Soil N2O Concentration Research Articles

The divergent role of straw return in soil O2 dynamics elucidates its confounding effect on soil N2O emission

The divergent effects of straw returns on nitrous oxide (N2O) emissions from soil require elucidation of the underlying mechanisms and factors that explain the inconsistency in in-situ conditions. We conducted a field experiment based on a long-term trial under different regimes of nitrogen (N) fertilization and straw management, complemented by laboratory incubation experiments involving visualized O2 dynamics imaging. In the field trial, we performed hourly basis high-time-resolution measurements of soil matrix oxygen (O2), N2O concentrations and fluxes during N2O “hot moment” events. We found that straw return increased cumulative N2O emissions by 32.7% under conventional high N input (Ncon), but showed no effect on N2O emission under optimized N input (Nopt). In situ O2 content and further microcosm experiments with visualized O2 spatiotemporal distribution suggested that long-term straw return increases porosity and soil O2 content, which reduced N2O emission under low N substrate conditions by improving soil pore structure and aeration during “hot moment” events. By contrast, straw return increased N2O emission via creating short-term O2 depletion zone and triggering denitrification in anoxic microsites when excess N substrate was available. Although straw return showed inconsistent effects on N2O emission under different N application rates, it consistently decreased N2O concentration in the soil matrix during the "hot moment" events, suggesting that straw return increases the transport of the produced N2O in soil matrix to the soil surface. Our study underscores the multifaceted role of straw return in soil O2 dynamics, i.e., stimulating O2 consumption in a short-term microscale of soil, but increasing soil porosity in a long-term mesoscale of soil. This explains the confounding effects of straw management on the production and transportation of soil N2O in situ and emphasizes the importance of optimized N fertilization for reducing the “hot moment” N2O emissions when straw is incorporated.

The electrochemical mechanism of biochar for mediating the product ratio of N2O/(N2O + N2) in the denitrification process

The biochar electrochemical properties and surface functional groups significantly impact N2O production and reduction during denitrification process. However, its effects on N2O emissions during the denitrification process and its electrochemical mechanisms remain unclear. The study examined the impact of pristine and oxidized biochar combined with two types of nitrogen fertilizers on the N2O/(N2O + N2) ratio and N2O emissions in an incubation experiment with seven treatments: (1) CK (no application of chemical fertilizer); (2) N1 (applying (NH4)2SO4); (3) N1B ((NH4)2SO4 + pristine biochar); (4) N1BO ((NH4)2SO4 + oxidized biochar); (5) N2 (applying KNO3); (6) N2B (KNO3 + pristine biochar); (7) N2BO (KNO3 + oxidized biochar). The study found that in comparison with applying nitrogen fertilizer alone, combining pristine biochar decreased soil N2O concentration by 7.1 %–85.8 %, while combining oxidized biochar increased it by 15.7 %–125.6 %. Applying pristine biochar reduced N2O/(N2O + N2) ratio by 10.4 %–86.2 %, whereas applying oxidized biochar increased it by 12.9 %–121.6 %. The application of pristine biochar increased the nosZ gene abundance and decreased the (nirS + nirK)/nosZ ratio, which contributed to reducing N2O to N2. Compared with oxidized biochar, the oxygen-containing functional groups of pristine biochar decreased by 46.6 %, and it possessed a higher specific surface area (23.01 m2 g−1) and electrical conductivity (0.003 mS cm−1). The correlation analysis showed that DOC and inorganic nitrogen were the key environmental factors affecting N2O emissions. Additionally, the electrical conductivity, specific capacitance, and oxygen-containing functional groups of the biochar were identified as the main factors driving N2O emissions. The SEM analysis suggested that the indirect influence of biochar electrochemical properties on N2O emissions was greater than its direct influence. Our work provides fresh perspectives on reducing soil N2O emissions and establishes a theoretical foundation for the subsequent preparation of biochar materials with enhanced N2O reduction capabilities.

