Soil Water-filled Pore Space Research Articles

Deficit irrigation interacting with biochar mitigates N2O emissions from farmland in a wheat–maize rotation system

Biochar application to agricultural fields is an effective carbon sequestration measure that has the potential to reduce N2O emissions and increase soil water holding capacity. However, the interaction mechanisms of biochar under deficit irrigation on N2O emissions remain unclear. A two-year field experiment is conducted in the Guanzhong Plain, China, in order to quantify the effects of biochar and deficit irrigation on N2O emissions from winter wheat–summer maize crop rotation and to investigate the potential mechanisms of nitrification and denitrification. According to the combination of biochar application and actual evapotranspiration-based irrigation scheduling, four treatments are designed (B1W100: biochar 30 t·ha−1 + ET; B1W80: biochar 30 t·ha−1+ 0.8 ET; B0W100: no biochar + ET; B0W80: no biochar + 0.8ET). The soil N2O flux, soil physical and chemical properties, and key functional gene abundance related to N2O emissions in nitrification and denitrification at different growth stages are investigated and discussed. Results show that the interaction between deficit irrigation and biochar significantly reduces soil N2O emissions. During the wheat and maize season, the application of biochar reduces the N2O emissions by an average of 12.9% and 15.2%, respectively. Deficit irrigation also reduces the N2O emissions by an average of 17.4% and 15.5%, respectively. Pearson correlation analysis shows that soil N2O is significantly correlated with soil water-filled pore space during the phase with intense N2O emissions. Soil functional gene abundance is determined at different growth stages for both wheat and maize. Maximum soil denitrification functional gene abundance is observed at the time when wheat and maize enter the stage of their peak growth at the jointing stage. With biochar addition and deficit irrigation, the abundance of nirK and nosZ genes increases and AOB amoA genes decreases. These results suggest that biochar with deficit irrigation is a better solution to reduce N2O emissions from agricultural soils.

Emission of Greenhouse Gases and Ammonia from the Excreta of Nellore Bulls Submitted to Energy and Tannin Supplementation

Animal supplementation during the background phase may increase greenhouse gas emissions (GHG). The inclusion of tannins in the diet of Nellore bulls can mitigate nitrous oxide (N2O), methane (CH4), and ammonia (NH3) production. The objective of this study was to quantify the effect of energy supplementation associated with sources of tannins in the diet of young Nellore bulls backgrounded in pastures with N2O, CH4, and NH3 emissions. Two experiments were conducted in a completely randomized design. The treatments were three supplementation strategies: (1) soybean hulls 0.3% of body weight (BW), (2) sorghum grain 0.3% of the BW, and (3) peanut peel 0.3% of BW, the last two being sources of tannin. The static closed chambers method was used to quantify N2O and CH4 emissions and the semi-open chamber technique to estimate NH3 volatilization. Supplementation strategies did not affect the N2O emissions (p = 0.9116). The soil water-filled pore space explained the variation in the N2O fluxes (p = 0.0071). The treatments did not change the total CH4 emissions (p = 0.3599), and no explanatory variable was correlated with the CH4 fluxes. The NH3 volatilization did not vary according to the supplements or tannin inclusion (p = 0.5170). However, the type of excreta affected the NH3 volatilization (p < 0.0001). Ammonia volatilization averaged 14.05, 4.16, and 2.25% of the applied N for urine, urine + dung, and dung, respectively. The energetic supplementation of Nellore bulls containing sources of tannins in the evaluated dosages was not a mitigation strategy for the emissions of N2O, CH4, and NH3.

Integrated impacts of irrigation and nitrogen management for balancing winter wheat yield and greenhouse gas emissions

