Nitrous Oxide (N2O) Emissions from Subsurface Soils of Agricultural Ecosystems

Exploring dissolved N2O characteristics and unearthing indirect N2O emission factors in the shallow groundwater of paddy and upland fields

Indirect emissions of nitrous oxide (N2O) stemming from nitrogen (N) leaching in agricultural fields constitute a significant contributor to atmospheric N2O. Groundwater nitrate (NO3--N) pollution is severe in the Ningxia Yellow River Irrigation Area (NYRIA), coupled with high NO3--N leaching, exacerbates the risk of indirect N2O emissions from groundwater. Over two years of field observations, this study investigated the characteristics and interannual variations of dissolved N2O (dN2O) concentrations and indirect N2O emission factors (EF5g) in shallow groundwater. The research focused on three typical farmlands in the NYRIA, each subjected to six levels of N fertilizer application. The mean dN2O concentrations in the groundwater of paddy, corn and vegetable fields were 5.17, 8.40 and 16.35 μg N·L-1, respectively. Notably, the dN2O concentrations in the shallow groundwater of upland fields exceeded those in paddy fields, with maximum levels in vegetable fields nearly an order of magnitude higher. Elevated N application significantly increased dN2O concentrations across various farmlands, showing statistically significant variation. However, differences in EF5g-A and EF5g-B within the same farmland were negligible. Denitrification was the primary process contributing to N2O production in groundwater, with nitrification also played a crucial role in upland fields. Factors such as NO3--N, NH4+-N, dissolved oxygen (DO), and pH critically influenced N2O production. EF5g-B, which considers the NO3--N consumption during denitrification processes in groundwater, was deemed more appropriate than EF5g-A for assessing the indirect N2O emission in the NYRIA. The EF5g of agricultural fields exhibited minimal sensitivity to N input but was significantly affected by other factors, such as the planting pattern. The study revealed the rationality of adopting EF5g-B in assessing indirect N2O emissions, providing valuable insights for N management strategies in regions with high NO3--N leaching. Minimizing N fertilizer application while ensuring crop yield, especially in upland fields, is beneficial for reducing N2O emissions.

Nitrous oxide and methane production and consumption at five full-size denitrifying bioreactors treating subsurface drainage water

Nitrate (NO3-) removal in denitrifying bioreactors is influenced by flow, water chemistry, and design, but it is not known how these widely varying factors impact the production of nitrous oxide (N2O) or methane (CH4) across sites. Woodchip bioreactors link the hydrosphere and atmosphere in this respect, so five full-size bioreactors in Illinois, USA, were monitored for NO3-, N2O, and CH4 to better document where this water treatment technology resides along the pollution swapping to climate smart spectrum. Both surface fluxes and dissolved forms of N2O and CH4 were measured (n = 7-11 sampling campaigns per site) at bioreactors ranging from <1 to nearly 5 years old and treating subsurface drainage areas from between 6.9 and 29 ha. Across all sites, N2O surface and dissolved volumetric production rates averaged 1.0 ± 1.6 mg N2O-N/m3-d and 24 ± 62 mg dN2O-N/m3-d, respectively, and CH4 production rates averaged 6.0 ± 26 mg CH4-C/m3-d and 310 ± 520 mg dCH4-C/m3-d for surface and dissolved, respectively. However, N2O was consistently consumed at one bioreactor, and only three of the five sites produced notable CH4. Surface fluxes of CH4 were significantly reduced by the presence of a soil cover. Bioreactor denitrification was relatively efficient, with only 0.51 ± 3.5 % of removed nitrate emitted as N2O (n = 48). Modeled indirect N2O emissions factors were significantly lower when a bioreactor was present versus absent (EF5: 0.0055 versus 0.0062 kg N2O-N/kg NO3-N; p = 0.0011). While further greenhouse gas research on bioreactors is recommended, this should not be used as an excuse to slow adoption efforts. Bioreactors provide a practical option for voluntary water quality improvement in the heavily tile-drained US Midwest and elsewhere.

