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Livestock slurry is a major source of atmospheric methane (CH4), but surface crusts harboring methane‐oxidizing bacteria (MOB) could mediate against CH4emissions. This study examined conditions for CH4oxidation by in situ measurements of oxygen (O2) and nitrous oxide (N2O), as a proxy for inorganic N transformations, in intact crusts using microsensors. This was combined with laboratory incubations of crust material to investigate the effects of O2, CH4, and inorganic N on CH4oxidation, using 13CH4to trace C incorporation into lipids of MOB. Oxygen penetration into the crust was 2 to 14 mm, confining the potential for aerobic CH4oxidation to a shallow layer. Nitrous oxide accumulated within or below the zone of O2depletion. With 102ppmv CH4there was no O2limitation on CH4oxidation at O2concentrations as low as 2%, whereas CH4oxidation at 104ppmv CH4was reduced at ≤5% O2. As hypothesized, CH4oxidation was in general inhibited by inorganic N, especially NO2–, and there was an interaction between N inhibition and O2limitation at 102ppmv CH4, as indicated by consistently stronger inhibition of CH4oxidation by NH4+and NO3–at 3% compared with 20% O2. Recovery of 13C in phospholipid fatty acids suggested that both Type I and Type II MOB were active, with Type I dominating high‐concentration CH4oxidation. Given the structural heterogeneity of crusts, CH4oxidation activity likely varies spatially as constrained by the combined effects of CH4, O2, and inorganic N availability in microsites. Oxygen penetration into surface crusts is shallow.Nitrous oxide accumulates at oxic–anoxic interfaces in surface crusts.Oxygen availability is important to high‐concentration CH4oxidation.Microbial CH4oxidation is affected by interactions of inorganic N and O2.Activity of Type I methanotrophs dominates under high CH4concentration.

Storages with liquid manure (slurry) may develop a surface crust of particulate organic matter, or an artificial crust can be established. Slurry storages are net sources of atmospheric methane (CH4), but a potential for bacterial oxidation of CH4in surface crusts was recently suggested in a study of experimental storages. The present study was conducted to investigate methanotrophic activity under practical storage conditions. Surface crusts from slurry storages at two pig farms and four dairy farms were sampled in late autumn. Mixed samples (0–4 cm depth) were used to determine changes in CH4, O2and CO2during incubation, while intact subsamples were used to characterize CH4oxidation as a function of CH4availability and moisture content. Methane oxidation was observed in all materials except for an expanded clay product (Leca) sampled from a pig slurry storage. Despite significant variation between replicate subsamples, there was a significant increase in methanotrophic activity when CH4concentrations increased from 500 to 50,000 ppmv. Maximum fluxes ranged from −1 to −4.5 g CH4m−2d−1. Surface crust samples were partly dried and then re-wetted in four steps to the original moisture content, each time followed by determination of CH4fluxes. Only one surface crust material showed a relationship between CH4fluxes and moisture content that would implicate gas diffusivity in the regulation of CH4oxidation. The occurrence of inducible CH4oxidation activity in slurry storage surface crusts indicates that there is a potential for stimulating the process by manipulation of gas phase composition above the stored slurry.

Replacement of high-input N fertilized pastures with low-input grass-legume pastures may provide a mitigation option to reduce agricultural N2O emissions. This study examined the relationship between N-cycling rates and N2O production and evolution from the root zone of grass-clover pastures of various ages (production year 1, 2 and 8). The experimental approach included cross-labelling pasture monoliths with 15N-enriched substrates to identify sources of N2O, in combination with assessment of gross N mineralization and nitrification. Nitrous oxide emissions were generally low, fluctuating between 82 and 136μg N2O–N m−2d−1, independent of pasture age. The 15N labelling indicated that at least 50% of the N2O was derived from the soil NH4+pool, approaching 100% in June. In the two year old pasture the NH4+pool contributions to N2O emissions varied significantly with sampling time. Emission rates of N2O correlated positively with soil NH4+concentrations and the NH4+supply as expressed by gross mineralization. The N2O emissions showed a significant inverse relationship with soil NO3−, but was not correlated with the supply of NO3−as expressed by gross nitrification. The ratio N2O vs. nitrification averaged 0.05% (range 0.004 to 0.29%) and varied with sampling time showing the lowest value in wet soil conditions.