Effect of mulching and biochar addition on the distribution and emission characteristics of N2O from furrow-ridge tillage soils

Mulching and biochar are increasingly used individually in agriculture, but little is known about their combined effects on N2O distribution and dispersion in ridge and furrow profiles. We conducted a 2-year field experiment in northern China to determine soil N2O concentrations using the in situ gas well technique and calculate N2O fluxes from ridge and furrow profiles by the concentration gradient method. The results showed that mulch and biochar increased soil temperature and moisture and altered the mineral nitrogen status, leading to a decrease in the relative abundance of nitrification genes in the furrow area and an increase in the relative abundance of denitrification genes, with denitrification remaining as the main source of N2O production. N2O concentrations in the soil profile increased significantly after fertiliser application, and N2O concentrations in the ridge area of the mulch treatment were much higher than those in the furrow area, where vertical and horizontal diffusion occurred. Biochar addition was effective in reducing N2O concentrations but had no effect on the N2O distribution and diffusion pattern. Soil temperature and moisture, but not soil mineral nitrogen, explained the variation in soil N2O fluxes during the non-fertiliser application period. Compared to furrow-ridge planting (RF), furrow-ridge mulch planting (RFFM), furrow-ridge planting with biochar (RBRF) and furrow-ridge mulch planting with biochar (RFRB) resulted in 9.2%, 11.8% and 20.8% increases in yield per unit area and 1.9%, 26.3% and 27.4% decreases in N2O fluxes per unit of yield, respectively. The interaction between mulching and biochar significantly affected the N2O fluxes per unit of yield. Biochar costs aside, RFRB is very promising for increasing alfalfa yields and reducing N2O fluxes per unit of yield.

Soil pore structure mediates the effects of soil oxygen on the dynamics of greenhouse gases during wetting–drying phases

The timing and magnitude of greenhouse gas (GHG) production depend strongly on soil oxygen (O2) availability, and the soil pore geometry characteristics largely regulate O2 and moisture conditions relating to GHG biochemical processes. However, the interactions between O2 dynamics and the concentration and flux of GHGs during the soil moisture transitions under various soil pore conditions have not yet been clarified. In this study, a soil-column experiment was conducted under wetting–drying phases using three pore-structure treatments, FINE, MEDIUM, and COARSE, with 0 %, 30 %, and 50 % coarse quartz sand applied to soil, respectively. The concentrations of soil gases (O2, nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4)) were monitored at a depth of 15 cm hourly, and their surface fluxes were measured daily. Soil porosity, pore size distribution, and pore connectivity were quantified using X-ray computed microtomography. The soil O2 concentrations were found to decline sharply as soil moisture increased to the water holding capacities of 0.46, 0.41, and 0.32 cm cm−3 in the FINE, MEDIUM, and COARSE, respectively. The dynamic patterns of the O2 concentrations varied across the soil pore structures, decreasing to anaerobic in FINE (<0.01 %) and MEDIUM (0.02 %), and to hypoxic (4.42 %) in COARSE. Correspondingly, the soil N2O concentration was the highest in FINE (101 μL L−1) and the lowest in COARSE (10 μL L−1), whereas the highest surface N2O flux was observed in MEDIUM (131 μg N m−2 h−1). As soil CO2 concentrations declined, CO2 fluxes increased from FINE to MEDIUM to COARSE. Most pores of FINE, MEDIUM, and COARSE were 15–80 μm, 85–100 μm, and 105–125 μm, respectively, in terms of diameter. The X-ray CT visible (>15 μm) porosity in FINE, MEDIUM and COARSE were 0.09, 0.17, and 0.28 mm3 mm−3, respectively. The corresponding Euler-Poincaré numbers were 180,280, 76,705, and −10,604, respectively, indicating higher connectivity in COARSE than in MEDIUM or FINE. In soil dominated by small air-filled porosity which limits gas diffusion and result in low soil O2 concentration, N2O concentration was increased and CO2 flux was inhibited as the moisture content increased. The turning point in the sharp decrease in O2 concentration was found to correspond with a moisture content, and a pore diameter of 95–110 μm was associated with the critical turning point between holding water and O2 depletion in soil. These findings suggest that O2-regulated biochemical processes are key to the production and flux of GHGs, which in turn are dependent on the soil pore structure and a coupling relationship between N2O and CO2. Improved understanding of the intense effect of soil physical properties provided an empirical foundation for the future development of mechanistic prediction models for how pore-space scale processes with high temporal (hourly) resolution up to GHGs fluxes at larger spatial and temporal scales.

