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A better understanding of the spatial distribution of denitrifiers and their activity may lead to an improved understanding of the &nitrification process in soil. This study determined the spatial distribution of the total bacterial community (16S rRNA), components of the denitrifier community ([cnorB.sub.p]. Pseudornonas mandelii and related species; nosZ), and denitrification activity across a range of soil aggregate size fractions (4-8, 1-2, and 0.25-0.5 mm) under permanent grassland (PG) and arable cropping (AC) systems. Aggregate size fraction had no significant effect on the abundance of the nosZ or [cnorB.sub.p] gene-bearing bacteria in soil from the AC system. The highest abundance of denitrifier bacteria was measured in the smallest size fraction in the PG system. Respiration did not differ among aggregate size fractions within the PG system; however, respiration was higher for the PG system than the AC system for all aggregate size fractions. For the AC system, higher respiration was measured in the 0.25- to 0.5-mm aggregate fraction than the 4- to 8-mm aggregate fraction. Denitrifying enzyme activity (DEA) was higher in the largest size fraction of the PG system than the AC system; however, DEA did not differ among aggregate size fractions within each management system. Cumulative denitrification during a 72-h incubation was significantly higher in the largest aggregate size fractions under both management systems. The results indicate that the differences among the aggregate size fractions were small in magnitude and that the spatial location of the denitrification activity and the abundance of the denitrifier bacteria were uncoupled across aggregate size fractions in the contrasting management systems. Abbreviations: AC, arable cropping; DEA, denitrifying enzyme activity; EOC, extractable organic carbon; PCR, polymerase chain reaction; PG, permanent grassland. doi: 10.2136/sssaj2008.0357

The influence of liquid dairy and swine manure, soil water-extractable C (WEC) and glucose on the amount of denitrification, the [N.sub.2]O molar ratio [[N.sub.2]O/([N.sub.2]O + [N.sub.2])], and the abundance of soil denitrifiers was investigated using repacked soil cores. Quantitative polymerase chain reaction was used to measure the abundance of the total bacteria (16S rRNA gene), two denitrifier guilds bearing the cnorB gene, and nosZ gene communities. The availability of both C and N oxides influenced the amount of denitrification and the [N.sub.2]O molar ratio for simple (glucose) and complex (liquid manure and WEC) C sources. A positive relationship ([R.sup.2] = 0.77) was measured between respiration (cumulative C[O.sub.2] emissions) and total denitrification. A negative relationship ([R.sup.2] = 0.70) was found between respiration and the [N.sub.2]O molar ratio, demonstrating that C availability in soil promotes the reduction of [N.sub.2]O to [N.sub.2]. The abundance of the 16S rRNA and cnor[B.sub.B] (Bosea/Bradyrhizobium/Ensifer spp.) gene bearing bacteria was not significantly affected by the addition of C. The abundance of cnor[B.sub.P] (Pseudomonas mandelii and related species) gene bearing bacteria increased in response to glucose and manure addition, but only when also amended with N[O.sub.3.sup.-]. Significant positive correlations were found between the abundance of the cnor[B.sub.P] gene bearing bacteria and respiration. The nosZ gene bearing bacteria significantly increased only in soil amended with liquid manures. No significant relationships were found between the abundance of denitrifiers and total denitrification, [N.sub.2]O emissions, or the [N.sub.2]O molar ratio. Changes in denitrifier community abundance appeared to reflect changes in C substrate availability and were uncoupled to denitrification activity. Abbreviations: EOC, extractable organic carbon; PCR, polymerase chain reaction; WEC, water-extractable carbon; WFPS, water-filled pore space.