How binomial (traditional rainfed olive grove-Crocus sativus) crops impact the soil bacterial community and enhance microbial capacities

Intercropping can favour the yield of the main crop. However, because of the potential competition among woody crops, this system is rarely used by farmers. To increase knowledge about the intercropping system, we have explored three different combinations of alley cropping in rainfed olive groves compared to conventional management (CP): (i) Crocus sativus (D-S); (ii) Vicia sativa/Avena sativa in annual rotation (D-O); and (iii) Lavandula x intermedia (D-L). Different soil chemical properties were analyzed to evaluate the effects of alley cropping, while 16S rRNA amplification and enzymatic activities were determined to study the changes that occurred in soil microbial communities and activity. In addition, the influence of intercropping on the potential functionality of the soil microbial community was measured. Data revealed that the intercropping systems highly affected the microbial community and soil properties. The D-S cropping system increased soil total organic carbon and total nitrogen that were correlated with the bacterial community, indicating that both parameters were the main drivers shaping the structure of the bacterial community. The D-S soil cropping system had significantly higher relative abundances of the phyla Bacteroidetes, Proteobacteria, and Patescibacteria compared to the other systems and the genera Adhaeribacter, Arthrobacter, Rubellimicrobium, and Ramlibacter, related to C and N functions. D-S soil was also related to the highest relative abundances of Pseudoarthrobacter and Haliangium, associated with the plant growth-promoting effect, antifungal activity, and a potential P solubilizer. A potentially increase of C fixation and N fixation in soils was also observed in the D-S cropping system. These positive changes were related to the cessation of tillage and the development of a spontaneous cover crop, which increased soil protection. Thus, management practices that contribute to increasing soil cover should be encouraged to improve soil functionality.

Characteristics of dissolved N2O and indirect N2O emission factor in the groundwater of high nitrate leaching areas in northwest China

Numerous studies have demonstrated high concentrations of dissolved N₂O and indirect N₂O emission factors in groundwater affected by agriculture. However, the characteristics of seasonal and vertical dimensional difference in groundwater in high nitrate leaching areas are relatively lacking. We monitored the concentrations of dissolved and wellhead N₂O of 23 groundwater wells over a one year period to understand the seasonal characteristics of dissolved and wellhead N₂O concentrations and indirect N₂O emission factors (EF₅) of the shallow and deep groundwater in a high nitrogen leaching area and analyze the reasons for their differences. The mean dissolved N₂O concentration in groundwater was 9.71 (9.03) μg/L, which was 1.5-fold higher during the wet season relative to the dry season. Furthermore, the leaching of soil N₂O caused by rainfall and irrigation could be a pivotal factor affecting seasonal variation in the dissolved N₂O. Shallow wells were found to have higher dissolved and wellhead N₂O concentrations compared with deep wells in all seasons. The low wellhead N₂O concentrations during the dry season were attributed to the seasonal decrease of the groundwater table and dissolved N₂O concentrations. We concluded that indirect N₂O emission factors did not vary in the vertical dimension but were higher during the wet season than that during the dry season. In addition, the mean indirect N₂O emission factor in the groundwater was 0.025 %, which was one order of magnitude below the current IPCC value (0.25 %). Thus, we proposed that such a low indirect N₂O emissions factor could imply a low indirect N₂O emission potential in groundwater with high dissolved oxygen and nitrogen loads. Our study further indicated that seasonal differences in dissolved N₂O concentrations and indirect N₂O emission factors should be considered when estimating the potential emissions of dissolved N₂O in groundwater.

Separating N2O production and consumption in intact agricultural soil cores at different moisture contents and depths

Agricultural soils are a major source of the potent greenhouse gas and ozone depleting substance, N2O. To implement management practices that minimize microbial N2O production and maximize its consumption (i.e., complete denitrification), we must understand the interplay between simultaneously occurring biological and physical processes, especially how this changes with soil depth. Meaningfully disentangling of these processes is challenging and typical N2O flux measurement techniques provide little insight into subsurface mechanisms. In addition, denitrification studies are often conducted on sieved soil in altered O2 environments which relate poorly to in situ field conditions. Here, we developed a novel incubation system with headspaces both above and below the soil cores and field-relevant O2 concentrations to better represent in situ conditions. We incubated intact sandy clay loam textured agricultural topsoil (0-10 cm) and subsoil (50-60 cm) cores for 3-4 days at 50% and 70% water-filled pore space, respectively. 15N-N2O pool dilution and an SF6 tracer were injected below the cores to determine the relative diffusivity and the net N2O emission and gross N2O emission and consumption fluxes. The relationship between calculated fluxes from the below and above soil core headspaces confirmed that the system performed well. Relative diffusivity did not vary with depth, likely due to the preservation of preferential flow pathways in the intact cores. Gross N2O emission and uptake also did not differ with depth but were higher in the drier cores, contrary to expectation. We speculate this was due to aerobic denitrification being the primary N2O consuming process and simultaneously occurring denitrification and nitrification both producing N2O in the drier cores. We provide further evidence of substantial N2O consumption in drier soil but without net negative N2O emissions. The results from this study are important for the future application of the 15N-N2O pool dilution method and N budgeting and modelling, as required for improving management to minimize N2O losses.

