2 2.8 21.4 5.3
north slope 33.6 1.16 1.3 2.2 17.5 3.2
south slope 39.5 ÿ1.75 0.18 1.6 7.16 3.2
¯at land 35.0 2.4 5.5 3.3 9.00 2.8
gateway 40.4 ÿ0.24 5.2 2.4 86.3 3.4
21 February gully bottom 39.5 ÿ0.059 7.1 2.0 16.1 3.2
north slope 33.9 ÿ0.410 3.3 1.6 7.1 2.3
south slope 36.4 0.170 1.5 3.2 10.4 0.73
¯at land 36.1 0.161 0.92 3.5 12.1 2.1
gateway 40.2 0.186 1.6 1.3 31.0 0.96
22 February gully bottom 36.7 0.493 7.1 1.9 23.4 2.6
north slope 30.9 ÿ0.338 1.2 1.6 12.1 0.88
south slope 34.3 ÿ0.484 1.2 2.0 14.8 1.9
¯at land 33.7 0.31 2.2 3.4 12.2 4.0
gateway 36.1 0.292 3.3 1.2 17.7 2.6
10 March gully bottom 30.6 ÿ0.507 10.7 2.0 7.01 3.9
north slope 25.9 0.173 4.1 1.9 4.7 2.9
south slope 30.9 ÿ1.289 5.6 2.1 6.5 0.76
¯at land 28.8 3.22 6.8 3.2 4.6 1.8
gateway 31.0 ÿ1.027 8.8 1.8 5.6 0.37
14 March gully bottom 48.9 1.39 6.7 2.0 17.0 3.6
north slope 43.8 0.17 1.9 1.8 8.1 2.6
south slope 44.4 0.22 2.1 2.1 8.9 0.7
¯at land 44.7 1.07 2.3 3.2 6.7 2.8
gateway 45.6 0.014 2.3 1.8 8.5 2.5
a
A rainfall (26 mm) started in the evening of 19 February 1993, stopped on 21 February. Another rainfall started on 12 March 1993.
Table 4
Eect of intensive grazing on denitri®cation and other soil variables (sampled on 5 August 1993)
Variable Denitri®cation Nitrate Moisture
rate
(mg N2O-N kgÿ 1
dÿ1 )
coecient of skewness concentration (mg NO3ÿ-N kgÿ
1 )
coecient of skewness content (% w wÿ1
)
coecient of skewness
Control site 21 1.58 0.79 2.56 47.03 0.44
Grazed sitea 42 2.67 3.12 3.88 48.54 0.56
a
soil NO3ÿ-N to become available to denitrifying
micro-organisms. Denitri®cation rates therefore increased as did the skewness of the distribution. A few days later, after the soil was saturated, the soil moisture became less skewed, and hence a more uniform distri-bution of anaerobic sites was achieved in the soil, resulting in less spatial variability in denitri®cation rates (Table 3).
As discussed above, high soil moisture contents are favourable to denitri®cation, and the variability of denitri®cation rate appeared to decrease because of a more even distribution of anaerobic sites in the soil after the rainfall in the dry season. Christensen et al. (1990) suggested that the distribution of denitri®cation rates appeared to be less strongly skewed on soil above ®eld capacity compared with soil at ®eld ca-pacity. In our study, soil was above the ®eld capacity for a period in the winter, but the variance and skew-ness of denitri®cation rate at this time did not tend to be less than the rest of the sampling dates in the study pasture (data not shown). It was found that the
varia-bility of NO3ÿ-N or available-C are more important
factors controlling the spatial variability of denitri®ca-tion and soil anaerobiosis alone may not determine the skewed distribution of denitri®cation rate in this par-ticular pasture when the soil was wet in the winter (Luo et al., 1999b).
3.2. Temporal variation of denitri®cation rate
3.2.1. Seasonal pattern
The rates of denitri®cation on the gully bottom, north-facing, south-facing, ¯at and gateway sites in the study paddock are shown in Fig. 2. Denitri®cation rates were highest in the winter (May to August), fol-lowed by a decrease during the spring (September to November). Denitri®cation rates were generally lowest in the summer (December to February) and then increased during the autumn (March to April).
The seasonal pattern in denitri®cation rate under ®eld conditions we observed in this study (Fig. 2) was similar to that found by Ruz-Jerez et al. (1994) on a freely-drained, ®ne sandy loam in the same locality. Both studies reveal that the highest N losses by denitri®cation occurred in the winter and lowest occurred during the summer. Changes in soil
aera-tion, supply of NO3ÿ-N and availability of C under
®eld conditions may all be implicated in seasonal variations of denitri®cation activity, as the dominant controlling factors aecting denitri®cation appeared to vary temporally in the study pasture (Luo et al., 1999b).