Irrigation and nitrogen (N) input are effective methods of balancing crop yields and greenhouse gas (GHG) emissions. However, the effects of irrigation and N management on the GHG emission of winter wheat production systems have not yet been well documented. Therefore, we conducted 2-year field experiments on winter wheat and studied the effects of irrigation and N reduction/replacement on GHG emissions, grain yield, global warming potential (GWP), and GHG emission intensity (GHGI). The winter wheat was irrigated once or twice with 60 or 90 ​mm each time. Five N reduction/replacement treatments were no N application (CK), 25% traditional urea (U) reduction (75%U), traditional U (U), 25% reduction in polymer-coated U (PCU), and 25% reduction in traditional U combined with organic fertilizers (U ​+ ​OM). The results indicated that N reduction/replacement significantly reduced N2O emissions, but had no significant effect on CH4 emissions. Soil water-filled pore space was the same key factor affecting the emission fluxes of N2O and CH4. Soil temperature significantly affected the fluxes of CH4 (P ​< ​0.001). Irrigation treatment did not exhibit a significant effect on grain yield, but high irrigation input increased the GWP and GHGI. Compared with U, the grain yield of CK, 75%U, PCU, and U ​+ ​OM increased by −46%, −9.2%, −1.8%, and 5.4%, respectively; GWP decreased by 35.9%, 11.6%, 7.3%, and 2.7%, respectively; and GHGI decreased by −22.1%, 3.1%, 6.0%, and 8.0%, respectively. Therefore, sustainable production goals of increasing grain yield and reducing GHG emissions were achieved with N reduction/replacement using U + OM and low irrigation input of 60 ​mm.

The effects of ectomycorrhizal and saprotropic fungi on soil nitrogen mineralization differ from those of arbuscular and ericoid mycorrhizal fungi on the eastern Qinghai-Tibetan Plateau.

Interactions between soil fungi and soil environmental factors regulate soil nitrogen (N) mineralization rates on the eastern Qinghai-Tibetan Plateau. Some studies have also illuminated differences in soil N mineralization rate based on different mycorrhizal forests, but the associated effect of soil fungal functional guilds and soil environmental factors underlying this process are not well-understood. Three primary forests respectively dominated by Abies fargesii var. faxoniana (ectomycorrhizal, EcM), Cupressus chengiana (arbuscular mycorrhizal, AM) and Rhododendron phaeochrysum (ericoid mycorrhizal, ErM) trees were selected in this area. Meanwhile, soil net N mineralization rate, soil fungal composition and soil enzyme activity among these three mycorrhizal forests were studied. Our results showed that there were significant differences in the seasonal variation of soil net N mineralization rates among three mycorrhizal forests. Soil net N mineralization rate in the AM forest was faster. EcM fungi and saprotroph are the main functional guilds in these three mycorrhizal forests. Meanwhile, the relative abundances of soil fungal functional guilds, soil temperature and soil peroxidase activity could explain 85.0% in the difference of soil net ammonification rate among three mycorrhizal forests. In addition, soil temperature, soil water-filled pore space and soil ammonium content play a central role in controlling the differing soil net nitrification rate among three mycorrhizal forests. Our results suggest differences in soil net mineralization among different mycorrhizal forest types are driven mainly by soil net ammonification. Soil fungal functional guilds and temperature regulate the rate of soil net ammonification by modulating soil peroxidase activity.

Effects of biochar addition and deficit irrigation with brackish water on yield-scaled N2O emissions under drip irrigation with mulching

Biochar has been proven to have great potential for mitigating greenhouse gas emissions and improving crop growth. However, the effects of biochar addition on yield-scaled N2O emissions in a mulched and drip-irrigated maize field using different irrigation water qualities and quantities remain unclear. A two-year field experiment was conducted to examine the effects of biochar addition, irrigation level and water salinity on soil N2O emissions, maize yield and grain N uptake in Northwest China. Eight treatments in total included the combination of two biochar addition rates of 0 t/ha (B0) and 60 t/ha (B1), two irrigation levels of full (W1) and deficit irrigation (W2, W2 = 1/2 W1) and two water salinity levels of freshwater (S0, 0.71 g/L) and brackish water (S1, 4 g/L). The soil properties, N2O emission fluxes, maize yield and grain N uptake, along with the abundances of N-cycle functional marker genes were measured during the two growing seasons. Soil water-filled pore space (WFPS), soil NH4+-N and NO3--N contents accounted for the majority of the variation in the dynamic changes of N2O fluxes. Deficit irrigation had lower N2O emissions compared with full irrigation, which was more pronounced in the second year. Relative to freshwater irrigation, brackish water irrigation increased N2O emissions under W1, but freshwater and brackish water irrigation had similar N2O emissions under W2 in both years. Biochar addition effectively mitigated N2O emissions by 29.4–31.0% and 17.9–29.2% under different irrigation treatments in 2020 and 2021, respectively. The continuous drought induced by deficit irrigation weakened the effectiveness of biochar in reducing N2O emissions. The differences in N-cycle functional marker genes could not reflect the variation in cumulative N2O emissions among treatments. Under all irrigation treatments excluding W2S1, biochar addition improved the total maize yield and grain N uptake over the two-year period by 4.06–10.74% and 5.57–8.63%, respectively. Brackish water and deficit irrigation increased grain and grain N yield-scaled N2O emissions, whereas biochar addition reduced both of them. Overall, these findings suggest that biochar addition can effectively reduce N2O emissions and increase yield and grain N uptake under brackish water and deficit drip irrigation with mulching in a maize field system, and thus adding biochar can help with achieving the sustainability of agricultural production in areas with limited freshwater resources.