Tree stems are a net source of CH4 and N2O in a hemiboreal drained peatland forest during the winter period

Nutrient-rich northern peatlands are often drained to enhance forest productivity, turning peatland soils into sinks of methane (CH4) and sources of nitrous oxide (N2O). However, further attention is needed on CH4 and N2O dynamics during the winter period to fully understand the spatio-temporal variability of fluxes. Besides soil, tree stems can also emit CH4 and N2O. However, stem contribution is not considered in most biogeochemical models. We determined the temporal dynamics of winter-time CH4 and N2O fluxes in a drained peatland forest by simultaneously measuring stem and soil fluxes and exploring the relationships between gas fluxes and soil environmental parameters. During sampling (October 2020–May 2021), gas samples from Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees were collected from different tree heights using manual static chambers and analysed using gas chromatography. Soil CH4 and N2O concentrations were measured using an automated dynamic soil chamber system. Tree stems were a net source of CH4 and N2O during the winter period. The origin of stem CH4 emissions was unclear, as stem and soil CH4 fluxes had opposite flux directions, and the irregular vertical stem flux profile did not indicate a connection between stem and soil fluxes. Stem N2O emissions may have originated from the soil, as emissions decreased with increasing stem height and were driven by soil N2O emissions and environmental parameters. Soil was a net sink for CH4, largely determined by changes in soil temperature. Soil N2O dynamics were characterised by hot moments—short periods of high emissions related to changes in soil water content. Tree stem emissions offset the soil CH4 sink by 14% and added 2% to forest floor N2O emissions. Therefore, CH4 and N2O budgets that do not incorporate stem emissions can overestimate the sink strength or underestimate the total emissions of the ecosystem.

Soil profile N2O efflux from a cotton field in arid Northwestern China in response to irrigation and nitrogen management

It remains uncertain how different N inputs as synthetic fertilizer or manure and irrigation types affect nitrous oxide (N2O) production and effluxes in the subsurface. A field trial was carried out in 2016 to evaluate the impacts of conventional urea, animal manure, and a 50/50 mix of urea and manure on N2O production/effluxes from a cotton (Gossypium hirsutum L.) field under flood or drip irrigation in northwestern China. Soil N2O concentrations were monitored at 5, 15, 30, and 60 cm depths to assess the production and diffusion rates of N2O in the soil profile. The results showed that N2O concentrations in 0–60 cm ranged between 221 and 532 nL L−1 and averaged 344 nL L−1, which was generally lower compared to other studies in the same region. Manure and flood irrigation significantly increased N2O production at 0–5 cm and 5–15 cm, respectively. That is, the effects of nitrogen management and irrigation types on the N2O production of the profile were reflected in the surface layers and subsurface layers, respectively. All N2O production occurred in the 0–15 cm layer, with the 0–5 cm depth contributing 87%–100% of the surface emissions. The response discrepancy of N2O production/diffusion to irrigation and nitrogen management in different soil depths should be fully considered in developing agricultural N2O emission reduction measures.

Differential contributions of electron donors to denitrification in the flooding-drying process of a paddy soil

Flooding-drying (FD) is an important process causing nitrous oxide (N2O) emission in paddy field, which is mainly driven by denitrification. However, information on the contributions of the electron donors to soil denitrification during FD is limited. In this study, the potential contributions of various electron donors to denitrification during FD in a paddy soil were estimated using a 9-d soil incubation experiment without rice plants cultivation in a chamber. The results showed that during continuous flooding (CF), the soil remained very low denitrifying activity, and CH4 was the major electron donor for the denitrification with contribution of 28.85–45.23%. In the FD treatment, the soil N2O concentration and flux in 0–5 d were significantly lower than that in 5–9 d. Therefore, the estimations were based on these two phases. The Fe2+ and Mn2+ contents rapidly decreased, while the NO3−-N content slowly increased during the first 5 d of drying. However, the outstanding feature during 5–9 d drying was that the soil CH4 concentration decreased, and the cumulative oxidation of CH4 increased. The stoichiometry of the electrons showed that the potential contributions of Fe2+, Mn2+, CH4, and other electron donors to the denitrification were 51.29%, 45.88%, 2.82%, and 0% in the first phase, and 12.15%, 1.35%, 6.17%, and 80.33% in the second phase, respectively. Thus, the autotrophic denitrification driven by Fe2+ and Mn2+ would be the main mechanisms for gaseous N-loss during the first phase of drying, whereas, the other electron donors, including organic matter, were the major contributors for denitrification in the second phase. These findings should be considered when implementing measures to mitigate N2O and CH4 emissions from paddy fields.

