Soil Emissions Research Articles

Soil Emissions of Reactive Oxidized Nitrogen Reduce the Effectiveness of Anthropogenic Source Control in China.

Nitrogen dioxide (NO2) has decreased by ∼33% across over 1200 monitoring sites in China during 2015-2023, following a series of clean air policies. However, most of these sites are located in or near cities, leading to uncertainties in NO2 trends beyond urban regions due to limited observations. Here, we used satellite measurements to examine the differences in NO2 trends between urban and rural China. In urban areas, NO2 columns decreased by 4.0% per annum (a-1) during summer 2011-2023, consistent with bottom-up anthropogenic emission inventory and in situ measurements. In contrast, rural NO2 columns showed a slower than expected reduction (-2.6 to -0.0% a-1) during the same period. Model simulations with updates in the soil reactive oxidized nitrogen (Nr) scheme indicated that increasing soil Nr emissions can be an important factor contributing to the observed slow NO2 decrease in rural areas. This unregulated source increased summertime pollutant levels, partially offsetting the national efforts to mitigate NO2, ozone (O3), and particulate nitrate (NO3-) levels by 20.9%, 15.4%, and 4.7%, respectively, from 2011 to 2020. In the agriculture-intensive North China Plain, the increase in soil Nr emissions offset 46.6% of the NO2 reductions achieved by clean air policies. Our results highlight the increasing significance of soil emissions and the need to control them in future air-quality policies.

Multifunctional effects of nitrification and urease inhibitors: decreasing soil herbicide residues and reducing nitrous oxide emissions simultaneously

Glyphosate pollution and greenhouse gas emissions are major problem in achieving sustainable soil management. It is necessary to develop effective strategies to simultaneously reduce herbicide residues and nitrous oxide (N2O) emissions in soil. This study aimed to: (1) quantitative analyze the effects of nitrogen (N) cycle inhibitors (nitrification inhibitors 3,4 dimethylpyrazole phosphate (DMPP) and dicyandiamide (DCD) and urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT)) on glyphosate degradation and reduction of N2O under different soil moistures; (2) identify the functional microbes and genes associated with glyphosate degradation and N2O emissions; and (3) decipher the main mechanisms of N cycle inhibitors affecting glyphosate degradation at different soil water contents. Compared to the control, the application of DMPP, DCD and NBPT reduced glyphosate residues in soil by 33.0%, 60.3% and 35.7%, respectively, under 90% water holding capacity (WHC). The application of DCD stimulated Acidobacteria and the phnX gene to degrade soil glyphosate. Further, soil glyphosate residues were significantly and negatively related to soil N2O emissions at both 60% and 90% WHC. Compared to the control, NBPT application decreased cumulative N2O emissions by 91.4% at 90% WHC by decreasing soil nitrate N (NO3--N) and inhibiting amoC and narG genes at 90%. The application of N cycle inhibitors could be a potential strategy to simultaneously reduce glyphosate residues and soil N2O emissions. Our study could provide technical support to reduce the risks of herbicide exposure and reduce greenhouse gas emissions.

Unprecedented N2O production by nitrate-ammonifying Geobacteraceae with distinctive N2O isotopocule signatures.

