Sources of N2O-N following simulated animal treading of ungrazed pastures

It has been previously hypothesised that the treading of pastures by grazing animals can increase nitrous oxide (N2O) emissions as the result of reduced plant N uptake. In addition, grazing animals urinate and defecate on to soils which can also increase the N2O emissions. To avoid these additional N inputs, a treading machine was used in two field experiments where the pre-grazing dry matter (DM) was present or removed to test the above hypothesis. The N2O emissions were measured for 47 and 30 days afterwards, respectively. The soil nitrate (NO3−–N) pool was 15N labelled prior to treading in Experiment 2. Treading induced greater N2O emissions and reduced herbage DM yields (by 31%–41%) and soil NO3−–N concentrations when soil gravimetric water contents ranged from 0.45 to 0.63 g water/g soil. A comparison of treading in the presence and absence of pre-grazing DM showed that reduced plant N uptake did not induce greater N2O emissions. Rather, the 15N labelling of the NO3−–N pool indicated this pool contributed to the N2O emissions under treading. In addition, 15N labelling showed other soil–N pools became available as a consequence of either, (1) treading inducing soil perturbation and releasing N from the soil organic matter–N and/or plant root–N pools, or (2) as a result of simulated grazing causing the release of plant root–N. Soil water soluble carbon (C) also increased under treading in the absence of pasture, supporting the theory that organic–N was released from either the soil organic matter (OM) or plant root pools. Further research should investigate the effect of treading on the turnover and contribution of organic–N pools (roots, OM) in pasture soils in order to more fully understand the contributions these make to background N2O emissions and effects on N and C cycling in pasture soils

Determining the nitrous oxide transfer velocity and emission factor of an agricultural drain

There have been few studies examining indirect nitrous oxide (N2O) emissions (EN2O) associated with nitrogen (N) leaching from agricultural soils. For agricultural drainage water, EN2O equals the water’s N2O concentration in excess of the atmospheric value multiplied by the N2O transfer velocity (VN2O). Using this equation, tracers and chamber methods, EN2O and VN2O were measured from an agricultural drain. Estimates of VN2O were made by measuring water speed and depth using the relationship developed by O’Connor and Dobbins [1958. Mechanisms of reaeration in natural streams. Transactions of the American Society of Civil Engineers. 123:641–684]. The measurements and estimates were not significantly different and VN2O averaged 5 m d−1. Alternatively, for the method developed by the Intergovernmental Panel on Climate Change, EN2O equals the mass of N flowing in the water multiplied by an emission factor (EF). By additional measurements of the drain’s width, the water’s flow rate and nitrate (NO−3 ) concentration, the estimated EF was 1.2 × 10−4 kg N2O − N kg−1 NO−3 − N

Increasing soil aeration reduces mitigation efficacy of dicyandiamide when targeted at ruminant urine-derived N2O emissions

Soil aeration effects on the efficacy of dicyandiamide (DCD) at reducing reactive nitrous oxide (N2O) fluxes from ruminant urine patches have not been assessed. We tested the null hypothesis that increasing soil aeration and dissolved organic carbon (DOC) levels, following ruminant urine deposition, would reduce DCD efficacy, as determined from measurements of N2O fluxes. Soil aeration was controlled (−1 or −10 kPa, the former being wetter and less aerated) within a factorial experiment that also included DCD and ruminant urine (700 kg N ha−1) treatments, destructively sampled over 40 days. Increased soil aeration corresponded with a decrease in DCD efficacy. However, contrary to the null hypothesis, increasing soil DOC concentrations following urine application did not correspond with greater DCD degradation rates. In fact, at −1 kPa applying urine increased the DCD half-life from 23.5 (±3.0) to 53.3 (±7.5) days, while at −10 kPa the DCD half-life was not affected by urine application (14.9 [±2.9] and 16.7 [±2.7] days, with or without urine, respectively). The efficacy of DCD at mitigating urine-induced cumulative N2O fluxeswas95%at−1 kPaand57%at−10 kPa.Thisstudyconfirmsthatthedegradation of DCD is slower, and thus its efficacy is better, in wet soils compared with well-aerated soils

Soil aeration affects the degradation rate of the nitrification inhibitor dicyandiamide