Effects of Surface Mulching on the Growth and Water Consumption of Maize

This study provides understandings of the effect of mulching on the growth, development, and water consumption of dry maize. Parameters including soil temperature, soil water-filled pore space (WFPS), water storage capacity, water consumption, grain yield, water-use efficiency, and biomass yields were followed and analyzed by applying straw mulching (SM), gravel mulching (GM), and plastic film mulching (FM). The results show that the soil temperature (0–20 cm) throughout the whole observation period (2011–2013) was significantly increased by applying GM and FM, while SM reduced the soil temperature. SM increased the WFPS, while FM and GM showed no significant effect. SM and FM increased the soil water storage and water-use efficiency in the early stages of maize growth (from sowing to vegetative growth) compared with using GM. With the progress of time, fewer differences between all treatments were observed. Water consumption of the three treatments was in the order of SM &lt; FM &lt; GM, indicating that SM was the most effective in preventing water evaporation. The resulting yields of corn also varied. Compared with the control, FM significantly increased the yields by 1.7, 0.5, and 2.2 ton/ha in the tested three years, respectively. In contrast, GM showed no significant difference in the three years, and SM showed no significant difference in 2011 and 2012 but increased the yield by 2.2 ton/ha in 2013. FM is shown to be an effective method for increasing the yields of corn for the studied region, GM is not recommended, and SM is the most effective in improving the water availability in the soil, while its effect on corn yields needs to be further explored.

Drying and rewetting cycles increased soil carbon dioxide rather than nitrous oxide emissions: A meta-analysis

The increased frequency of extreme weather variations worldwide has resulted in dramatic changes in the soil water content via pronounced drying and rewetting cycles (DWCs). A comprehensive exploration of carbon dioxide (CO2) and nitrous oxide (N2O) emissions in response to DWCs can help summarize the existing results and better estimate terrestrial greenhouse gas emissions under the intensified drought and precipitation variations. This meta-analysis based on soil emissions of CO2 (868 observations, 29 studies) and N2O (52 observations, 19 studies) at the global scale investigated the direction and intensity of the changes in soil CO2 and N2O emissions in response to DWCs as controlled by experimental variables including land use type, soil texture, soil nutrients, and frequency and duration of DWCs. The results showed that, compared to the constant soil water content, DWCs led to the increase in CO2 emissions by 35.7% (95% confidence intervals ranging from 0.300 to 0.415), whereas it had no significant effect on N2O emissions (−0.2638 to 1.4751). The random-effects model indicated that soil water-filled pore space during wetting, soil clay content, days of drying and wetting, and frequency of DWCs significantly affected CO2 and N2O emissions in response to DWCs. Furthermore, potential biotic and abiotic factors affecting soil CO2 and N2O emissions under DWCs are also summarized, and it was proposed that mobility and availability of carbon substrate as well as enhanced microbial activity and abundance are the main drivers facilitating soil CO2 and N2O emissions in response to DWCs. However, soil gas diffusion or oxygen availability also dominated soil N2O emissions under DWCs. Overall, this study improves our understanding of soil CO2 and N2O emissions in response to various DWC scenarios and facilitates the development of better greenhouse gas mitigation strategies against the background of a rapidly changing climate.