Nitrogen addition increases CO2, CH4, and N2O concentrations of topsoils and subsoils in a subtropical Moso bamboo forest

Forest soils play a vital role in regulating the balance between the production and consumption of global soil greenhouse gases (GHGs), such as CO2, CH4, and N2O. However, the effect of N addition on GHGs concentrations at different soil depths in subtropical forests has never been elucidated. We conducted a two-year field study to investigate the effects of different N addition rates (0, 30, 60, and 90 kg N ha−1 year−1, defined as Control, N30, N60, and N90) on CO2, CH4, and N2O concentrations in the topsoil (0–20 cm) and subsoil (20–40 cm) in a subtropical Moso bamboo (Phyllostachys edulis) forest. Soil CO2 and N2O concentrations increased with soil depth and showed patterns of seasonal variation, being higher in summer and lower in winter. Soil CH4 concentrations declined with soil depth and showed erratic patterns of seasonal variations. N addition significantly increased the annual topsoil concentrations of CO2 (23.2–81.5%), CH4 (28.1–93.8%), and N2O (52.1–210.9%) and the annual subsoil concentrations of CO2 (21.5–74.2%), CH4 (36.8–109.2%), and N2O (54.5–249.2%), but did not change the seasonal patterns of the three GHGs. The CO2 concentration of the topsoil was more responsive to N addition than that of the subsoil, whereas the CH4 and N2O concentrations showed the opposite response pattern. Soil CO2 concentration was significantly and positively correlated with soil temperature and volumetric water content (VWC). Soil CH4 concentration was significantly negatively correlated with soil pH. Soil N2O concentration was significantly positively correlated with soil temperature, VWC, and soil total nitrogen. These findings indicate that N addition increased CO2, CH4, and N2O concentrations at different soil depths in Moso bamboo forests. These results contribute to a better understanding of GHG fluxes from forest ecosystem, and their responses to the increasing atmospheric N deposition.

Soil oxygen depletion and corresponding nitrous oxide production at hot moments in an agricultural soil

Hot moments of nitrous oxide (N2O) emissions induced by interactions between weather and management make a major contribution to annual N2O budgets in agricultural soils. The causes of N2O production during hot moments are not well understood under field conditions, but emerging evidence suggests that short-term fluctuations in soil oxygen (O2) concentration can be critically important. We conducted high time-resolution field observations of O2 and N2O concentrations during hot moments in a dryland agricultural soil in Northern China. Three typical management and weather events, including irrigation (Irr.), fertilization coupled with irrigation (Fer.+Irr.) or with extreme precipitation (Fer.+Pre.), were observed. Soil O2 and N2O concentrations were measured hourly for 24 h immediately following events and measured daily for at least one week before and after the events. Soil moisture, temperature, and mineral N were simultaneously measured. Soil O2 concentrations decreased rapidly within 4 h following irrigation in both the Irr. and Fer.+Irr. events. In the Fer.+Pre. event, soil O2 depletion did not occur immediately following fertilization but began following subsequent continuous rainfall. The soil O2 concentration dropped to as low as 0.2% (with the highest soil N2O concentration of up to 180 ppmv) following the Fer.+Pre. event, but only fell to 11.7% and 13.6% after the Fer.+Irr. and Irr. events, which were associated with soil N2O concentrations of 27 ppmv and 3 ppmv, respectively. During the hot moments of all three events, soil N2O concentrations were negatively correlated with soil O2 concentrations (r = −0.5, P < 0.01), showing a quadratic increase as soil O2 concentrations declined. Our results provide new understanding of the rapid short response of N2O production to O2 dynamics driven by changes in soil environmental factors during hot moments. Such understanding helps improve soil management to avoid transitory O2 depletion and reduce the risk of N2O production.