Riparian ecosystems have the capacity to lower [NO.sub.3.sup.-] concentrations in groundwater entering from nonpoint agricultural sources. The processes responsible for decreases in riparian groundwater N[O.sub.3.sup.-] concentrations in the Willamette Valley of Oregon are not well understood. Our objective was to determine if denitrification and/or dissimilatory N[O.sub.3.sup.-] reduction to N[H.sub.4.sup.+] (DNRA) could explain decreases in groundwater N[O.sub.3.sup.-] moving from a perennial ryegrass cropping system into a mixed-herbaceous riparian area. In situ denitrification rates (DN) were not different between the riparian area (near-stream or near-cropping system) and cropping system the first year. In the second year, during the transition to a clover planting, DN was highest just inside of the riparian/cropping system border. Median denitrification enzyme activity (DEA) rates ranged from 29.5 to 44.6 mg [N.sub.2]O-N [kg.sup.-1] [d.sup.-1] for surface soils (0-15 cm) and 0.7 to 1.7 [micro]g [N.sub.2]O-N [kg.sup.-1] [d.sup.-1] in the subsoil (135-150 cm). Denitrification enzyme activity rates were not different among the zones and were most often correlated to soil moisture and N[H.sub.4.sup.+]. Nitrate additions to surface soils increased DEA rates, indicating a potential to denitrify additional N[O.sub.3.sup.-]. Based on groundwater velocity estimates, N[O.sub.3.sup.-] (3.8 mg N[O.sub.3.sup.-]-N [L.sup.-1]) entering the riparian surface soil could have been consumed in 0.2 to 7 m by denitrification and 0.03 to 1.0 m by DNRA. Denitrification rates measured in the subsoil could not explain the spatial decrease in [NO.sub.3.sup.-]. However, with the potentially slow movement of water in the subsoil, denitrification and DNRA (0 to 264 [micro]g N [kg.sup.-1] [d.sup.-1]) together could have completely consumed N[O.sub.3.sup.-] within 0.5 m of entering the riparian zone. Abbreviations: CS, cropping system; DEA, denitrification enzyme activity; DN, in situ denitrification rates; DNRA, dissimilatory nitrate reduction to ammonium; DOC, dissolved organic carbon; ECD, electron capture detector; EOC, extractable organic carbon; [[theta].sub.g], gravimetric soil moisture; MBC, microbial biomass carbon; MinC, mineralizable carbon; Rip-CS, riparian area near the cropping system; Pdp-Str, riparian area near the stream.

The importance of spring thaw nitrous oxide ([N.sub.2]O) fluxes to the total [N.sub.2]O emission budget in cold climates has been recognized recently. Two mechanisms have been proposed to explain the burst in [N.sub.2]O fluxes due to soil freezing and thawing: enhanced microbial activity due to increased nutrient availability at spring thaw, and release of [N.sub.2]O trapped at depth during winter. The objective of this study was to determine whether increased surface N20 fluxes were due to physical release at spring thaw of [N.sub.2]O accumulated all winter at depth in the soil profile, or whether fluxes were due to rapid [N.sub.2]O production in the surface layer during the thaw process. Micrometeorological flux measurements and a chamber method applied to in situ soil columns receiving [sup.15]N tracer were used in Ontario, Canada during winters of 2003 and 2004. Labeled K[sup.15]N[O.sub.3] fertilizer (60% excess [sup.15]N) at the rate of 100 kg N [ha.sup.-1] was applied to two layers, that is, surface layer 0 to 5 cm (SL) and deep layer 12 to 17 cm (DL) in nondisturbed soil columns placed in the field during the winter. The burst in [N.sub.2]O fluxes from the soil surface measured by both methods occurred within the same period of soil thawing. Denitrification was the main mechanism responsible for [N.sub.2]O production, and conditions conducive to [N.sub.2]O and [N.sub.2] production occurred both in the SL and DL during thawing. Despite high [sup.15] [N.sub.2]O concentrations at depth, the burst in [N.sub.2]O fluxes from DL soil columns were 1.5 to 5 times lower than that from SL soil columns as more [N.sub.2]O from DL was converted to [N.sub.2] before diffusing out of the soil profile. Comparison of [N.sub.2]O fluxes originating from SL and DL soil columns indicates that the source of [N.sub.2]O burst at spring thaw is mostly 'newly' produced [N.sub.2]O in the surface layer, and not the release of [N.sub.2]O trapped in the unfrozen soil beneath the frozen layers. Abbreviations: DL, deep layer 12 to 17 cm; SL, surface layer 0 to 5 cm