Holistic view of biological nitrogen fixation and phosphorus mobilization in Azotobacter chroococcum NCIMB 8003

Nitrogen (N) and phosphorus (P) deficiencies are two of the most agronomic problems that cause significant decrease in crop yield and quality. N and P chemical fertilizers are widely used in current agriculture, causing environmental problems and increasing production costs. Therefore, the development of alternative strategies to reduce the use of chemical fertilizers while maintaining N and P inputs are being investigated. Although dinitrogen is an abundant gas in the atmosphere, it requires biological nitrogen fixation (BNF) to be transformed into ammonium, a nitrogen source assimilable by living organisms. This process is bioenergetically expensive and, therefore, highly regulated. Factors like availability of other essential elements, as phosphorus, strongly influence BNF. However, the molecular mechanisms of these interactions are unclear. In this work, a physiological characterization of BNF and phosphorus mobilization (PM) from an insoluble form (Ca₃(PO₄)₂) in Azotobacter chroococcum NCIMB 8003 was carried out. These processes were analyzed by quantitative proteomics in order to detect their molecular requirements and interactions. BNF led to a metabolic change beyond the proteins strictly necessary to carry out the process, including the metabolism related to other elements, like phosphorus. Also, changes in cell mobility, heme group synthesis and oxidative stress responses were observed. This study also revealed two phosphatases that seem to have the main role in PM, an exopolyphosphatase and a non-specific alkaline phosphatase PhoX. When both BNF and PM processes take place simultaneously, the synthesis of nitrogenous bases and L-methionine were also affected. Thus, although the interdependence is still unknown, possible biotechnological applications of these processes should take into account the indicated factors.

Agriculture-Induced N2O Emissions and Reduction Strategies in China

Greenhouse gases are one of the most important factors in climate change, their emissions reduction is a global problem. Clarifying the spatial patterns of N₂O, as an important component of greenhouse gases, it is of great significance. Based on the planting and breeding data of China from 2000 to 2019, this paper measures the N₂O emissions of agricultural systems, and uses kernel density to explore the spatial distribution differences between the eight major economic zones. Finally, the proposed emissions reduction countermeasures are provided. The research results show that the N₂O emissions of China's agricultural system showed a trend of increasing first and then decreasing, and in 2019, the national N₂O emissions were 710,300 tons, agricultural land emissions and animal husbandry emissions were the main sources of N₂O emissions. The difference in N₂O emissions by province was significant, the concentration trend was more prominent, and the differences of N₂O emissions between provinces and regions were diverse. In order to achieve the reduction in N₂O emissions, it is necessary to carry out low-carbon production of staple grains for different parts and economic zones, and focusing on low-carbon production in the Central Part and the West Part, as well as the Northeast and the Greater Southwest zones, is essential.

Identification of nosZ-expressing microorganisms consuming trace N2O in microaerobic chemostat consortia dominated by an uncultured Burkholderiales

Microorganisms possessing N₂O reductases (NosZ) are the only known environmental sink of N₂O. While oxygen inhibition of NosZ activity is widely known, environments where N₂O reduction occurs are often not devoid of O₂. However, little is known regarding N₂O reduction in microoxic systems. Here, 1.6-L chemostat cultures inoculated with activated sludge samples were sustained for ca. 100 days with low concentration (<2 ppmv) and feed rate (<1.44 µmoles h⁻¹) of N₂O, and the resulting microbial consortia were analyzed via quantitative PCR (qPCR) and metagenomic/metatranscriptomic analyses. Unintended but quantified intrusion of O₂ sustained dissolved oxygen concentration above 4 µM; however, complete N₂O reduction of influent N₂O persisted throughout incubation. Metagenomic investigations indicated that the microbiomes were dominated by an uncultured taxon affiliated to Burkholderiales, and, along with the qPCR results, suggested coexistence of clade I and II N₂O reducers. Contrastingly, metatranscriptomic nosZ pools were dominated by the Dechloromonas-like nosZ subclade, suggesting the importance of the microorganisms possessing this nosZ subclade in reduction of trace N₂O. Further, co-expression of nosZ and ccoNO/cydAB genes found in the metagenome-assembled genomes representing these putative N₂O-reducers implies a survival strategy to maximize utilization of scarcely available electron acceptors in microoxic environmental niches.