Clear-cut and forest regeneration increase soil N2O emission in Cunninghamia lanceolata plantations

Forest regeneration after clear-cut has been practiced extensively to establish a new stand. However, the effects of forest regeneration on soil N2O emission and the related changes in soil environment factors have been rarely examined in managed plantation forests. A 21-month field trial was conducted to investigate the effects of two different regeneration practices following clear-cut on soil N2O emission and related environmental factors in a Cunninghamia lanceolata plantation. The trial included three treatments, clear-cut and natural regeneration (CN), clear-cut and artificial overall regeneration (CAO), and uncut control (UC). Compared with the control, CN, and CAO increased cumulative soil N2O emissions (P < 0.05) by 76%, and 170%, respectively. Soil N2O fluxes were positively (P < 0.05) correlated with soil temperature, moisture, microbial biomass C and N, NO3−-N, water-soluble organic C and N, and NH4+-N concentrations. We conclude that clear-cut and associated natural regeneration (CN) increased N2O fluxes due to the increased soil temperature, and the increased concentrations of NO3−-N, microbial biomass C and N, and water-soluble organic C and N; and artificial overall regeneration increased soil N2O fluxes due to the increased concentrations of soil NO3−-N, microbial biomass C and N. The results highlighted that artificial overall regeneration with higher soil disturbance intensity in clear-cut stands would exacerbate soil N2O emission.

Linking soil profile N2O concentration with surface flux in a cotton field under drip fertigation

It remains unclear how the source and rate of nitrogen (N) fertilizers affect N2O concentration and effluxes along the soil profile under the drip-fertigated agricultural system. A plot-based field study was performed in 2017 and 2018 in a cotton field in arid northwestern China, with an objective to elucidate the impact of the applications of conventional urea (Urea), polymer-coated urea (ESN) and stabilized urea (SuperU) at rates of 120 and 240 kg N ha−1 on concentration and efflux of N2O in the soil profile and its relationship with N2O surface emissions. The in-situ N2O concentrations at soil depths of 5, 15, 30 and 60 cm were measured and used to estimate soil profile N2O effluxes. Estimates of surface N2O flux using the concentration gradient-based (GM) were compared with those measured using the chamber-based (CM) method. In both years, soil N2O concentrations at all depths increased in response to basal N application at planting or in-season fertigation events. However, N rate or source did not affect soil N2O concentrations or effluxes at each depth. Surface emissions of N2O were mostly associated with that presented in the top layer of 0–15 cm. Surface N2O efflux determined by GM was poorly or not associated with those of chamber measurements, which was attributed to the low N2O production restricted by soil moisture condition under the drip-fertigated condition. These results highlight the challenge of applying the enhanced efficiency N fertilizer products in the drip-fertigated agricultural system.

Regulation of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O emissions from acid organic soil drained for agriculture

Abstract. Organic soils drained for crop production or grazing land are agroecosystems with potentially high but variable emissions of nitrous oxide (N2O). The present study investigated the regulation of N2O emissions in a raised bog area drained for agriculture, which is classified as potentially acid sulfate soil. We hypothesised that pyrite (FeS2) oxidation was a potential driver of N2O emissions through microbially mediated reduction of nitrate (NO3-). Two sites with rotational grass, and two sites with a potato crop, were equipped for monitoring of N2O emissions and soil N2O concentrations at the 5, 10, 20, 50 and 100 cm depth during weekly field campaigns in spring and autumn 2015. Further data acquisition included temperature, precipitation, soil moisture, water table (WT) depth, and soil NO3- and ammonium (NH4+) concentrations. At all sites, the soil was acidic, with pH ranging from 4.7 to 5.4. Spring and autumn monitoring periods together represented between 152 and 174 d, with cumulative emissions of 4–5 kg N2O-N ha−1 at sites with rotational grass and 20–50 kg N2O-N ha−1 at sites with a potato crop. Equivalent soil gas-phase concentrations of N2O at grassland sites varied between 0 and 25 µL L−1 except for a sampling after slurry application at one of the sites in spring, with a maximum of 560 µL L−1 at the 1 m depth. At the two potato sites the levels of below-ground N2O concentrations ranged from 0.4 to 2270 µL L−1 and from 0.1 to 470 µL L−1, in accordance with the higher soil mineral N availability at arable sites. Statistical analyses using graphical models showed that soil N2O concentration in the capillary fringe (i.e. the soil volume above the water table influenced by tension saturation) was the strongest predictor of N2O emissions in spring and, for grassland sites, also in the autumn. For potato sites in autumn, there was evidence that NO3- availability in the topsoil and temperature were the main controls on N2O emissions. Chemical analyses of intact soil cores from the 0 to 1 m depth, collected at adjacent grassland and potato sites, showed that the total reduction capacity of the peat soil (assessed by cerium(IV) reduction) was much higher than that represented by FeS2, and the concentrations of total reactive iron (TRFe) were higher than those of FeS2. Based on the statistical graphical models and the tentative estimates of reduction capacities, FeS2 oxidation was unlikely to be important for N2O emissions. Instead, archaeal ammonia oxidation and either chemodenitrification or nitrifier denitrification were considered to be plausible pathways of N2O production in spring, whereas in the autumn heterotrophic denitrification may have been more important at arable sites.