The influence of organic C quantity and quality on denitrification in riparian environments is poorly understood. We measured denitrification potential (DNP), organic matter, and several fractions of organic C in surface soils and subsurface sediments in a river riparian zone. Surface soils in conifer forest peat, mixed forest, and marsh sites had similar DNP, although mean organic matter ranged from 9.4% (marsh) to 19.6% (mixed forest) and 36.6% (peat). These soils also differed widely in organic C, water-extractable C, and anaerobic mineralizable C. Mean DNP in peat at depths of 0.8 to L4 m was four times lower than in the surface peat. Mean organic utter and organic C were significantly greater in the deep peat than at the surface, whereas the other C fractions were similar. Mean organic matter content of buried channel sediments at depths of 2 to 3 m was 3.6%; however, mean DNP was 75 to 80 times lower than in the surface mixed forest and marsh soils. When the three surface soil sites were considered separately, anaerobic mineralizable C showed the highest correlation with DNP in the marsh soils (r = 0.87) and the conifer peat soil (r = 0.82). Water-extractable C was also highly correlated with DNP in the marsh soils (r = 0.81). Correlations between DNP and either organic matter or the three C fractions were not significant in the deep peat, whereas the former channel sediments showed a significant relationship between DNP and both organic matter (r = 0.81) and water-extractable C (r = 0.81). These results show that C quantity and quality influence DNP, but no single index was a good predictor for all soil types.

The C quality of four aquatic plants found in wetlands explains the differences in their relative capacity to fuel denitrification. In replicated (n = 4) flow-through microcosm experiments we compared nitrate removal using equal C additions of two emergent plants (Scirpus acutus, Typha latifolia) and two floating plants (Hydrocotyle umbellata, Lemna minor). As expected, plant fitter with the highest initial C/N and lignin content (Scirpus acutus), required significantly (p < 0.05) higher dry matter addition to achieve the nitrate removals of the other plants studied. For all plants, losses in C content and significantly (p < 0.05) higher C/N ratios in leached litter is consistent with preferential hydrolysis of low molecular weight (MW) carbohydrates. Interestingly, plant-specific differences in this denitrification potential were removed when nitrate removal data was compared on an add-soluble carbohydrate (ASC) basis. As an efficiency measure, high ratios of applied C to N[O.sub.3]-N decreased denitrification potentials, whereas increasing N[O.sub.3]-N loading at equal C additions increased denitrification potentials. Differences in denitrification potential resulting from carbohydrate and lignin content of the plants studied were generally smaller than relative differences in reported productivity. That much of the plant C left the microcosms before oxidation is consistent with overall electron acceptor limitation in wetlands and suggests observable differences in field-scale denitrification performance may still arise with the presence or absence of highly productive reed (e.g., Scirpus sp., Typha sp.) stands.

Nitrous oxide emissions increase because of denitrification in the first few days after cattle (Bos taurus) slurry (CS) is applied to grassland soils fertilized with N[O.sub.3]. Denitrifying conditions are created when the readily decomposable C in the CS is oxidized by the soil microbial biomass when N[O.sub.3] is present and [O.sub.2] is deficient. Half of the readily decomposable C in CS can be volatile fatty acids (VFAs) that take up to 4 d to degrade. The timing of CS application relative to fertilizer-N[O.sub.3] application could therefore affect the losses of [N.sub.2]O and [N.sub.2]. We used the [sup.15]N gas-flux method to measure [N.sub.2]O and [N.sub.2] fluxes from grassland when CS containing 60 kg N[H.sub.4]-N [ha.sup.-1] was applied 4, 3, 2, 1, and 0 d before the application of 60 kg N [ha.sup.-1] of [K.sup.15]N[O.sub.3]. For a field experiment repeated in April, May, August, and October 1998, CS applied 3 or 4 d before KN[O.sub.3] had no significant effect in any month on the flux of [N.sub.2]O in the 124 h after KN[O.sub.3] application. On average over all months, the extra emission of [N.sub.2]ON over the control was equivalent to 0.8, 1.1, and 2.9% of KN[O.sub.3]-N for prior applications of CS at 2, 1, and 0 d, respectively. When CS was applied 4 d prior to KN[O.sub.3] there was no significant effect on the flux of [N.sub.2] in any month. The maximum loss of [N.sub.2]O + [N.sub.2] was 8.3% of the KN[O.sub.3] applied (5 kg N [ha.sup.-1]) when CS and KN[O.sub.3] were applied at the same time in April.