Peaks of denitri®cation occurred in late February and mid-March, after rainfall events (Fig. 2). For-mation of anaerobic sites by receipt of water from
rainfall was probably a fundamental requisite for deni-tri®cation in those periods. Increased denideni-tri®cation rates following rainfall events can also be attributed to increased availability of C and NO3ÿ-N in the soil (Luo
et al., 1999b).
3.2.2. Site dierences in denitri®cation rate
There were dierences in denitri®cation rate between sampling sites, but these were not always consistent (Fig. 2). Soil near the gateway exhibited slightly (but not signi®cantly at P< 0.05) greater rates of denitri®-cation than the other sites on most sampling dates through the year. When the denitri®cation rate was
generally low during the summer (December to Febru-ary), no dierences in the rates among the ¯at, slopes and gully bottom were found. However, from July to October the denitri®cation rates in both sloping sites were low compared to the rates at other sites. This ob-servation emphasises the importance of animal eects on denitri®cation activity at various points in a pas-ture. More excreta from animals is deposited in the paths of animal movement (Barrow, 1967) and on hill pasture the animals can transport signi®cant quantities of nutrients to ¯at areas, because they tend to camp there (Saggar et al., 1988). Loss of N by denitri®cation may, thus, be enhanced around gateways or in
site areas, due to higher amounts of deposition of urine and dung.
A slightly higher (not signi®cantly atP< 0.05) deni-tri®cation rate was observed in the gully bottom than in ¯at and sloping sites after a rainfall event in Febru-ary (Fig. 2). A slightly higher peak of denitri®cation rate was also observed in the gully bottom compared with the other sites after a rainfall event in March, but it was not as high as that in February (Fig. 2). These dierent responses of denitri®cation to soil wetting in various areas of landscape could be a result of
sub-strate (NO3ÿ-N or C) redistribution. Substrates were
most likely to accumulate in the gully bottom after rainfall events (Fig. 3); presumably they were trans-ported from slopes to the gully bottom in water.
3.3. Characteristics of denitri®cation and its regulators
3.3.1. Factors related to denitri®cation rates measured in individual soil cores
Examination of the correlation-coecient matrix revealed that at some individual sampling times and in some topographical sites, denitri®cation rates were clo-sely related to one or more of the other measured vari-ables. However, these relationships were not consistent over time or between topographical sites (data not pre-sented). When the data for each sampling time were combined for each individual sites or the whole pad-dock, denitri®cation rates were always weakly, but
sig-ni®cantly (P < 0.01), correlated to soil moisture
content. The correlations between denitri®cation rates
and soil NO3ÿ-N concentrations and soil respiration
rate (CO2emission rate) were not consistent. When the
entire data sets for the whole year were evaluated,
cor-relations between denitri®cation rate and NO3ÿ-N
con-centration, or respiration rate, were improved, when the pooled data were partitioned according to soil moisture (Table 5). The highest correlation between
denitri®cation rate and NO3ÿ-N concentration was 0.38
and this was obtained when the soil moisture content
was over 45% (w wÿ1) (about ®eld capacity). In
con-trast, the highest correlation coecient between deni-tri®cation rate and soil respiration rate was 0.44, and this was obtained when the soil moisture content was
less than 30% (w wÿ1). That the strength of
corre-lation between denitri®cation rate and soil NO3ÿ-N
concentration or soil respiration rate were dependent on the soil moisture content suggests that the
avail-ability of NO3ÿ-N or readily biodegradable C can be
aected by soil moisture in this particular pasture.
3.3.2. Associations among means of measured variables Among the edaphic conditions, soil moisture content had a pattern similar to the denitri®cation rate (Figs. 2
and 3). Changes in soil NO3ÿconcentration and
respir-ation rate had opposite temporal patterns to changes in denitri®cation rate (Figs. 2 and 3). Correlation coef-®cients were computed between the mean values of denitri®cation rate and the other edaphic properties for each sampling date for both the individual sites and the whole paddock. In all cases, the closest re-lationships were obtained between denitri®cation rate
and soil moisture content (Table 6). Soil NO3ÿ-N
con-centration, respiration rate and temperature appeared to be negatively correlated with denitri®cation rate, however, the signi®cance of the correlations varied among sites (Table 6).