Oxygen Regulates Nitrous Oxide Production Directly in Agricultural Soils.

Oxygen (O2) plays a critical and yet poorly understood role in regulating nitrous oxide (N2O) production in well-structured agricultural soils. We investigated the effects of in situ O2 dynamics on N2O production in a typical intensively managed Chinese cropping system under a range of environmental conditions (temperature, moisture, ammonium, nitrate, dissolved organic carbon, and so forth). Climate and management (fertilization, irrigation, precipitation, and temperature), and their interactions significantly affected soil O2 and N2O concentrations (P < 0.05). Soil O2 concentration was the most significant factor correlating with soil N2O concentration (r = -0.71) when compared with temperature, water-filled pore space, and ammonium concentration (r = 0.30, 0.25, and 0.26, respectively). Soil N2O concentration increased exponentially with decreasing soil O2 concentrations. The exponential model of N treatments and fertilization with irrigation/precipitation events predicted 74-90% and 58% of the variance in soil N2O concentrations, respectively. Our results highlight that the soil O2 status is the proximal, direct, and the most decisive environmental trigger for N2O production, outweighing the effects of other factors and could be a key variable integrating the aggregated effects of various complex interacting variables. This study offers new opportunities for developing more sensitive approaches to predicting and through appropriate management interventions mitigating N2O emissions from agricultural soils.

Effects of freeze-thaw cycles on soil N2O concentration and flux in the permafrost regions of the Qinghai-Tibetan Plateau

Effects of freeze-thaw cycles on soil N2O concentration and flux in the permafrost regions of the Qinghai-Tibetan Plateau

Dynamics of CO2 and N2O in Seasonal Frozen Soil Profiles for a Typical Steppe in Inner Mongolia

In order to evaluate the effect of grazing on the mechanism for greenhouse gas emissions in the seasonal frozen soils for a typical steppe in Inner Mongolia, variations of N2O and CO2 concentrations in different soil layers were monitored by an in situ gas collection system. Three conditions were selected:ungrazed since 1979 (UG79), ungrazed since 1999 (UG99), and continuously grazed (CG). The results showed that the profile soil N2O and CO2 concentrations demonstrated a significant spatio-temporal distribution. ① The average concentrations of CO2 in the soil profile of the three conditions were:CO2 in the growth period > in the freezing-thawing period > in the freezing period. The CO2 concentrations in the growing period were much higher than in the freezing-thawing period and freezing period. The CO2 concentration was the highest in the UG79, and the lowest was in the CG. The concentration of CO2 in different soil layers was ordered as 20 cm ≥ 50 cm ≥ 35 cm ≥ 10 cm ≥ 5 cm at the UG79 and UG99 sites, and 50 cm ≥ 35 cm ≥ 20 cm ≥ 10 cm ≥ 5 cm at the CG site. ② The spatial and temporal variation of N2O concentration in the soil profile was different from that of CO2. While the UG79 and UG99 sites showed a "single peak type" change, the CG site had a "bimodal" pattern. The N2O concentration of the three conditions increased sharply during the soil freezing-thawing period, and the N2O was also released weakly during the growing season at the CG site (P<0.05). The average N2O concentration of the CG was significantly higher than that of UG79 and UG99 (P<0.05). The results show that the concentration of N2O in different soil layers was ordered as follows:UG79:20 cm ≥ 50 cm ≥ 35 cm ≥ 10 cm ≥ 5 cm; CG:50 cm ≥ 35 cm ≥ 20 cm ≥ 10 cm ≥ 5 cm, and UG99:35 cm ≥ 50 cm ≥ 20 cm ≥ 10 cm ≥ 5 cm. The results concluded that grazing decreased the profiled soil CO2 concentration and increased N2O concentration, which provides a basis for the accurate estimation of greenhouse gas emissions in the seasonal frozen soil in grasslands.