Cover crop residue influence on soil N2O and CO2 emissions under wetting‐drying intensities: An incubation study

AbstractUnder increasing frequency and intensity of extreme weather events, the natural wet‐dry cycles of different intensities can induce soil carbon (C) and nitrogen (N) transformation and may contribute to increased nitrous oxide (N2O) and carbon dioxide (CO2) emissions from cultivated soils. While cover crop residue addition is a viable strategy to improve soil health, their impacts on soil greenhouse gas (GHG) emissions, especially N2O emissions, in response to wetting‐drying intensities have received limited attention. A 3‐factorial laboratory incubation was conducted using soils from a long‐term experiment to examine cover crop residue roles on N2O and CO2 emissions when exposed to different intensities of wetting‐drying cycles. These factors included (i) cover crop residue (3 levels): no cover crop, winter wheat (Triticum aestivum L.), and crimson clover (Trifolium incarnatum L.), (ii) wetting intensity (2 levels): 70% and 90% water‐filled pore space (WFPS), and (iii) drying intensity (2 levels): 10% and 40% WFPS. All treatments received the same amount of N fertiliser as urea equivalent to 100 kg N ha−1. We measured N2O and CO2 fluxes, N2O isotopomer composition, as well as dynamics of soil WFPS and biogeochemical changes after wetting. While cover crop residue addition generally exhibited higher CO2 emissions than the no cover crop treatment under all wetting‐drying intensities, their impact on N2O was more nuanced and contingent on cover crop residue type and wetting‐drying intensity. When wetted to 70% WFPS, crimson clover amended soils produced the largest daily and cumulative N2O emissions, whereas soil amended with winter wheat residues had comparable emissions to the no cover crop treatment. In contrast, cover crop residues, especially winter wheat, significantly decreased N2O emissions under extreme wetting to 90% WFPS which was often dominated by denitrification and nitrifier denitrification processes (except in winter wheat when wetted from 10% WFPS). Our results suggest that winter wheat cover crop residues mixed with N fertiliser could provide reliable N2O mitigation benefits under variable soil moisture conditions.Highlights Cover crop effects on N2O emissions from soil with varied wetting‐drying intensities were studied Increased risks of N2O emissions with increasing intensity of wetting of dry soils Crimson clover produced higher N2O emissions than no cover crop when wetted to 70% WFPS Cover crops mixed with N fertiliser reduced N2O emissions than no cover crop when wetted to 90% WFPS

Greenhouse gas emissions associated to sprinkler-irrigated alfalfa under semi-arid Mediterranean conditions

Aim of the study: Alfalfa is one the most important forage legume crop worldwide but little information is available regarding to greenhouse gas emissions (GHG) under Mediterranean sprinkler-irrigated conditions. Area of study: Middle Ebro valley, Spain Materials and methods: The GHG emissions during two alfalfa growing periods (4th and 5th stands) were evaluated using both the static method chambers and two automatic chambers coupled with a photoacoustic equipment that measured short-term gas emissions. Main results: Year-average CH4 fluxes were -0.71 g C ha-1 day-1, generally no significantly different from zero. Year-average N2O flux was 3.96 g N ha-1 day-1 with higher fluxes associated to some specific large rainfall or irrigation events. Average cumulative emissions of 865 g N ha-1 year-1 were found. We found short-term peaks of N2O (up to 160 g N ha-1 day-1) associated with high values of soil water filled pore space (WFPS) that can go unnoticed using the static chamber procedure. In spite of the higher soil NO3‾ concentration in the alfalfa-precedent field compared to the maize-precedent field, no significant differences in cumulative N2O emissions were observed in the two-month period after alfalfa or maize residues incorporation. Research highlights: Low GHG emissions were found in an irrigated alfalfa crop compared to N-fertilized crops but a deeper knowledge of the limiting factors of denitrification observed during some anoxic events (WFPS&gt;90%) is necessary to properly quantify N2O emissions in irrigated alfalfa.

Effects of water allocation process on greenhouse gas emissions in drip-irrigated apple orchards on the Loess Plateau, China

Apple tree (Malus pumila Mill.) is a perennial and deciduous tree with large water demand in the growth period. Drip irrigation, as a mature and high-efficiency water-saving irrigation technology, has been widely used in apples, grapes and other economic crops around the world, achieving huge economic benefits. However, studies addressing the effects of drip irrigation on soil greenhouse gas emissions from apple orchards on the Loess Plateau in China are limited, and little is known about the potential drivers of the global warming potential based on the water allocation processes in apple orchards under drip irrigation technology. Hence, the goal of this study was to evaluate the impact of water allocation processes on soil greenhouse gas emissions and potential drivers of the global warming potential in apple orchards based on different irrigation water treatments under drip irrigation technology for a two-year consecutive field experiment in 2019 and 2020. The results showed that soil water-filled pore space, daily mean soil surface evaporation and plant transpiration were all significantly increased by drip irrigation. The peaks of carbon dioxide and nitrous oxide fluxes showed a lagged effect, with fluxes proportional to irrigation amount (with emissions occurring throughout the growth period) while methane fluxes were consumptive. Overall, using higher drip irrigation water in apple orchards increased the growth-period mean emissions of carbon dioxide and nitrous oxide and decreased the growth-period mean uptake of methane. Therefore, optimization of drip irrigation water is needed in practice to mitigate greenhouse gas emissions from apple orchard production. Furthermore, the global warming potential was directly driven by soil water-filled pore space, soil surface evaporation, plant transpiration and indirectly driven by rainfall and drip irrigation. The different water allocation processes were closely linked and interacted with each other, thereby affecting the global warming potential directly or indirectly.