The rates of respiration in light and darkness, Ci/Caand carbon isotope fractionation were investigated in glycine decarboxylase-deficient plants of barley, potato and Arabidopsis thalianagrown in climate chambers with controlled light intensity, temperature, humidity, irradiation and different CO2concentrations (360, 700 and 1400 µl l−1) and compared to the wild-type plants. All photorespiration-impaired plants exhibited higher Ci/Caand corresponding lower apparent water-use efficiencies, which were more expressed under high irradiance and elevated temperature. The mutants were depleted in 13C as compared to the wild-type plants, with a difference of up to 6‰ following growth in 360 µl l−1CO2. We determined the carbon isotope content at different CO2concentrations to calculate the contribution of both Ci/Caand photorespiration for 13C/12C fractionation. The direct effect of photorespiration was in the range of 0.7–1.0‰, from which we calculated the value of fractionation at the site of glycine decarboxylation as being 10–13‰, which is in agreement with the previously reported carbon isotope discrimination exerted by the glycine decarboxylase. Respiratory rates, particularly in the light, were increased in the glycine decarboxylase mutants. The necessity of the maintenance of a high CO2concentration near the site of carboxylation in chloroplasts in plants deficient in photorespiratory enzymes, requires an increased opening of the stomata with a corresponding decrease in water-use efficiency. It is concluded that photorespiration participates in the regulation of Ci/Caand contributes to carbon isotope fractionation, both via effects on stomata and via discrimination of 13C in the glycine decarboxylase reaction.

The distribution of moisture, degradable C, and N after direct injection of slurry can affect the turnover and plant availability of slurry N. This study examined effects of injection method, soil conditions, and slurry properties on the infiltration of several slurry components under practical conditions. The water retention capacity of 22 pig and cattle slurries was quantified by dialysis at −0.016, −0.047, and −0.100 MPa. All slurries followed the relationship: relative water loss = 1/(1 + aVS[volatile solids]), indicating that retention of liquids in the slurry injection zone can be predicted from slurry VS and soil water potential. Two‐disc injection and harrow tine injection were simulated (no slurry applied) in five trials. Two trials indicated that disc injection resulted in higher permeability compared with harrow tine injection. In a separate experiment, soil moisture and dissolved ions were monitored in and around injection slits amended with pig or cattle slurry. Moisture gradients, which were recorded with small printed‐circuit‐board (PCB) time‐domain‐reflectometry (TDR) probes, were temporally stable and reestablished following rainfall. Slit sections with pig and cattle slurry containing 13C‐acetate and 15N‐ammonium showed a shift in the 13C to 15N ratio of the injection zone within 24 h, which was explained by removal of dissolved C and/or retention of NH+4Cattle slurry was more concentrated around the injection slit than pig slurry, and greater contact between slurry and soil was obtained with harrow tine injection. The heterogeneity of slurry C and N distribution after direct injection should be accounted for in models describing slurry N turnover.

Gross N mineralization and immobilization was examined in soil amended with compost and sewage sludge on seven occasions during a year using $^{15}$N pool dilution and enrichment techniques. Gross N mineralization was initially stimulated with both wastes and accelerated through the first 112 days of incubation, peaking at 5 mg N.kg$^{-1}$.d$^{-1}$ with compost compared with 4 mg N.kg$^{-1}$.d$^{-1}$ in control and sludge-treated soil. The magnitudes of mineralization rates exceeded those of immobilization by on average 6.3 (compost) and 11.4 (sludge) times, leading to a persistent net N mineralization cumulating up to 160 mg N.kg$^{-1}$ soil (compost) and 54 mg N.kg$^{-1}$ soil (sludge) over the season from May to November. The numerical model FLUAZ comprehensively predicted rates of gross mineralization and immobilization. Sludge exhibited an early season N-release, whereas compost released only 10% of the N during the first two months of incubation. This indicates that compost should be applied well in advance of sowing in order to match crop N demands.