We present and test a new approach for measuring denitrification in unsaturated subsoils in situ. The procedure consists of applying a test solution containing [sup.15]N[O.sup.-.sub.3] to the subsoil, measuring steady-state [sup.15]([N.sub.2] + [N.sub.2]O) concentration in the pore space of the [sup.15]N-labeled soil, and determining denitrification rates by fitting measured and simulated concentrations. We adapted and evaluated a numerical, spherical diffusion model to simulate production and diffusion of [sup.15]([N.sub.2] + [N.sub.2]O) evolved from a [sup.15]([N.sub.2] + [N.sub.2]O)-producing subsoil volume surrounded by an infinite nonproducing soil volume. The effects of gas diffusivity, gas production rate, and spatial distribution of [sup.15]N[O.sup.-.sub.3] on steady-state [sup.15]([N.sub.2] + [N.sub.2]O) concentrations were investigated with computer simulations, which proved the methodological principle theoretically. Field experiments were done using two different probe designs. Initially, multipurpose probes were used for applying [sup.15]N[O.sup.-.sub.3] solution to the subsoils and subsequent collection of gas samples, but denitrification rates were unrealistically high, probably because of depressed soil gas diffusivity caused by the probe installation procedure. Nondisturbing capillary probes were used for two subsequent in situ experiments. Here, in situ [sup.15]([N.sub.2] + [N.sub.2]O) concentrations were near the detection limit and resulted in denitrification rates between 0 and 7 g N [ha.sup.-1] [d.sup.-1] 10 [cm.sup.-1], which were comparable to rates determined by laboratory reference methods. We concluded that the in situ [sup.15]N gas emission method is a promising tool for measuring subsoil denitrification because it overcomes some of the limitations of the alternative techniques.

Microbial respiration and denitrification are greatly affected by abiotic factors, but they are difficult to assess in natural environments. Under controlled conditions the interactions between temperature and soil water content on microbial respiration, N2O production, and denitrification in soil amended with animal slurries were studied. The effects of the abiotic factors on the biological processes were monitored for 8 wk in repacked soil cores amended with pig or cattle slurry. The soil cores were incubated at 43, 57, and 72% water-filled pore space (WFPS) and at 10, 15, and 20 degrees C with or without addition of 10% acetylene. The amount of N2O lost at 72% WFPS corresponded to 8 to 22% of the slurry's NH+4 content, but for only 0.01 to 0.9% at 43 to 57% WFPS. Denitrification losses at 72% WFPS accounted for 17 to 58% of the slurry's NH+4 content, but for only 0.01 to 1.2% at 43 to 57% WFPS. The amount of available C accounted for by denitrification was 8 to16% of total respiration at 72% WFPS, but only 0.03 to 0.4% at 43 to 57% WFPS. Both N2O production and denitrification peaked earlier in the cattle-slurry treated soil than in the pig-slurry treated soil, whereas the total N loss was greatest from the latter. Neither amendments nor soil water contents seemed to affect the Q(sub 10)-values for the CO2 production, resulting in values between 1.6 and 2.6. At 72% WFPS, N20 production and denitrification had Q(sub 10)-values ranging between 3.3 and 5.4. High temperatures enhanced both aerobic respiration and denitrification, and aerobic respiration further enhanced denitrification by consuming oxygen, resulting in strong sensitivity of denitrification to temperature.

Various long-term crop management strategies are known to have differing effects on soil organic C. This laboratory study explored the effect of long-term (35 yr) fertilization and crop rotation on soil organic C and denitrification capacity at different depths of a Brookston clay loam soil (fine-loamy, mixed, mesic Typic Argiaquoll). We related denitrification capacity to soil biochemical (C[O.sub.2] production, organic C, microbial biomass C, soluble organic C) and soil structural properties. Denitrification capacity was determined as the increase in [N.sub.2]O that occurred when [Mathematical Expression Omitted]-amended soils were incubated anaerobically in the presence of acetylene. Treatments included fertilized and nonfertilized plots of continuous corn (Zea mays L.), continuous bluegrass (Poa pratensis L.), and rotation corn (corn-oat [Avena sativa L.]-alfalfa [Medicago sativa L.]-alfalfa). Soils from an adjacent mixed deciduous woodlot were also sampled. Soils from the woodlot had higher denitrification capacities than the continuous or rotation corn treatments. Among the agricultural treatments, the soil under bluegrass had the greatest denitrification capacity followed by the soil under corn rotation, with the continuous corn having the lowest capacity. Long-term fertilization resulted in 35% higher denitrification capacity and 65% higher C[O.sub.2] production than nonfertilized soils. Denitrification capacity across all depths in the agricultural soils was correlated with C[O.sub.2] production ([r.sup.2] = 0.76), microbial biomass C ([r.sup.2] = 0.60), and organic C ([r.sub.2] = 0.54); however, the relationship between denitrification capacity and soil structure was not as strong ([r.sup.2] = 0.28).