Nitrogen Addition Affects Nitrous Oxide Emissions of Rainfed Lucerne Grassland.

Nitrous oxide (N2O) is a potent greenhouse gas. Assessing the N2O emission from lucerne grasslands with nitrogen addition will aid in estimating the annual N2O emissions of such agriculture areas, particularly following summer rainfall events in light of precipitation variation associated with global change. Here, we measured soil N2O emissions, soil temperature and water content of lucerne grasslands with four levels of nitrogen addition over 25 days, which included 10 rainfall events. Results showed that nitrogen addition was observed to increase soil NO3−-N content, but not significantly improve dry matter yield, height or leaf area index. Nitrogen addition and rainfall significantly affected N2O emissions, while the response of N2O emissions to increasing nitrogen input was not linear. Relative soil gas diffusivity (Dp/Do) and water-filled pore space (WFPS) were good indicators of N2O diurnal dynamics, and Dp/Do was able to explain slightly more of the variation in N2O emissions than WFPS. Collectively, nitrogen addition did not affect lucerne dry matter yield in a short term, while it induced soil N2O emissions when rainfall events alter soil water content, and Dp/Do could be a better proxy for predicting N2O emissions in rainfed lucerne grasslands.

Response of nitrous oxide emissions to individual rain events and future changes in precipitation.

Changing precipitation has the potential to alter nitrous oxide (N2 O) emissions from agricultural regions. In this study, we applied the Coupled Model Intercomparison Project Phase 5 end-of-century RCP 8.5 (business as usual) precipitation projections for the U.S. Upper Midwest and examined the effects of mean precipitation changes, characterized by increased early-season rainfall and decreased mid- to late-season rainfall, on N2 O emissions from a conventionally managed corn (Zea mays L.) cropping system grown in an indoor mesocosm facility over four growing seasons. We also assessed the response of N2 O emissions to over 1,000 individual rain events. Nitrous oxide emissions were most strongly correlated with water-filled pore space (WFPS) and soil nitrogen (N) status. After rain events, the change in N2 O emissions, relative to pre-rain emissions, was more likely to be positive when soil NO3 - was>40mg N kg-1 soil and soil NH4 + was>10mg N kg-1 soil and was more likely to be negative when soil NO3 - was>40mg N kg-1 soil and soil NH4 + was<10mg N kg-1 soil. Similarly, hourly N2 O emissions remained <5nmol m- 2 s-1 when combined NH4 + + NO3 - was<20mg N kg-1 soil or NH4 + and NO3 - were<5 and 20mg N kg-1 soil, respectively. Rain event magnitude did not substantially affect the change in N2 O flux. Finally, growing-season N2 O emissions, soil moisture, and inorganic N content were not affected by the future precipitation pattern. Near-optimal soil WFPS combined with soil N concentrations above the identified thresholds favor higher N2 O emissions.

Straw strip mulching in a semiarid rainfed agroecosystem achieves carbon sequestration and emission reduction from winter wheat fields