3.3.3. Periods of relatively high denitri®cation rates A relatively high denitri®cation rate was observed in the winter (Fig. 2), although the soil temperature was
only about 88C (Fig. 1). The active denitri®cation in
the winter appears to have been associated mainly with high soil moisture contents. Due to frequent rain-fall and low evaporation (Fig. 1), the moisture content in the soil can readily reach amounts greater than `®eld capacity' in this poorly-drained soil in the winter (Fig. 3). Most soil pores would then be ®lled with
water and O2 diusion through water is considerably
slower than through air. It has long been recognised
that O2 concentrations can aect both synthesis and
activity of the denitri®cation enzyme system (Firestone, 1982). Therefore, soil denitri®cation can be increased by an increase in the number of anaerobic sites in the soil in the winter. Previous ®eld studies have demon-strated that the rate of denitri®cation often remains negligible during dry periods, but then increases con-siderably when soil water content exceeds a certain critical amount (e.g. Aulakh and Rennie, 1985; De
Table 5
Correlations between soil denitri®cation (mg N2O-N kgÿ1dÿ1) and measured variables using data from individual soil coresa,b
Moisture content (w wÿ1) Sample number Nitrate (mg NO
3
Klein and Van Logtestijn, 1996; Nelson and Terry, 1996).
In the study pasture, the low concentration of soil
NO3ÿ-N, especially during wet periods of the year, was
an important factor which limited the rate of denitri®-cation (Luo et al., 1999b). The good correlation
obtained between denitri®cation rates and NO3ÿ-N
concentrations at the high soil moisture contents (Table 5) also suggests that in this unfertilised pasture
NO3ÿ-N become a more important limiting factor for
denitri®cation when the potential rate of denitri®cation is increased at high soil moisture contents. Other stu-dies also indicate that soil NO3ÿ-N availability
gener-ally limit denitri®cation in unfertilised grassland soils (e.g. Tenuta and Beauchamp, 1996). The availability of
NO3ÿ-N could be in¯uenced by soil NO3ÿ-N
concen-trations or NO3ÿ-N movement to active denitri®cation
sites. In the study pasture, high soil water contents in the winter provide an optimum medium for diusion
and, therefore, NO3ÿ originating from nitri®cation can
more easily move to anaerobic denitri®cation sites. It has been suggested that nitri®cation and denitri®cation can occur simultaneously on opposite sides of an
aerobic±anaerobic interface (Knowles, 1978). So NO3ÿ
-N could be denitri®ed rapidly and would not accumu-late in a soil with high moisture content. The weak
re-lationships between denitri®cation rate and CO2 at
high soil moisture contents may suggest that soil C was not an important regulatory factor for denitri®ca-tion in the winter, as there would be plenty of
anaero-bic sites and low concentrations of NO3ÿ-N in the wet
soil in this pasture.
The rate of denitri®cation can undoubtedly be lim-ited by low temperature in the winter (Luo et al., 1999b). However, the mean soil temperature (Fig. 1) in the winter in which the study was carried out was always above the critical temperature for denitri®ca-tion, as the lowest temperature at which ®eld
denitri®-cation can occur has been reported to be 58C (Ryden,
1986). The eect of temperature on denitri®cation in the natural environment is complicated by other fac-tors. In the present study, high denitri®cation rates at relatively low temperature in the winter were caused by an opposing, seasonal relationship between
tem-perature and water content in the soil. The tempera-ture eect on denitri®cation may also be aected by simultaneous changes in plant growth in the ®eld. The relatively high denitri®cation rate observed during the winter may have been aected by the limited uptake of
available NO3ÿ from the soil by pasture, since the
growth of grass slowed as daylight and temperature decreased in the winter.
3.3.4. Periods of relatively low denitri®cation rates Rates of denitri®cation were generally very low in
the summer (Fig. 2), although soil NO3ÿ-N
concen-tration and respiration rate were relatively high during this period (Fig. 3). The most obvious factor limiting denitri®cation was the low moisture content of the soil (Fig. 3). Other studies have also found that soil moist-ure content can limit denitri®cation rates in grassland
soils, in which NO3ÿ-N and C availability would be
considered adequate for denitri®cation (e.g. Jarvis et al., 1991). The better correlation between
denitri®ca-tion and CO2 production at low soil moisture content
than at high soil moisture content in the present study (Table 5) may suggest that the role of C in relatively
dry soil may involve O2 consumption by respiration,
which would produce increased number of anaerobic sites suitable for denitri®cation.