The measurement of denitrification in soil by the 15N gas emission method requires determination of the [N.sub.2] mole masses 28, 29, and 30. Direct determination of 30[N.sub.2], which is not possible with all isotope mass spectrometers, can be omitted if gas samples are equilibrated prior to isotope analysis. Because a previously published off-line equilibration procedure was inaccurate and laborious, a continuous-flow procedure was developed and evaluated in this study. Analysis of 29[N.sub.2] and 28[N.sub.2] was performed with an isotope ratio mass spectrometer coupled to an elemental analyzer. The [N.sub.2] fraction of the samples was equilibrated in an electrodeless discharge unit placed within the interface connecting both instruments. Measuring sensitivity for the fraction of 15N-pool-derived [N.sub.2] (d) and 15N atom fraction of the 15N pool ([a.sub.p]) was evaluated by analyzing 15N standard gases at d = 5 x [10.sup.-6] to 5 x [10.sup.-4] and [a.sub.p] = 0.4903, corresponding to [Delta]15N = 1.3 to 68.3[per thousand]. Measured values of d and [a.sub.p] satisfactorily coincided with theoretical values. Coefficients of variation of d and [a.sub.p] ranged from 0.13 to 19.59% and from 0.66 to 12.86%, respectively. Precision was higher than the off-line equilibration procedure and also higher than methods including direct measurement of 30[N.sub.2].

Nitric oxide reacts with [C.sub.2][H.sub.2] at ambient temperature under aerobic conditions with relatively large quantities of [O.sub.2] present. We used a gas-flow system at 20 [degrees] C to determine the extent, if any, of reaction of NO with [C.sub.2][H.sub.2] under nearly anaerobic conditions with addition of trace (

Nitrification and denitrification in soil often occur in close proximity so that a substantial part of the N[[O.sub.3].sup.-] formed by nitrification afterwards diffuses to the anaerobic denitrification zone where it is reduced to [N.sub.2]. The stable N isotope based isotope pairing method was used to study nitrification and coupled nitrification-denitrification associated with a manure hot-spot in soil contained within a diffusion chamber. During the 3-wk incubation period, a very active nitrification zone developed within 0.7 to 1 mm from the aerobic-anaerobic soil-manure interface, and within this thin zone, all N[[H.sub.4].sup.+] liberated from the manure was oxidized. The short distance between the nitrification zone and the anaerobic manure facilitated coupled nitrification-denitrification, which turned out to be the main sink for liberated inorganic N with maximum rates of 140 nmol N-[N.sub.2] [cm.sup.-2] [h.sup.-1], thereby removing 90% of the inorganic N liberated during the incubation period. The applicability of the isotopic pairing method was verified by using variable concentrations of [Mathematical Expression Omitted] and by selectively inhibiting nitrification with acetylene. The change in 14N[[O.sub.3].sup.-] profile following addition of acetylene was simulated by a diffusion reaction model to obtain estimates of the underestimation of denitrification by use of the commonly used acetylene blockage technique. Incubation periods with acetylene of 1, 3, 6, and 12 h reduced the coupled nitrification-denitrification rate to 46, 25, 16, and 10%, respectively, of the original rate, and use of the acetylene technique for determining denitrification in manure-amended soils thus cannot be recommended.