Mulching practices improve soil hydrothermal conditions, which cause changes in soil respiration. Conventional straw mulching has low productivity in areas with relatively low temperatures. Meanwhile, the responses of soil respiration to mulching practices and the relationship between changes in environmental factors caused by mulching and carbon (C) components are not clear in dryland agroecosystem. In this study, we monitored seasonal changes in soil respiration flux and heterotrophic respiration flux of winter wheat fields under different mulching practices in a semiarid rainfed agroecosystem of northern China. The fields were designed for the following three treatments: straw strip mulching (SSM), plastic film mulching (PFM), and non-mulching (CK). Compared to CK, SSM decreased cumulative soil respiration and heterotrophic respiration by 11.6% and 13.3%, whereas PFM increased these values by 12.8% and 8.5%, respectively. SSM significantly decreased soil temperature but increased soil water-filled pore space (WFPS), while PFM significantly increased soil WFPS without affecting soil temperature. Respiration flux was more highly correlated with soil temperature and air temperature than with WFPS and rainfall. SSM and PFM respectively increased by 5.7–9.1% and 10.8–35.5% for grain yield, by 8.1–45.6% and 16.2–69.5% for straw yield, by 8.6–28.6% and 15.1–36.4% for net primary production, and by 23.1–80.6% and 20.3–95.2% for net ecosystem production. Furthermore, mulching practices had a great potential of soil C sequestration and SSM significantly increased soil organic carbon. Changes in ecosystem C components were primarily driven by biomass, followed by soil temperature and air temperature rather than WFPS and rainfall. Based on the sustainability of agroecosystems, SSM increased grain yield of winter wheat while reduced carbon emissions, which showed great potential for C sequestration and may create an environmentally friendly C sink system in a semiarid rainfed agroecosystem of northern China.

Climate Overrides Effects of Fertilizer and Straw Management as Controls of Nitrous Oxide Emissions After Oilseed Rape Harvest

Oilseed rape (Brassica napus L.) is an important bioenergy crop that contributes to the diversification of renewable energy supply and mitigation of fossil fuel CO2 emissions. Typical oilseed rape crop management includes the use of nitrogen (N) fertilizer and the incorporation of oilseed rape straw into soil after harvest. However, both management options risk increasing soil emissions of nitrous oxide (N2O). The aim of this 2-years field experiment was to identify the regulating factors of N cycling with emphasis on N2O emissions during the post-harvest period. As well as the N2O emission rates, soil ammonia (NH4+) and nitrate (NO3−) contents, crop residue and seed yield were also measured. Treatments included variation of fertilizer (non-fertilized, 90 and 180 kg N ha−1) and residue management (straw remaining, straw removal). Measured N2O emission data showed large intra- and inter-annual variations ranging from 0.5 (No-fert + str) to 1.0 kg N2O-N ha−1 (Fert-180 + str) in 2013 and from 4.1 (Fert-90 + str) to 7.3 kg N2O-N ha−1 (No-fert + str) in 2014. Cumulative N2O emissions showed that straw incorporation led to no difference or slightly reduced N2O emissions compared with treatments with straw removal, while N fertilization has no effect on post-harvest N2O emissions. A process-based model, CoupModel, was used to explain the large annual variation of N2O after calibration with measured environmental data. Both modeled and measured data suggest that soil water-filled pore space and temperature were the key factors controlling post-harvest N2O emissions, even though the model seemed to show a higher N2O response to the N fertilizer levels than our measured data. We conclude that straw incorporation in oilseed rape cropping is environmentally beneficial for mitigating N2O losses. The revealed importance of climate in regulating the emissions implies the value of multi-year measurements. Future studies should focus on new management practices to mitigate detrimental effects caused by global warming, for example by using cover crops.

A global synthesis of soil denitrification: Driving factors and mitigation strategies

Dinitrogen (N2) and nitrous oxide (N2O) produced via denitrification may represent major nitrogen (N) loss in terrestrial ecosystems. A global assessment of soil denitrification rate, N2O/(N2O+N2) ratio, and their driving factors and mitigation strategies is lacking. We conducted a global synthesis using 225 studies (3367 observations) to fill this knowledge gap. We found that daily N loss through soil denitrification varied with ecosystems and averaged 0.25 kg N ha−1. The average emission factor of denitrification (EFD) was 4.8%. The average N2O/(N2O+N2) ratio from soil denitrification was 0.33. Soil denitrification rate was positively related to soil water-filled pore space (WFPS) (p < 0.01), nitrate (NO3-) content (p < 0.05) and soil temperature (p < 0.01), and decreased with higher soil oxygen content (p < 0.01). N2 emissions increased with latitude (p < 0.05), WFPS (p < 0.01) and soil mineral N (p < 0.05) but decreased with soil oxygen content (p < 0.05). The N2O/(N2O+N2) ratio increased with soil oxygen content (p < 0.01) but decreased with organic carbon (C) (p < 0.05), C/N ratio (p < 0.01), soil pH (p < 0.05) and WFPS (p < 0.01). We also found that optimizing N application rates, using ammonium-based fertilizers compared to nitrate-based fertilizers, biochar amendment, and application of nitrification inhibitors could effectively reduce soil denitrification rate and associated N2 emissions. These findings highlight that N loss via soil denitrification and N2 emissions cannot be neglected, and that mitigation strategies should be adopted to reduce N loss and improve N use efficiency. Our study presents a comprehensive data synthesis for large-scale estimations of denitrification and the refinement of relevant parameters used in the submodels of denitrification in process-based models.