Changes in snow cover alter nitrogen cycling and gaseous emissions in agricultural soils

Climate change-related increases in winter temperatures and precipitation, as predicted for eastern Canada, may alter snow cover, with consequences for soil temperature and moisture, nitrogen cycling, and greenhouse gas fluxes. To assess the effects of snow depth in a humid temperate agricultural ecosystem, we conducted a two-year field study with (1) snow removal, (2) passive snow accumulation (via snow fence), and (3) ambient snow treatments. We measured in situ N2O and CO2 fluxes and belowground soil gas concentration, and conducted denitrification and potential nitrification laboratory assays, from November through May. Snow manipulation significantly affected winter N2O dynamics. In the first winter, spring thaw N2O fluxes in snow removal plots were 31 and 48 times greater than from ambient snow and snow accumulation plots respectively. Mid-winter soil N2O concentration was also highest in snow removal plots. These effects may have been due to increased substrate availability due to greater soil frost, along with moderate gas diffusivities facilitating N2O production, in snow removal plots. In the second winter, spring thaw N2O fluxes and soil N2O concentration were greatest for ambient snow plots. Peak fluxes in ambient snow plots were 19 and 24 times greater than in snow accumulation and snow removal plots, respectively. Greater soil moisture in ambient snow plots overwinter could have facilitated denitrification both through decreased O2 availability and increased disruption of soil aggregates during freeze-thaw cycles. Overall, results suggest that effects of changing snow cover on N cycling and N2O fluxes were not solely a direct effect of snow depth; rather, effects were mediated by both soil water content and temperature. Furthermore, the fact that treatments with greatest mid-winter belowground N2O accumulation also had greatest spring thaw N2O fluxes in both years suggests the hypothesis that high spring thaw fluxes were due not only to spring soil conditions, but also to an effect of soil conditions in frozen soils that had facilitated N2O production throughout winter.

Effects of Plastic Film Mulching on Nitrous Oxide Emissions from a Vegetable Field

Considering the common cropping system (chili-radish rotation) in Southwest China, the objective of this study is to explore the effect of plastic film mulching on N2O flux, N2O concentrations in the soil profile, soil temperature, and humidity from a vegetable field. The method of the static opaque chamber was used and the experimental period lasted for one year. The results showed that the average flux of N2O was 1000.0 μg·(m2·h)-1 in the conventional treatment and 400.6 μg·(m2·h)-1 in the film mulching treatment during the chili growing period. The N2O flux of the film mulching treatment was significantly lower than that of conventional treatment (P<0.05). However, in the radish growing season, the N2O flux of the film mulching treatment was higher than that of conventional treatment, but the difference between the two treatments was not significant (P>0.05). N2O concentrations in the soil profile of the two treatments both increased with the increase in soil depth. The N2O concentrations showed significant correlations among the different soil profiles of each treatment; meanwhile, there was also a significant correlation between soil N2O concentrations at the same soil depth in different treatments. There was a significant positive correlation between the N2O concentration in different profiles and N2O fluxes in surface soils in the conventional treatment, but the N2O flux under the plastic mulching film treatment was only positively correlated with the N2O concentrations of 30 cm deep soil. The soil moisture and temperature observations showed that the effect of plastic film mulching on soil temperature was more significant in summer and the soil moisture was more prominent in autumn and winter. However, the results of the correlation analysis and principal component analysis showed that N2O emissions were mainly determined by the distribution of nitrogen in the soil and were affected by the change in soil total nitrogen content in the conventional treatment but N2O emissions were more sensitive to the variation in soil inorganic nitrogen in the film mulching treatment.