The denitrification potential of subsoils and aquifers must be characterized to assess the ultimate fate of soil N. This experiment was conducted to study changes in the subsurface distribution and activity of denitrifying bacteria as a result of cultivation. We examined soil profiles for their capacity (N addition) and potential (N + C addition) to denitrify as well as for denitrifying bacterial numbers in a Reading silt loam (fine, mixed, mesic Typic Argiudoll) soil under grassland and cultivation. Denitrifying enzyme activity was undetectable throughout the subsurface of soil profiles at both sites. Overall, the cultivated site had significantly higher N[[O.sub.3].sup.-] concentrations, denitrifier populations, and denitrification potential. Denitrifying bacteria were stratified in the vadose zone at the prairie site, with lowest numbers and lowest denitrification potential occurring at the interface of the water table. Although C addition enhanced denitrification at this site, the profile was not nearly as limited by C as that of the cultivated site. The results demonstrated that increased N inputs and cultivation during approximately 50 yr changed the denitrifying population and denitrification potential in the vadose and saturated zones of soils.

Acetylene inhibition of [N.sub.2]O reduction to [N.sub.2] is traditionally used to estimate denitrification. Although NO is also produced in the denitrification process, the effect of [C.sub.2][H.sub.2] on NO production in denitrification has not previously been examined, mainly because [C.sub.2][H.sub.2] interferes with chemiluminescent analysis of NO. We used a gas-flow system to study the effect of [C.sub.2][H.sub.2] on NO and [N.sub.2]O production in soil columns under denitrifying conditions and developed a method to avoid [C.sub.2][H.sub.2] interference of NO analysis. Nitric oxide, [N.sub.2]O, and [C.sub.2][H.sub.2] were measured in [Mathematical Expression Omitted]-amended Brookston clay loam (fine-loamy, mixed, mesic Typic Argiaquoll) and Fox sandy loam (fine-loamy over sandy or sandy-skeletal, mixed, mesic Typic Hapludalf) with various moisture contents with and without added glucose during 50-h anaerobic incubations. In glucose-amended Brookston soil with [approximately equal to]8% [C.sub.2][H.sub.2] added to the [N.sub.2] carrier gas, net NO production rates were increasingly inhibited after the first 24 h, and by 50 h were only 2 to 7% of the rates obtained in the absence of [C.sub.2][H.sub.2]. Without glucose amendment and with [C.sub.2][H.sub.2], net NO production rates decreased by 24 to 62% in the Brookston soil. In the Fox soil inhibition occurred almost immediately on [C.sub.2][H.sub.2] addition and net NO production during 50 h was reduced by 36 to 62% with glucose amendment, and by 65 to 76% without glucose amendment. In most cases, net [N.sub.2]O production rates were decreased by [C.sub.2][H.sub.2] due to the decrease in production of its precursor, NO, and in some cases, increased due to inhibition of [N.sub.2]O reduction to [N.sub.2]. Our data suggest that the [C.sub.2][H.sub.2] inhibition method may underestimate total denitrification rates under low water contents or high levels of C substrate.

Previous research suggested that using controlled drainage to elevate the water table reduces [Mathematical Expression Omitted] contamination of surface water by enhancing denitrification. The [C.sub.2][H.sub.2] inhibition technique was used to study denitrification and [N.sub.2]O evolution using 56-cm-long, undisturbed cores of Cape Fear loam (clayey, mixed, thermic Typic Umbraquult) maintained in the field and subjected to three static water table levels (15, 30, and 45 cm). Results showed that intermittent [C.sub.2][H.sub.2] exposure did not (i) affect soil inorganic N distribution between [Mathematical Expression Omitted] and [Mathematical Expression Omitted], (ii) diminish inhibition of [N.sub.2]O reduction during subsequent exposure, or (iii) induce [C.sub.2][H.sub.2] decomposition. Denitrification from 1 Nov. 1993 through 21 Apr. 1994 (172 d) was 341 kg N [ha.sup.-1] for the 15-cm water table treatment, 260 kg N [ha.sup.-1] for the 30-cm water table treatment, and 86 kg N [ha.sup.-1] for the 45-cm water table treatment. Denitrification was maximum at the lowest moultored zone (36-54 cm) for each water table treatment. Total [N.sub.2]O evolution was 9 kg N [ha.sup.-1] for the 15-cm water table treatment, 4 kg N [ha.sup.-1] for the 30-cm water table treatment, and 2 kg N [ha.sup.-1] for the 45-cm water table treatment. Nitrous oxide evolution was positively correlated with mean soil temperature (10-cm depth) until low [Mathematical Expression Omitted] levels appeared to limit denitrification. Since steady-state diffusion was not reached, estimates of [N.sub.2] evolution using [N.sub.2]O evolution in the presence of [C.sub.2][H.sub.2], were underestimated 12-fold. Evolved [N.sub.2]O-N represented only 2% of denitrification in the soil core (0-54 cm) for each water table treatment. Drainage control to elevate the water table enhanced denitrification and [N.sub.2] evolution, reducing the potential for N transport with subsurface drainage to surface water.