Dissimilatory nitrate reduction to ammonium (DNRA), driven by nitrate-ammonifying bacteria, is an increasingly appreciated nitrogen-cycling pathway in terrestrial ecosystems. This process reportedly generates nitrous oxide (N2O), a strong greenhouse gas with ozone-depleting effects. However, it remains poorly understood how N2O is produced by environmental nitrate-ammonifiers and how to identify DNRA-derived N2O. In this study, we characterize two novel enzymatic pathways responsible for N2O production in Geobacteraceae strains, which are predominant nitrate-ammonifying bacteria in paddy soils. The first pathway involves a membrane-bound nitrate reductase (Nar) and a hybrid cluster protein complex (Hcp-Hcr) that catalyzes the conversion of NO2- to NO and subsequently to N2O. The second pathway is observed in Nar-deficient bacteria, where the nitrite reductase (NrfA) generates NO, which is then reduced to N2O by Hcp-Hcr. These enzyme combinations are prevalent across the domain Bacteria. Moreover, we observe distinctive isotopocule signatures of DNRA-derived N2O from other established N2O production pathways, especially through the highest 15N-site preference (SP) values (43.0‰-49.9‰) reported so far, indicating a robust means for N2O source partitioning. Our findings demonstrate two novel N2O production pathways in DNRA that can be isotopically distinguished from other pathways.IMPORTANCEStimulation of DNRA is a promising strategy to improve fertilizer efficiency and reduce N2O emission in agriculture soils. This process converts water-leachable NO3- and NO2- into soil-adsorbable NH4+, thereby alleviating nitrogen loss and N2O emission resulting from denitrification. However, several studies have noted that DNRA can also be a source of N2O, contributing to global warming. This contribution is often masked by other N2O generation processes, leading to a limited understanding of DNRA as an N2O source. Our study reveals two widespread yet overlooked N2O production pathways in Geobacteraceae, the predominant DNRA bacteria in paddy soils, along with their distinctive isotopocule signatures. These findings offer novel insights into the role of the DNRA bacteria in N2O production and underscore the significance of N2O isotopocule signatures in microbial N2O source tracking.

Drivers of greenhouse gas emissions in agricultural soils: the effect of residue management and soil type

Developing successful mitigation strategies for greenhouse gases (GHGs) from crop residue returned to the soil can be difficult due to an incomplete understanding of factors controlling their magnitude and direction. Therefore, this study investigates the effects of varying levels of wheat residue (WR) and nutrient management on GHGs emissions (CO2, N2O, and CH4) across three soil types: Alfisol, Vertisol, and Inceptisol. A combination of laboratory-based measurements and a variety of data analysis techniques was used to assess the GHG responses under four levels of WR inputs (0, 5, 10, and 15 Mg/ha; WR0, WR5, WR10, and WR15) and three levels of nutrient (NP0: no nutrient, NP1: nutrients (N and P) were added to balance the residue C/nutrient stoichiometry of C/N/P= 100: 8.3: 2.0 to achieve 30% stabilization of added residue C input at 5 Mg/ha (R5), and NP2: 3 × NP1). The results of this study clearly showed that averaged across residue and nutrient input, Inceptisol showed negative N2O flux, suggesting consumption which was supported by its high legacy phosphorus (19.7 mg kg⁻1), elevated pH (8.49), and lower clay content (13%), which reduced microbial activity, as indicated by lower microbial biomass carbon (MBC) and alkaline phosphatase (Alk-P) levels. N2O emissions were more responsive to nutrient inputs, particularly in Vertisol under high WR (15 Mg/ha) input, while CH4 fluxes were significantly reduced under high residue inputs, especially in Vertisol and Inceptisol. Alfisol exhibited the highest total carbon mineralization and GWP, with cumulative GWP being 1.2 times higher than Vertisol and 1.4 times higher than Inceptisol across residue and nutrient input. The partial least square (PLS) regression revealed that anthropogenic factors significantly influenced CO2 and N2O fluxes more than CH4. The anthropogenic drivers contributed 62% and 44% of the variance explained for N2O and CH4 responses. Our study proves that different biogeochemical mechanisms operate simultaneously depending on the stoichiometry of residue C and nutrients influencing soil GHG responses. Our findings provide insight into the relative contribution of anthropogenic and natural drivers to agricultural GHG emissions, which are relevant for developing process-based models and addressing the broader challenge of climate change mitigation through crop residue management.