Elevated carbon dioxide stimulates nitrous oxide emission in agricultural soils: A global meta-analysis

Elevated carbon dioxide (CO 2 ) (eCO 2 ) has been shown to affect the nitrous oxide (N 2 O) emission from terrestrial ecosystems by altering the interaction of plants, soils, and microorganisms. However, the impact of eCO 2 on the N 2 O emission from agricultural soils remains poorly understood. This meta-analysis summarizes the effect of eCO 2 on N 2 O emission in agricultural ecosystems and soil physiochemical and biological characteristics using 50 publications selected. The eCO 2 effect values, which equal to the percentage changes of N 2 O emission under eCO 2 , were calculated based on the natural logarithm of the response ratio to eCO 2 . We found that eCO 2 significantly increased N 2 O emission (by 44%), which varied depending on experimental conditions, agricultural practices, and soil properties. In addition, eCO 2 significantly increased soil water-filled pore space (by 6%), dissolved organic carbon content (by 11%), and nitrate nitrogen content (by 13%), but significantly reduced soil pH (by 1%). Moreover, eCO 2 significantly increased soil microbial biomass carbon (by 28%) and soil microbial biomass nitrogen (by 7%) contents. Additionally, eCO 2 significantly increased the abundances of ammonia-oxidizing bacteria (AOB) amoA (by 21%), nirK (by 15%), and nirS (by 15%), but did not affect the abundances of ammonia-oxidizing archaea (AOA) amoA and nosZ . Our findings indicate that eCO 2 substantially stimulates N 2 O emission in agroecosystems and highlight that optimization of nitrogen management and agronomic options might suppress this stimulation and aid in reducing greenhouse effect.

Effects of slow and fast pyrolysis biochar on N2O emissions and water availability of two soils with high water-filled pore space

Abstract Biochars, depending on the types of feedstocks and technological conditions of pyrolysis, can vary significantly in their properties and, therefore, it is difficult to predict biochar-induced effects on nitrous oxide (N2O) emissions from various soils, their physical properties and water availability. The objectives of this study were (1) to quantify effects of slow pyrolysis biochar (BC) and fast pyrolysis biochar (PYRO) on physical and hydro-physical properties of sandy soil (Haplic Arenosol) and clayey loam soil (Gleyic Fluvisol), and (2) to assess corresponding N2O emissions from these two soils. The study included a 63-day long laboratory investigation. Two doses of BC or PYRO (15 t ha−1 and 30 t ha−1) were applied to the soils in combination or without nitrogen fertilizer (NH4NO3, 90 kg N ha−1). The obtained results have shown a significant decrease in the bulk density of sandy soil after it was amended with either rate of BC or PYRO. Water retention capacity of the soils in all the treatments with BC or PYRO increased considerably although no changes was found in the soil water-filled pore space (WFPS) which was higher than 60%. BC was increasing N2O emission rates from the sandy soil treated with N fertilizer, and reducing N2O emission rates from the clayey loam soil treated with N fertilizer. PYRO was more efficient and was reducing N2O emissions from both fertilized soils, but for the sandy soil the reduction was statistically significant only at higher dose (30 t ha−1) of the biochar.

Interactions between maize plants and nitrogen impact on soil profile dynamics and surface uptake of methane on a dryland farm