Dicyandiamide (DCD) is a nitrification inhibitor of variable efficacy. In soils, DCD biodegradation rate is known to be a function of temperature; however, microbial activity can also be affected by soil aeration and substrate availability. Studies determining the effects of soil aeration on DCD degradation are few. We tested the null hypothesis that the rate of degradation of DCD in soil would be the same under aerobic and anaerobic conditions. Soils from two sites with different organic matter concentrations but the same parent material were sampled to the same depth, sieved, and repacked into tubes (‘soil cores’). These were saturated with a DCD solution (30 mgmL–1) and placed under controlled aeration conditions by imposing five levels of matric potential (0, –1, –3, –6, and –10 kPa) at a constant temperature (228C). The relative O2 diffusivity (O2 diffusion coefficient in soil/O2 diffusion coefficient in air, Dp/Do) was measured, along with periodic destructive sampling of soil cores over 40 days, to assess the DCD concentrations. Fitting first-order exponential functions to plots of soil DCD concentration v. time showed that the DCD degradation rate was greater (P < 0.05) when the soil was aerobic (Dp/Do _0.01). Consequently, the null hypothesis was rejected. These results show that soil aeration determines the degradation rate of DCD

Temporal in situ synmics of N20 reductase activity as affected by nitrogen fertilization and implications for the N20/(N20+N2) product ratio and N20 mitigation.

In vitro, high nitrate (NO3) concentrations significantly inhibit N2O reductase activity. However, little information is available on the in situ temporal effects of excessive N fertilization on soil N2O reductase activity and the regulation of the N2O/(N2 + N2O) product ratio in agricultural soil. This study examined the monthly in situ dynamics of NO3 − concentration, N2O reductase activity, and N2O/(N2 + N2O) product ratio for 2 years in loamy soil that had received either continuous N fertilizer at 400 kg N ha−1 year−1 for 15 years (N400) or no N fertilizers (CK). N2O reductase activity was significantly lower under the N400 treatment than under the CK and correlated negatively with soil NO3 − concentration. The decrease in N2O reductase activity resulted in the N2O/(N2 + N2O) product ratio increasing. These results demonstrate that excessive N fertilization has the potential to increase N2O emissions by reducing N2O reductase activity in soils. These results highlight the need for N2O mitigation options to embrace the reduction of soil NO3 − concentrations

Irrigation of DOC-rich liquid promotes potential denitrification rate and decreases N2O/(N2O+N2) product ratio in a 0–2 m soil profile

Lack of dissolved organic carbon (DOC) is generally one of the key factors limiting denitrification in subsoil beneath the root zone. Despite a number of laboratory DOC amendment studies, the effects of in situ DOC infiltration on subsoil denitrification, and on subsequent end product composition, are less understood. Here, we report on the effects of in situ infiltration of a DOC-rich liquid, derived from decomposing straw, on potential denitrification rate (PDR), N2O/(N2O + N2) product ratio, and nitrate stock in a 0–2 m soil profile. The results showed that in situ infiltration with a DOC-rich liquid (100 mm, 2 ton DOC ha−1) significantly increased the DOC concentration and PDR, and significantly decreased the N2O/(N2O + N2) product ratio in the soil profile. Up to 70% of the nitrate accumulated in the 0–2 m soil profile disappeared within three weeks following the infiltration of the DOC-rich liquid. The majority of the nitrate removed could be accounted for by denitrification. The predominant end product of denitrification was N2. The mass ratio between the consumed DOC and nitrate-N was about 5. Our results demonstrate the significant potential for removing subsoil nitrate by in situ introduction of DOC generated from the above-ground crop biomass

Ammonia oxidising populations and relationships with N2O emissions in three New Zealand soils

Nitrous oxide (N2O) emissions from intensively grazed pasture systems contribute significantly to total greenhouse gases. A series of field-plot experiments were conducted at three geographically separated sites to study the ammonia oxidising microbial populations and their relationships with N2O emissions. Our results support the findings from earlier incubation experiments, showing that growth of ammonia oxidising bacteria (AOB), and not ammonia oxidising archaea (AOA), was stimulated by the high concentration of urine. There was substantial spatial variability of the AOB and AOA amoA gene copy numbers at different sites and within replicate samples. AOB abundance was generally higher in the top 2.5 cm compared with the top 10 cm of soil, whereas the opposite trend was observed for AOA. Positive relationships were found between the N2O emissions and AOB abundance, although the relationship varied at the three sites

Compaction influences N2O and N2 emissions from 15N-labeled synthetic urine in wet soils during successive saturation/drainage cycles.