Key biochar properties linked to denitrification products in a calcareous soil

Meta-analyses show an overall decrease in soil N2O emissions after biochar (BC) amendment. Nonetheless, N2O mitigation with BC cannot be extrapolated to every BC-soil combination, inasmuch as an increase in soil N2O release has been occasionally reported. We hypothesized that BC characteristics are key, and performed two microcosm experiments to advance in the understanding of the properties associated. We first investigated how 22 well-characterized BCs affect N2O emissions in a calcareous soil under denitrification conditions. Whereas most BCs decreased N2O emissions, some substantially increased N2O emissions. In a second experiment, we selected and further characterized eight of the 22 previous BCs. We applied the 15N-gas-flux method to study how these BCs affect denitrification products (N2O and N2) in the same soil. Results indicate that the interaction between BC and the denitrification process depends on the temperature of pyrolysis. Whereas BCs produced at 400 °C tended to increase total denitrification (N2O+N2) by an average of 28%, BCs produced at 600 °C significantly reduced total denitrification by 53%. Nevertheless, this decline in overall denitrification did not result in a decrease of N2O emissions, as there was a strong shift in the N2O/(N2+N2O) ratio favoring N2O. A redundancy analysis revealed a direct correlation between carboxylic groups on BCs surface and N2O emissions. This research enhances our understanding of the interaction of BC with denitrification, particularly concerning the relevance of the temperature of pyrolysis, and opens up new paths for investigation, crucial for optimizing the application of BCs in different soil environments.Graphical

Navigating the complex policy landscape for carbon farming in The Netherlands and the EU

Background The EU carbon removal and carbon farming (CRCF) framework sets the quality requirements for voluntary greenhouse gas certification schemes for i) permanent removal, ii) temporary carbon storage in long-lasting products, iii) temporary carbon storage from carbon farming, and iv) soil emission reduction. The next step is to develop “tailored certification methodologies for different types of carbon removal activities.” Via CRCF compliant certification schemes, certificates can be issued and sold to generate extra funding for carbon farming via voluntary carbon markets or other (future) performance-based incentive schemes (e.g., Agri sector Emissions Trading Scheme). Method We provide a qualitative assessment of the complex landscape of carbon farming incentive schemes and their associated monitoring, reporting, verification, and certification (MRV-C) frameworks. To illustrate these complexities, we take the farmers’ perspective of a Dutch dairy farmer that wants to implement different carbon farming activities (i.e., a portfolio), while aiming to secure a decent farm-level income. We discuss the farmers’ challenge by analyzing different options and routes for the financial valorization for several carbon farming activities. Results We find that the incremental MRV-C and incentive schemes, like the EU CRCF and potential agri-ETS are likely to add to the existing administrative burden for (carbon) farmers in the EU. Proper alignment, integration and standardization of the EU CRCF with existing MRV-C infrastructure(s) is needed to limit the administrative burden and transaction costs. Also, there is a need to provide better ex-ante clarity to (carbon) farmers and investors on the complementarity of incentive schemes from different policy domains (e.g., agriculture, climate, energy, bioeconomy). Conclusion We argue that the complexity of the MRV-C and incentives landscape ads to the administrative burden and flag this a potential source of inertia for investments in carbon farming. We conclude with recommendations to simplify and streamline the policy mix and MRV-C processes.

Application of straw-derived biochar: a sustainable approach to improve soil quality and crop yield and reduce N2O emissions in paddy soil.

The burning of agricultural straw is a pressing environmental issue, and identifying effective strategies for the rational utilization of straw resources is decisive for achieving sustainable development. Owing to its high carbon content and exceptional stability, straw biochar produced via pyrolysis has emerged as a key focus in multidisciplinary research. However, the efficacy of biochar in mitigating nitrous oxide (N2O) emissions from paddy soils is not consistent. A 2-year field experiment was conducted and investigated the impact of biochar derived from two feedstocks (rice straw and wheat straw, pyrolyzed at 450°C) on N2O emissions, global warming potential (GWP), greenhouse gas intensity (GHGI), nitrogen use efficiency (NUE), crop yield, and soil quality. The static chamber technique was used for collecting N2O gas samples, and concentrations were analyzed through gas chromatography methods. The treatment combinations included BR0 (control), BR1 (NPK at the recommended dose, 120:60:40kgha-1), BR2 (wheat straw biochar, 5 t ha-1), and BR3 (rice straw biochar, 5 t ha-1). The results exhibited that cumulative N2O emissions from BR2 and BR3 treatments decreased by 10.55% and 13.75% respectively, compared to BR1. Lower GWP and GHGI were observed under both biochar treatments compared with BR1. The highest rice grain yield (3.48Mgha-1) and NUE (76.72%) were recorded from BR3, which also exhibitedthe lowest yield-scaled N2O emission. We observed positive correlations between soil nitrate, ammonia and water-filled pore spaces, while NUE showed negative correlations with N2O emissions. Significant (p < 0.05) improvements in soil quality were also detected in both the biochar treated plots, indicated by increased soil pH, water holding capacity, porosity, and nutrient contents. Overall, the results suggest that applying biochar at a rate of 5 t ha-1 in paddy soil is a viable nutrient management strategy with the potential to reduce reliance on inorganic fertilizers, mitigate N2O emissions, and contribute to sustainable food production.