To determine the role of maize plants and their interaction with nitrogen (N) fertilizer in regulating subsoil methane (CH4) fluxes, we conducted a two-year field experiment on a semiarid farm in northwestern China. Four treatments were evaluated: unplanted soil without (C0) or with (CN) N fertilization and planted maize without (P0) or with (PN) N fertilization. Soil surface fluxes and profile dynamics of CH4 at depths of 0–50 cm in situ were monitored. The CH4 concentrations decreased with increasing soil depth. Seasonal and vertical variations in CH4 diffusive fluxes varied with maize phenology. The CH4 fluxes in the 0–20 cm layers accounted for 48.9–57.5% of the total fluxes in the 0–50 cm layers. The surface CH4 uptake was positively correlated with soil temperature during the fallow season and negatively correlated with soil water-filled pore space (WFPS) and exchangeable NH4+ concentration during the growing season. Compared with the unplanted treatments, the presence of maize plants significantly decreased soil WFPS and increased surface CH4 uptake during the growing season. The two-year mean cumulative diffusive CH4 fluxes were also significantly increased by 33.1–36.3% at depths of 10–30 cm and by 15.8–66.4% at depths of 10–40 cm in the P0 and PN treatments, respectively. Compared with the P0 treatment, the PN treatment significantly increased the surface CH4 uptake and cumulative CH4 uptake at depths of 0–40 cm in 2015. There was no significant difference in the CH4 surface and subsoil fluxes between the C0 and CN treatments. Our results suggested that N fertilization stimulated CH4 diffusion and uptake mainly by promoting maize growth and subsequently the soil water consumption on a dryland farm.

Plants are a natural source of nitrous oxide even in field conditions as explained by 15N site preference

Plants are either recognized to produce nitrous oxide (N2O) or considered as a medium to transport soil-produced N2O. To date, it is not clear whether in their habitat plants conduit N2O produced in soil or are a natural source. We aimed to understand role of plants in N2O emissions in field conditions. Therefore, rubber plants (Ficus elastica) were planted in the field; then plant and soil chambers were deployed simultaneously to collect gas samples, and 15N site preference (SP) of N2O was evaluated. The mean SP values of plant and soil emitted N2O were −20.85 ± 2.8‰ and −8.85 ± 1.08‰, respectively, and were significantly different (p < 0.0001); while bulk 15N of plant and soil emitted N2O were −10.83 ± 3.33‰ and −22.56 ± 3.37‰, respectively and were similar (p = 0.06). In the current study, soil always acted as a source of N2O, while plants were both source and sink. Plant and soil N2O fluxes had significant positive exponential relationship with both soil and air temperature. Soil water-filled pore space (WFPS) had significant negative linear relationship with only soil N2O fluxes. Plant N2O fluxes had significant positive linear relationship with plant respiration rates and negative linear relationship with plant surface areas. Based on the relationship between plant respiration rates and N2O fluxes, we suggest that mitochondria are the possible sites of N2O formation in plant cells while the relationship between plant surface areas and N2O fluxes suggests that roots are the parts of its formation in natural and field conditions. Our results suggest that plants are a natural source of N2O even at field conditions and challenge a view that plants are a medium to transport soil-produced N2O into the atmosphere.

Diurnal variability in soil nitrous oxide emissions is a widespread phenomenon.

Manual measurements of nitrous oxide (N2 O) emissions with static chambers are commonly practised. However, they generally do not consider the diurnal variability of N2 O flux, and little is known about the patterns and drivers of such variability. We systematically reviewed and analysed 286 diurnal data sets of N2 O fluxes from published literature to (i) assess the prevalence and timing (day or night peaking) of diurnal N2 O flux patterns in agricultural and forest soils, (ii) examine the relationship between N2 O flux and soil temperature with different diurnal patterns, (iii) identify whether non-diurnal factors (i.e. land management and soil properties) influence the occurrence of diurnal patterns and (iv) evaluate the accuracy of estimating cumulative N2 O emissions with single-daily flux measurements. Our synthesis demonstrates that diurnal N2 O flux variability is a widespread phenomenon in agricultural and forest soils. Of the 286 data sets analysed, ~80% exhibited diurnal N2 O patterns, with ~60% peaking during the day and ~20% at night. Contrary to many published observations, our analysis only found strong positive correlations (R>0.7) between N2 O flux and soil temperature in one-third of the data sets. Soil drainage property, soil water-filled pore space (WFPS) level and land use were also found to potentially influence the occurrence of certain diurnal patterns. Our work demonstrated that single-daily flux measurements at mid-morning yielded daily emission estimates with the smallest average bias compared to measurements made at other times of day, however, it could still lead to significant over- or underestimation due to inconsistent diurnal N2 O patterns. This inconsistency also reflects the inaccuracy of using soil temperature to predict the time of daily average N2 O flux. Future research should investigate the relationship between N2 O flux and other diurnal parameters, such as photosynthetically active radiation (PAR) and root exudation, along with the consideration of the effects of soil moisture, drainage and land use on the diurnal patterns of N2 O flux. The information could be incorporated in N2 O emission prediction models to improve accuracy.