Prairie Strips and Lower Land Use Intensity Increase Biodiversity and Ecosystem Services

Agricultural landscapes can be managed to protect biodiversity and maintain ecosystem services. One approach to achieve this is to restore native perennial vegetation within croplands. Where rowcrops have displaced prairie, as in the US Midwest, restoration of native perennial vegetation can align with crops in so called “prairie strips.” We tested the effect of prairie strips in addition to other management practices on a variety of taxa and on a suite of ecosystem services. To do so, we worked within a 33-year-old experiment that included treatments that varied methods of agricultural management across a gradient of land use intensity. In the two lowest intensity crop management treatments, we introduced prairie strips that occupied 5% of crop area. We addressed three questions: (1) What are the effects of newly established prairie strips on the spillover of biodiversity and ecosystem services into cropland? (2) How does time since prairie strip establishment affect biodiversity and ecosystem services? (3) What are the tradeoffs and synergies among biodiversity conservation, non-provisioning ecosystem services, and provisioning ecosystem services (crop yield) across a land use intensity gradient (which includes prairie strips)? Within prairie strip treatments, where sampling effort occurred within and at increasing distance from strips, dung beetle abundance, spider abundance and richness, active carbon, decomposition, and pollination decreased with distance from prairie strips, and this effect increased between the first and second year. Across the entire land use intensity gradient, treatments with prairie strips and reduced chemical inputs had higher butterfly abundance, spider abundance, and pollination services. In addition, soil organic carbon, butterfly richness, and spider richness increased with a decrease in land use intensity. Crop yield in one treatment with prairie strips was equal to that of the highest intensity management, even while including the area taken out of production. We found no effects of strips on ant biodiversity and greenhouse gas emissions (N2O and CH4). Our results show that, even in early establishment, prairie strips and lower land use intensity can contribute to the conservation of biodiversity and ecosystem services without a disproportionate loss of crop yield.

Water, Energy and Carbon Tradeoffs of Groundwater Irrigation-Based Food Production: Case Studies from Fergana Valley, Central Asia

In arid environments, water shortages due to over-allocation of river flow are often compensated by lift irrigation or pumping groundwater. In such environments, farmers using pumped irrigation can deploy on-farm energy-efficient and water-saving technologies; however, pumping water requiring extra energy is associated with carbon emissions. This study explores how to increase crop production using pumped irrigation with minimal energy and carbon emissions. The purpose of this research is twofold: first, to examine on-farm energy consumption and carbon emissions in gravity and groundwater irrigation systems; and second, to explore system-level alternatives of power generation and water management for food production based on the results from the farm-level analysis. This study employs a novel system-level approach for addressing water, energy, and carbon tradeoffs under pumped irrigation using groundwater. These tradeoffs are assessed at farm and system levels. On-farm level estimates showed that farm-level interventions were insufficient to produce mutual gains. According to the results of the system-level evaluation, system-level interventions for water and energy conservation, the use of renewable energy to pump water for irrigation, and river basin scale cooperation are all required to maintain crop production while reducing energy consumption and carbon emissions.

Influence of the shallow groundwater table on the groundwater N2O and direct N2O emissions in summer maize field in the North China Plain