Triple impact: Biochar, no-tillage, and cover crops for soil carbon enhancement and climate resilience in soybean farming

With the dissemination of conservation agriculture, no-tillage (NT) and cover crops have widely been adopted globally. However, their effects on greenhouse gas (GHG) emissions are controversial. Biochar is posited to mitigate climate change by increasing carbon (C) sequestration and decreasing GHG emission in soil. To investigate the comprehensive effect of NT, cover crops, and biochar on soil organic carbon (SOC) sequestration and net global warming potential (GWP), a split-split-plot experiment was conducted. The experiment involved various combinations of two tillage methods (NT; Moldboard plowing, MP), two cover crop treatments (Fallow, FA; Rye, RY), and two biochar treatments (with biochar application, WB; no biochar application, NB). NT and RY demonstrated a trend of increasing N2O emission, while WB tended to reduce the N2O emission in NT plots. NT–RY–WB increased the SOC stock (0–30 cm) by 23.2% in 2020 and 30.2% in 2021 compared with MP–FA–NB, indicating that this combination promoted C sequestration. Due to the heightened SOC stock, the net carbon dioxide (CO2) retention effectively compensated the GWP arising from non-CO2 emissions. Consequently, the triple combination of NT, RY and WB positively contributed to a decreased net GWP in the soybean field (−1231 kg CO2 equivalent ha−1 year−1 in 2020 and −2767 kg CO2 equivalent ha−1 year−1 in 2021). These findings highlight the considerable potential of the combined NT–RY–WB for SOC sequestration and net GWP decrease, positioning it as an environmentally beneficial agricultural system for mitigating climate change in Asia’s long-term food production.

Characterizing multifaceted environmental risks of oil and gas well leakage through soil and well methane and hydrogen sulfide emissions

Oil and gas wells (OGWs) can lead to soil and well emissions of methane (CH4), a potent greenhouse gas, and hydrogen sulfide (H2S), a highly toxic gas, both of which reduce air quality and can cause explosions when emitted into confined spaces. Developments have been occurring over OGWs, posing larger health and safety risks. However, to our knowledge, previous studies have not conjunctively analyzed well and soil emissions while considering development on or near OGWs. In this paper, we characterize 343 CH4 and H2S emission rate measurements from 67 non-producing (abandoned) and 35 producing (active) OGWs, including 205 measurements from soils surrounding 81 OGWs in Ontario and Quebec. We also provide the first emission rate estimates from an abandoned water and OGW-linked explosion and map OGWs in urban and built-up areas in Ontario and Quebec. We estimate the explosion-linked emissions to be 3,000 g CH4/hour and 7 g H2S/hour. Moreover, we find that 7,264 and 161 OGWs in Ontario and Quebec, respectively, are in urban and built-up areas, with 94% of these wells being abandoned. For the 102 wells we measured, we find OGW emission rate ranges of -16 to 47,000 mg CH4/hour and 0.001 to 3,300 mg H2S/hour, with 9% of the wells having positive detections of H2S. Although soil CH4 emissions at a 1-meter distance from the wells are most correlated with well emissions, the highest soil emission rate was observed at a 3-meter distance, indicating the potential for OGW-related emissions into buildings to occur away from the well. Overall, our multi-faceted measurement dataset provides a basis for conjunctive analysis of the broad range of environmental risks of OGWs to climate, indoor and outdoor air quality, and explosions.