The role of maize plants in regulating soil profile dynamics and surface emissions of nitrous oxide in a semiarid environment

To study the effects of maize plants on nitrous oxide (N2O) fluxes from a dryland experimental farm, in situ soil profiles of N2O concentrations and surface emissions were investigated in a field experiment from 2014 to 2015 in the semiarid areas of northwestern China. The experiment included four treatments: unplanted and N-unfertilized soil (C0), unplanted soil fertilized with 225 kg N ha−1 (CN), maize-planted and N-unfertilized soil (P0), and maize-planted soil fertilized with 225 kg N ha−1 (PN). Surface N2O emissions and soil N2O concentrations at depths of 0, 10, 20, 30, 40, and 50 cm were measured weekly. Nitrogen fertilization significantly increased the soil N2O concentrations and surface emissions. Compared to the unplanted soil, the presence of maize plants significantly decreased the N2O concentrations at depths of 10–40 cm during the maize growing season. The modeled N2O fluxes at a depth of 10 cm presented a similar pattern to the chamber measurements. However, there was a discrepancy between the concentration gradient and chamber methods when the fluxes were high, mainly because the gradient method could not detect N2O production and consumption process above the uppermost gas sampler (0–10 cm). Soil moisture and temperature were critical factors affecting the N2O concentrations and surface emissions. The respective cumulative surface emissions and effluxes at a depth of 10 cm during the maize growing season (PN treatment) were decreased by 8.9 and 17.9% in 2014 and by 14.7 and 17.5% in 2015 compared to values of the CN treatment. This was mainly due to the decrease in soil moisture caused by the growth of the maize plants, which resulted in a soil condition less suitable for N2O production.

Nitrous oxide emissions from stems of alder, beech and spruce in a temperate forest

Although plants are recognized as conduits for soil-produced N2O, little is known about N2O fluxes from mature trees under field conditions as well as their contribution to total (soil + stem) N2O fluxes. Exclusion of tree-mediated N2O may thus underestimate total forest N2O fluxes. In the present study, our aims were to quantify tree-mediated N2O emissions and their seasonal patterns. We investigated in situ stem and soil N2O fluxes from mature alder trees on poorly-drained soil and mature beech and spruce trees on well-drained soils during March–October 2015. Alder, beech and spruce consistently emitted N2O via stems and all displayed clear seasonal patterns. Soil and air temperature, vapor pressure deficit, and soil N2O concentration influenced the temporal variability of stem N2O fluxes. Stem and soil N2O fluxes from the alder stand were higher than beech and spruce stands. Stem N2O fluxes accounted 8.8–9.8% of the total N2O fluxes in the beech and spruce stands but only 1.1% in the alder stand. Our study reveals the importance of field-based measurements across seasons in order to come up with reliable estimates of stem N2O fluxes. N2O estimates from temperate forests that are solely based on soil N2O flux measurements are probably conservative.

Weed communities and their negative impact on rice yield in no-input paddy fields in the northern part of Japan

Although weeds cause serious yield losses in agriculture, relatively few studies have been conducted on the relationship between weeds and yield loss in no-input rice production systems, where no herbicides are used. This study investigated weed communities in rice paddy fields and their influence on rice yield, by comparing 16 no-input paddy fields in northern Japan. The weed seed community in the paddy field soils, before transplanting, were analyzed using a spontaneous germination test, and the established weed communities in the fields were examined at the full heading growth stage of the rice. The weed community structures showed a marked difference between the seeds in the soil and the established weed communities. Although eight weed species germinated from the soil, only four weed species were observed in the fields at the full heading stage of the rice. The two dominant weed species, Monochoria vaginalis and Cyperaceae, accounted for 84% of the total weed biomass in the field. The multiple linear regression analysis revealed that during the period of no-input cultivation, soil NO3 concentrations, air temperature, and sunlight hours had significant effects on the biomass of M. vaginalis, but not on Cyperaceae. Furthermore, the two dominant weed species had different impact on rice yield; Cyperaceae had a large negative effect on rice yield, whereas M. vaginalis did not. These results indicated that weed management in no-input paddies should be directed towards the control of specific weed species such as Cyperaceae sp., rather than the entire weed community.