A study by Luo et al. (1999b) at this site determined that denitri®cation was limited by the slow rate of
dif-fusion of NO3ÿ-N to active denitri®cation sites when
soil was relatively dry in the summer. It appears that the weak correlations between denitri®cation rate and
NO3ÿ-N concentration observed at low soil water
con-tents (Table 5) is a further evidence that the NO3ÿ-N
concentration was not a major limiting factor for deni-tri®cation when soil moisture was low. Other workers have also reported that slow diusion rates can limit
NO3ÿ-N availability to denitri®cation even at a high
concentration of soil NO3ÿ-N (Ryden, 1983). The low
denitri®cation rates in the summer may have also been aected by the activity of plant roots, since water and
NO3ÿ-N uptake by rapidly-growing pasture would be
expected to be substantial. High rates of plant uptake
decrease the availability of NO3ÿ-N to denitrifying
Table 6
Correlations between denitri®cation (mg N2O-N kgÿ1dÿ1) and measured variables using means from individual datesa,b
Site Gully bottom N-facing slope S-facing slope Flat site Gateway Whole paddock
Nitrate (mg NO3ÿ-N kgÿ1) ÿ0.41 ÿ0.64 ÿ0.44 ÿ0.48 ÿ0.61 ÿ0.33
microorganisms, particularly in the rhizospheric zones, where the availability of C may be high.
3.4. Nitrogen loss through denitri®cation
Denitri®cation rates measured on soil cores were expressed on an areal basis, using bulk density values,
and were interpolated over the period between
sampling dates to estimate annual N loss by denitri®-cation. Cumulative annual N losses monitored in this
study were 4.70, 3.56, 3.70, 4.54 and 5.80 kg N haÿ1
at the gully bottom, north-facing slope, south-facing slope, ¯at and gateway sites, respectively (Table 7).
The overall estimate was 4.5 kg N haÿ1 yÿ1 for the
study pasture. This value was determined by using weighted averages for the losses at the dierent sites in
this paddock. This annual N loss of 4.5 kg N haÿ1 by
denitri®cation from the pasture ®eld in the current study is of the same order as the N losses found by both Ruz-Jerez et al. (1994) and Ledgard et al. (1997) in pastures without fertiliser in New Zealand. The N loss by denitri®cation does not appear to be substan-tial in terms of N balances for pasture and is lower than the expected values for denitri®cation that have been derived from previous N mass balance studies (Ball et al., 1979; Field et al., 1985).
As the study area is in a temperate region, high soil water conditions necessary for the denitri®cation pro-cess are typically found in the winter and therefore as-sociated with low temperatures. During this period, a soil NO3ÿ-N supply for denitri®cation is also restricted
in this unfertilised pasture. Therefore, denitri®cation is limited. Although soil temperature increases in the summer, the low soil moisture content limits denitri®-cation and therefore N loss. Most of the soil mineral N in this paddock was associated with excreta from the grazing animals. So the eects of animal grazing on denitri®cation would be signi®cant. The direct eects of grazing on denitri®cation in the study area were reported by Luo et al. (1999a). Overall, the lim-ited availability of soil NO3ÿ-N or low soil water
con-tent during most times of the year are probably the main cause of the small loss of N by denitri®cation observed from this legume-based pasture.
4. Conclusions
Denitri®cation rates exhibited marked spatial varia-bility in this pasture, with coecient of variation fre-quently being larger than 100%. The distribution of rates was generally skewed. A log-normal distribution was the most appropriate for describing the spatial variation among denitri®cation rates measured in the various topographical areas. Rainfall and animal graz-ing events aected the spatial variation of denitri®ca-tion rates. Rainfall in the warm-dry season increased the skewness of the denitri®cation rate initially, but then decreased as the soil became more uniformly wet. An intensive grazing event in the winter increased the skewness of the frequency distribution of denitri®ca-tion rates.
Denitri®cation rates in this legume-based dairy-farm pasture were highest in the wet winter and lowest during the dry summer and early autumn. However, high denitri®cation rates did occur for brief periods after rainfall events in the dry season. This pasture showed a characteristic relationship between denitri®-cation rate and soil moisture content. Low soil
moist-ure content was the primary factor limiting
denitri®cation during the summer and early autumn.
Changes in soil NO3ÿ concentration, respiration rate
and soil temperature had opposite temporal patterns
to changes in denitri®cation rate. About 4.5 kg N haÿ1
annual N loss by denitri®cation was estimated. Deni-tri®cation does not appear to be a major pathway for loss of N from this pasture.
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