Cocoa pod husk-derived organic soil amendments differentially affect soil fertility, nutrient leaching, and greenhouse gas emissions in cocoa soils

The depletion of soil nutrients and decline in yields on cocoa farms in west Africa over time force farmers to abandon their farms and look for new fertile land, thereby contributing to deforestation. Cocoa pod husks (CPH) are a major farm waste representing considerable export of nutrients from cocoa soils, particularly P and K. Here, the impacts of soil amendment with raw CPH residues, CPH compost, CPH biochar, or a CPH compost-biochar mixture on soil fertility, greenhouse gas (GHG) emissions, and nutrient losses via leaching were assessed in two laboratory experiments using two major soil types used for cocoa cultivation in Ghana (an acidic Ferralsol and an alkaline Nitisol). In Experiment 1, soil nutrient availability and CO2 and N2O emissions were quantified, whereas simulation and quantification of nutrient leaching were conducted in Experiment 2. Soil pH increased from 4.8 and 8.6 by 1.4- and 1.1-folds on average in amended Ferralsols and Nitisols, respectively. Soil electrical conductivity increased in soils amended with CPH compost and/or biochar. Addition of raw CPH caused remarkable microbial immobilisation of N and reduced N availability and leaching, whereas CPH compost and/or biochar addition increased soil nitrate availability but reduced soil ammonium availability. Leaching of Ca in Nitisols was reduced when CPH biochar was included in the soil amendment. While soil K availability increased in all amended soils, most notably when CPH biochar was included, soil P and Ca availabilities were greatest where CPH compost was included. Soil total GHG emission (CO2 plus N2O) increased in all amended Ferralsols and the Nitisols amended with CPH compost or raw CPH, with the latter remarkably increasing soil CO2 emission by up to 14.8-folds. Compared to sole CPH compost amendment, CPH compost-biochar mixture amendment reduced soil total GHG emission and N and P leaching. These findings show that composted and/or pyrolysed CPH can be judiciously used to enhance soil fertility in cocoa farms, particularly in acidic soils. Pyrolysed CPH is especially beneficial for reducing soil nutrient leaching and GHG emissions and thus for increasing the sustainability of cocoa production in Ghana and west Africa.

Organic fertilizer significantly mitigates N2O emissions while increase contributed of comammox Nitrospira in paddy soils

Nitrification is the dominant process for nitrous oxide (N2O) production under aerobic conditions, but the relative contribution of the autotrophic nitrifiers (the ammonia-oxidising archaea (AOA), the ammonia-oxidising bacteria (AOB) and the comammox) to this process is still unclear in some soil types. This is particularly the case in paddy soils under different fertilization regimes. We investigated active nitrifiers and their contribution to nitrification and N2O production in a range of unfertilized and fertilized paddy soils, using 13CO2-DNA based stable isotope probing (SIP) technique combined with a series of specific nitrification inhibitors, including acetylene (C2H2), 3, 4-dimethylpyrazole phosphate (DMPP) and 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO). The soils had a long-term history of fertilizer application, including chemical fertilizer only, a mixture of chemical fertilizers (70 %) and chicken manure (30 %) or a mixture of rice straw and chemical fertilizers. 13CO2-DNA-SIP and Illumina MiSeq sequencing demonstrated that comammox clades A.1 and B were active nitrifiers in all fertilized paddy soils. Inhibitor experiment showed that AOB largely contributed to nitrification activity and N2O emission in all paddy soils, while comammox contribution was more significant than AOA. Fertilization considerably altered nitrifiers' relative contribution to nitrification activity and N2O emissions. Applying organic fertilizers significantly decreased the N2O emissions but increased the contribution of comammox to the process. These findings expand the functional ecological niche of comammox, revealing their nitrification role and N2O production in other ecosystems than oligotrophic habitats.