The analysis of two soil samples (1, 2) from two Minnesota Histosols suggest that the net N-mineralization rates of 1 range between 0.30 to 2.0 mg N per kg per d at 4 to 22 degree centigrade respectively, indicating the advantage of post-harvest mineralization to provide N to the next crop. Concurrently, nitrification and denitrification occurred with higher nitrification rates rising from 4 to 22 degree centigrade. In 1 nitrification is vigorous than other mineral soils but that in 2 is similar to many mineral soils.

A process of simultaneous measurement of CO2, N2, N2O and NO formation in soil cores to study denitrification in soil cores reveals that N2O production rates can be calculated correctly up to 0.04 micro g N/kg per measuring interval with calculation of N2, CO2 and O2 production rates being less sensitive. The technique does not affect the biochemical process in the soil and is completely automated to facilitate highly time-resolved measurements for longer periods.

The use of N2O and O2 microsensors enables the study of denitrification in soil microenvironments, which had never been possible by conventional methods. The modified method using microsensors also enables the determination of the spatial distribution of oxygen respiration. The method reveals that aggregates having diameters of 20 to 25 mm possessed anaerobic centers though they had low denitrification rates. Anoxia and high denitrifying activity rates are observed when organic matter is added to the soil microenvironment.

An evaluation of the effect of litter water content on NH3 volatilization and denitrification during storage involves the use of incubated 2 litter samples from broiler houses. N2O emission is calculated to measure denitrification, while ammonia volatilization is evaluated by calculating NH3 obtained from incubated samples without C2H2. Denitrification losses of 41% to 79% of initial NO3 and ammonia volatilization loss of 32% to 139% of initial NH4 suggest that N losses can be minimized by storing poultry litter in dry conditions.

Biological denitrification is affected by many environmental factors that control the amount of N2 and N2O entering the atmosphere. This study was conducted to measure the effect of water-filled pore space (WFPS), available C, and soil NO3 concentration on total denitrification (N2 + N2O), using acetylene (C2H2) inhibition, and to ascertain if denitrification could be estimated from N2O measurements in the field using an average N2/N2O ratio. Repacked cores of four benchmark soils were brought to 60, 75, and 90% WFPS by applying treatments of glucose-C (0, 180, and 360 kg ha to the -1) and NO3-N (0, 50, and 100 kg ha to the -1). The cores were incubated at 25 degrees C, with and without 100 mL C2H2 L to the -1, for 5 d during which daily gas samples of the headspace were analyzed for N2O and CO2. Total N loss due to denitrification generally increased as soil texture became finer and WFPS increased. The only exception to this was the C-amended sand, where N losses up to 26 and 66% were recorded at 60 and 75% WFPS, respectively. Denitrification rates at high N concentrations were quite small in the absence of an available C source but increased with increasing available C (glucose). The N2/N2O ratio generally increased with time of incubation after the initial treatment application. The largest ratios (up to 549) were found at the highest available C rate and generally at the highest soil water content. The presence of high NO3 concentrations apparently inhibited the conversion of N2O to N2, resulting in lower N2/N2O ratios. Using an average N2/N2O ratio for estimation of denitrification from N2O field measurements cannot be recommended because of the variation in this ratio due to the many environmental factors altered by field management that influence denitrification and the relative production of N2 and N2O.

A set of formulas for use in mass spectrometric studies of nitrogen and nitrous oxide emissions is presented. These formulas can be used regardless of the initial conditions in the setup. The calculations assume a homogenous soil N pool and are expressed in terms of fractions of 15N and 30N(sub2) in the original and final atmospheres. Algorithms for the conversion of direct instrumental readings into the required parameters are detailed.