Nitrous oxide emitted from urine patches is a key source of agricultural greenhouse gas emissions. A better understanding of complex soil environmental and biochemical regulation of urine-N transformations in wet soils is needed to predict N2O emissions from grazing and also to develop mitigation technologies. Soil aeration, gas diffusion and drainage are key factors regulating N transformations and are affected by compaction during grazing. To understand how soil compaction from animal treading influences N transformations of urine in wet soils we applied pressures of 0, 220 and 400 kPa to repacked soil cores, followed by 15N-labelled synthetic urine and then subjected the cores to three successive saturation-drainage cycles on tension tables from 0 to 10 kPa. Compaction had a relatively small effect on soil bulk density (increasing from 0.81 to 0.88 Mg m-3), but strongly affected the pore size distribution. Compaction reduced the total soil porosity and macroporosity. It also affected the pore size distribution, principally by decreasing the proportion of 30-60 µm and 60-100 µm pores and increasing the proportion of micropores (30 µm). Rates of urine-N transformations, emissions of N2 and N2O, and the N2O to N2 ratio were affected by the saturation/drainage cycles and degree of compaction. During the first saturation-drainage cycle, production of both N2O and N2 was low (< 0.4 mg N m-2 hr-1), probably because of anaerobic conditions inhibiting nitrification. In the second saturation/drainage cycle, the predominant product was N2 at all compaction rates. By the third cycle, with increasing availability of mineral-N substrates, N2O was the dominant product in the uncompacted (max = 4.70 mg N m-2 h-1) and 220 kPa compacted soil (max = 7.65 mg N m-2 h-1) with lower amounts of N2 produced, while N2 was produced in similar quantities to N2O (max = 3.11 mg N m-2 h-1) in the 400 kPa compacted soil. Reduced macroporosity in the most compacted soil contributed to more sustained N2 and N2O production as the soils drained. In addition, compaction affected the rate of change of pH and DOC, both of which also affected the N2O to N2 ratio. Denitrification during drainage and re-saturation may make a large contribution to N2O emissions. Improving soil drainage and adopting grazing management practices that avoid soil compaction while increasing macroporosity will reduce total N2O and N2 emissions

Electron shuttle potential of biochar promotes dissimilatory nitrate reduction to ammonium in paddy soil

Enhancing dissimilatory nitrate reduction to ammonium (DNRA) is environmentally and agronomically beneficial due to DNRA improving nitrogen (N) retention in soil. However, the rate of DNRA is generally considerably lower than that of denitrification because DNRA requires more electron donors than denitrification. Biochar has been increasingly reported to act as an “electron shuttle” to facilitate electron transfer and to promote redox reactions in soil. Thus, this study aimed to investigate whether and how biochar could enhance the DNRA process in a paddy soil. The results showed that, compared with the no-biochar control, the application of rice straw biochar increased the DNRA rate from 0.2 to 0.7 mg NH4+-N kg−1 dry soil d−1. As well, biochar simultaneously, increased the relative abundance of DNRA functional microbes (nrfA-type microbes) and functional gene (nrfA) expression levels. Biochar's enhancement of DNRA was positively correlated with the biochar properties relevant to electron shuttling (e.g., specific capacitance). In contrast, the application of electron shuttle-weakened biochar (oxidized by H2O2) did not increase, or even decreased, the DNRA rate in the paddy soil. These results demonstrate that biochar can act as an electron shuttle to enhance electron availability for DNRA functional microorganisms and consequently promote the DNRA process in paddy soil. Our results indicate that amendment of paddy soil with biochar containing a high-capacity electron shuttle function is beneficial for preserving N by transforming the mobile nitrate anion into the less mobile ammonium cation in paddy soils

Determining EF1 for farm dairy effluent and urea fertiliser

Presentation to MPI and fellow N2O and CH4 scientists at NzOnet/Methanet meeting, 7-8 May 2015