Fluxes of N2O and CH4were measured monthly from 1990 through 1992 along a topogradient in a spruce forest (Picea abiesL.), beech forest (Fagus silvaticaL.), riparian grassland, coastal grassland, abandoned farmland, upland arable soil, and drained arable soil in Denmark. Spatial coefficints of variation (CV) in N2O emissions ranged between 106 and 617%. A consistent spatial pattern occurred across a temporary flood water boundary in the coastal grassland. Although inorganic N levels were low at this site, N2O emission rates compared with those at the N‐fertilized sites. Maximum N2O emission generally occured in the spring and autumn. Spatial CVs in CH4fluxes ranged between 166 and 1787%, and consistent spatial patterns were found at the riparian and coastal grassland sites where CH4was produced in temporarily flooded areas at rates of 7877 and 112 mg C m−2yr−1, respectively. Uptake, as well as emission of CH4, reached maximum rates when soils dried up, presumably because CH4diffusion became unconstrained. Annual site means of N2O emission were inversely correlated with CH4uptake, and were 77 and 91 mg N and C m−2in the spruce forest, 80 and 37 mg N and C m−2in the beech forest, 66 and 69 mg N and C m−2in the riparian, 124 and 7 mg N and C m−2in the coastal grassland, 26 and 80 mg N and C m−2in the abandoned farmland, 361 and 22 mg N and C m−2in the upland arable, and 467 and 11 mg N and C m−2in the drained arable. In this study, spatial variability in trace‐gas fluxes was high and poorly explained at all sites. The results suggest large‐scale integrative measurement techniques are necessary to obtain accurate flux estimates of N2O and CH4from these ecosystems.

The factors controlling NO3removal in riparian buffer systems are poorly understood. We measured denitrification rates for two Coastal Plain, forested riparian zone soils: Kinston fine loamy sand (fine‐loamy, siliceous, acid, thermic Typic Fluvaquent) and Alapaha loamy sand (loamy, siliceous, thermic Arenic Plinthic Paleaquult). Kinston soils are more poorly drained and have higher organic matter than Alapaha soils. Surface soil and shallow aquifer samples were treated with solutions that contained chloramphenicol with either distilled water, NO3‐N, glucose‐C, or NO3plus glucose. Denitrification potentials (N2O production in the presence of acetylene) were significantly higher in Kinston soil for both depths. Surface samples from both soils showed significant responses to NO3additions but no response to C additions without NO3. Subsurface samples, taken from the top of the aquifer, showed no significant response to either NO3or C treatments for either soil. Both soils showed a high degree of stratification within the top 10 cm, with 88 and 68% of denitrification potential in the top 2 cm for Alapaha and Kinston soils, respectively. Denitrification rates in cores were much lower than in slurries but rates in cores with NO3or NO3‐plus‐glucose additions were significantly higher than unamended or C‐amended cores for the Kinston soil. Although both soils respond to NO3additions, Kinston soils are better able to reduce incoming NO3. These results indicate that denitrification in the shallow aquifer is a more important NO3removal mechanism at the Kinston site than at the Alapaha site.

Gross rates of N mineralization, assimilation, nitrification, and NOin3sup-reduction were determined in soil from a wet riparian fen by 1-day incubations of soil cores and slurries with 15N-labelled substrates. N mineralization transformed 0.1% of the total organic N pool daily in the soil cores, of which 25% was oxidized through autotrophic nitrification and 53%–70% was incorporated into microorganisms. N mineralization and nitrification were markedly inhibited below 5 cm in soil depth. At least 80% of the NOin3sup-reduction in aerated cores occurred through dissimilatory processes. Dissimilatory reduction to NHin4sup+(DNRA) occurred only below 5 cm in depth. The results show that NHin4sup+oxidation was limited by available substrate and was itself a strong regulator of NOin3sup--reducing activity. NOin3sup-reduction was significantly increased when the soil was suspended under anaerobiosis; adding glucose to the soil slurries increased NOin3sup-reduction by 2.4–3.7 times. Between 3% and 9% (net) of the added NOin3sup-was reduced through DNRA in the soil slurries. The highest percentage was observed in soil samples from deeper layers that were pre-incubated anaerobically.