Soil properties in the sample area exhibited three patterns, random, diagonal and depression-centered, which was the pattern shown by denitrification. Topography had a significant influence on denitrification but was not the controlling factor. Soil aeration was the most important parameter affecting denitrification at the landscape and the landform-element scales. Strong correlations between volumetric water content, standard redox potential and denitrification rate suggest that water concentration was another significant factor. Landscape analysis assisted in the identification and analysis of causal relationships between the measured parameters and denitrification.

The effects of water on nitric oxide (NO) and nitrous oxide (N2O) production under conditions favoring denitrification were studied. The total NO + N2O production increased with increasing water content, with more N2O being produced at higher water content. Increasing water content favored the conversion of NO to N2O. Oxygen decreased NO production, specially at low water levels. Production of NO at the three oxygen concentrations converged at higher soil water contents.

Denitrification N losses from soils under no-tillage (NT) can reduce N-use efficiency and destroy the ozone layer if appreciable amounts of N oxides ([N.sub.2]O, NO) are lost. The objective of this study was to evaluate the effect of surface-applied urea rate (0, 70, and 210 kg N [ha.sup.-1]) and different application times [planting (PL) and six-leaf stage (V6)] on denitrification losses (DLs) in NT irrigated maize (Zea mays L.). The field experiment was carried out in the 1996 and the 1998 growing seasons, at Balcarce (37 [degrees] 45'S; 58 [degrees] 18'W), Argentina, on a Typic Argiudoll and a Petrocalcic Paleudoll. The annual mean rainfall in the area is 870 mm and annual mean temperature is 13.7 [degrees] C. Denitrification rates (DRs; [N.sub.2]O) were measured using [C.sub.2][H.sub.2] blockage core method. The DRs were highest between PL and the V6 because of higher water filled pore space (WFPS), and were closely associated with the WFPS ([r.sup.2] equal to 0.66 and 0.76 for 1996 and 1998, respectively). On average, accumulated DLs for the PL application were 7.6 and 9.8 kg [N.sub.2]O-N [ha.sup.-1] (5.5 and 2.6% of N applied for 70 and 210 kg N [ha.sup.-1], respectively). However, for application at the V6, accumulated DLs were 2.0 and 2.1 [N.sub.2]O-N kg [ha.sup.-1] (1.0 and 0.4% of N applied for 70 and 210 kg N [ha.sup.-1], respectively). The results indicate that for this soil and climatic zone, denitrification N losses were greater when the fertilizer was applied at PL than when fertilizer was applied at the V6.

Readily decomposable C in organic manures could enhance denitrification of [NO.sub.3] existing in soil. The worst scenario occurs when farmers apply manures and [NO.sub.3]-containing fertilizers at the same time to meet the nutrient requirements of the next crop. If the denitrification produced [N.sub.2]O rather than [N.sub.2], the emission factor of 1.25% of fertilizer N used to calculate national inventories for [N.sub.2]O would be an underestimate for this farming practice. We used the [sup.15]N gas-flux method to measure [N.sub.2]O and [N.sub.2] fluxes from grassland when cattle slurry (CS) containing 60 kg [NH.sub.4]-N [ha.sup.-1] and [KNO.sub.3] (60 kg N [ha.sup.-1]) were applied at the same time. By labeling the [KNO.sub.3] and the [NH.sub.4] in CS, we quantified the processes producing [N.sub.2]O and checked for [N.sub.2] production by microbial processes other than denitrification. On average over field experiments replicated in March, May, August, and September 1997, CS increased the flux of [N.sub.2]O by 0.63% of the applied [NO.sub.3]-N in the 104 h after application, but had no significant effect on the flux of [N.sub.2]. The maximum flux of [N.sub.2]O was always observed in the first measurement period (5-7 h) after CS application. All of the [N.sub.2]O was formed by reduction from [NO.sub.3] apart from in August when 10% was formed by nitrification in the CS treatment. There was no evidence for production of [N.sub.2] by other processes such as heterotrophic nitrifier denitrification or anaerobic [NH.sub.4] oxidation.