Agriculture is an important N₂O emissions source. Water cycle and nitrogen cycles have important effects on N₂O in farmland ecosystems. The changes in the groundwater table can lead to changes in farmland the water and nitrogen cycle processes. However, how this such changes will affect N2O emissions from farmland remains unclear. In this study, a two-year volume lysimeter experiment (2019-2020), including four controlled groundwater tables (i.e., 40, 70, 110, and 150 cm), was performed to monitor the variations in the NO₃^- and N₂O concentrations in shallow groundwater as well as the direct N₂O emissions due to surface soil and groundwater evaporation. Our results showed that N₂O emissions during fertilization accounted for 80%-90% of the total N₂O emissions throughout the maize growing period. Direct N₂O emissions increase with the increase in the groundwater table. The total N₂O emissions in 2020 were 96.44, 9.75, 6.46, and 6.22 kg ha^-1 y^-1 at a groundwater table of 40, 70, 110, and 150 cm, respectively. The high water-filled pore space (WFPS) value resulting from the elevated groundwater table increased the groundwater-atmosphere connectivity, leading to significantly increased N₂O emissions after fertilization. Increased precipitation (454.90 mm in 2020 vs. 180.30 mm in 2019) accelerated the hydrological processes in agroecology, reducing the retention time of N₂O (6 weeks in 2020 vs. 7.5 weeks in 2019) and NO₃^- (6.75 weeks in 2020 vs. 7.25 weeks in 2019) in shallow groundwater. Studying the influence of shallow groundwater tables on direct N₂O emissions will provide insights into the interaction between the water and nitrogen cycles in agroecosystems. The results of this study suggest that direct N₂O emissions can be effectively reduced by controlling the groundwater table in agricultural fields in the North China Plain.

Nitrification is a minor source of nitrous oxide (N2 O) in an agricultural landscape and declines with increasing management intensity

The long-term contribution of nitrification to nitrous oxide (N₂ O) emissions from terrestrial ecosystems is poorly known and thus poorly constrained in biogeochemical models. Here, using Bayesian inference to couple 25 years of in situ N₂ O flux measurements with site-specific Michaelis-Menten kinetics of nitrification-derived N₂ O, we test the relative importance of nitrification-derived N₂ O across six cropped and unmanaged ecosystems along a management intensity gradient in the U.S. Midwest. We found that the maximum potential contribution from nitrification to in situ N₂ O fluxes was 13%-17% in a conventionally fertilized annual cropping system, 27%-42% in a low-input cover-cropped annual cropping system, and 52%-63% in perennial systems including a late successional deciduous forest. Actual values are likely to be <10% of these values because of low N₂ O yields in cultured nitrifiers (typically 0.04%-8% of NH₃ oxidized) and competing sinks for available NH4+ in situ. Most nitrification-derived N₂ O was produced by ammonia-oxidizing bacteria rather than archaea, who appeared responsible for no more than 30% of nitrification-derived N₂ O production in all but one ecosystem. Although the proportion of nitrification-derived N₂ O production was lowest in annual cropping systems, these ecosystems nevertheless produced more nitrification-derived N₂ O (higher V_max ) than perennial and successional ecosystems. We conclude that nitrification is minor relative to other sources of N₂ O in all ecosystems examined.

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.

Effects of fertilizer and biochar applications on the relationship among soil moisture, temperature, and N2O emissions in farmland

Background

Di-nitrogen oxide (N₂O) emissions from soil may lead to nonpoint-source pollution in farmland. Improving the C and N content in the soil is an excellent strategy to reduce N₂O emission and mitigate soil N loss. However, this method lacks a unified mathematical index or standard to evaluate its effect.

Methods

To quantify the impact of soil improvement (C and N) on N₂O emissions, we conducted a 2-year field experiment using biochar as carbon source and fertilizer as nitrogen source, setting three treatments (fertilization (300 kg N ha⁻¹), fertilization + biochar (30 t ha⁻¹), control).

Results

Results indicate that after biochar application, the average soil water content above 20 cm increased by ∼26% and 26.92% in 2019, and ∼10% and 12.49% in 2020. The average soil temperature above 20 cm also increased by ∼2% and 3.41% in 2019. Fertigation significantly promotes the soil N2O emissions, and biochar application indeed inhibited the cumulation by approximately 52.4% in 2019 and 33.9% in 2020, respectively. N₂O emissions strongly depend on the deep soil moisture and temperature (20–80 cm), in addition to the surface soil moisture and temperature (0–20 cm). Therefore, we established an exponential model between the soil moisture and N₂O emissions based on theoretical analysis. We find that the N₂O emissions exponentially increase with increasing soil moisture regardless of fertilization or biochar application. Furthermore, the coefficient a < 0 means that N₂O emissions initially increase and then decrease. The a_RU < a_CK indicates that fertilization does promote the rate of N₂O emissions, and the a_BRU > a_RU indicates that biochar application mitigates this rate induced by fertilization. This conclusion can be verified by the sensitivity coefficient (SC_B of 1.02 and 14.74; SC_U of 19.18 and 20.83). Thus, we believe the model can quantify the impact of soil C and N changes on N₂O emissions. We can conclude that biochar does significantly reduce N₂O emissions from farmland.