Optimising Nitrogen Fertilisation in a Potato–Oat Rotation and Implications for Nitrous Oxide Emissions in Volcanic Soils

High nitrogen (N) fertiliser rates are usually applied to increase agricultural yields, leading to high nitrous oxide (N2O) emissions. This is a greenhouse gas that contributes to climate change and depletes the ozone layer. This study aimed to optimise N use efficiency and quantify N2O emission factors (EF1) by measuring the effect of N rates on the yield of a potato-cover crop rotation, apparent N use efficiency (NUE) and N2O emissions. The two-year experiment was carried out on volcanic soils (1.6% carbon, 1.4% N) in southern Chile (40°52′ S, 73°03′ W). Three N application rates were evaluated (80, 150 and 300 kg N ha−1), 35% of which was applied at the planting stage (granular) and 65% at the tubering stage. A control treatment with no N addition was also included. Reducing N fertilisation to 80 kg N ha−1 increased NUE by three times, reduced N2O-N emissions by 33% and reduced emission intensity by 27% without a detrimental impact on crop yield and marketable tuber calibre. No significant difference (p &lt; 0.05) was observed in the N2O emission factor (EF1) because of a low rainfall year. The results suggest that in rainfed agriculture systems, N fertiliser application can be significantly reduced without sacrificing potato yield, favouring the economic and environmental sustainability of potato production.

Biochar Applied in Places Where Its Feedstock Was Produced Mitigated More CO2 Emissions from Acidic Red Soils

Agricultural soil is the main source of greenhouse gas emissions, among which carbon dioxide (CO2) is an important greenhouse gas, impacting the global climate. In China, as a large rice-producing country, carbon sequestration and CO2 mitigation in paddy soil are crucial for the mitigation of global climate change. While biochar has been widely used in the mitigation of soil greenhouse gas emissions, the application site of biochar, i.e., whether or not it is the same as its feedstocks, may generate different effects on soil CO2 emissions due to the differences in the element and nutrient concentrations in its feedstocks, especially when applied in fertilized soil. In order to explore the effects of biochar application with different feedstocks on the mitigation of CO2 emissions from paddy soil, this experiment took paddy soil in a red soil area as the research object, using rice straw and Camellia oleifera fruit shell as raw materials to produce biochar (adding an amount of 40 g kg−1 soil) and urea as an external nitrogen source (adding an amount of 200 mg kg−1 soil). The effects of two different types of biochar derived from feedstocks with different producing origins on the CO2 emissions from paddy soil were studied via laboratory control incubation studies. The results showed that (1) the effects of rice straw biochar addition on the soil pH, NO3−-N and total available nitrogen (AN) content were significantly higher than those of Camellia oleifera fruit shell biochar (the scale of the increase was higher by 6.40%, 579.7% and 180.1%, respectively). (2) The CO2 emission rate and cumulative emissions of soil supplemented with rice straw biochar were significantly lower than in that supplemented with Camellia oleifera fruit shell biochar (decreases of 28.0% and 27.5%, respectively). Our findings suggest that the efficiency of emission mitigation of rice straw biochar is better than that of Camellia oleifera fruit shell applied to paddy soil. While future studies considering more types of greenhouse gases will be necessary to expand these findings, this study indicates that biochar prepared from in situ feedstock can be used to reduce greenhouse gas emissions in rice fields, so as to ensure sustainable development and achieve energy conservation and emission reduction goals. This study will benefit future agricultural practices when choosing biochar as a greenhouse gas mitigation strategy in the context of global warming, as well as other global changes following global warming, caused by elevated atmospheric greenhouse gases.

Dicyandiamide Applications Mitigate the Destructive Effects of Graphene Oxide on Microbial Activity, Diversity, and Composition and Nitrous Oxide Emission in Agricultural Soil.

The widespread production and utilization of graphene oxide (GO) raise concerns about its environmental release and potential ecological impacts, particularly in agricultural soil. Effective nitrogen (N) management, especially through nitrification inhibitors like dicyandiamide (DCD), might mitigate the negative effects of GO exposure on soil microbes via N biostimulation. This study quantified changes in soil physicochemical properties, nitrous oxide (N2O) emissions, microbial activity, biomass, and community after treatments with GO and DCD. The GO exposure significantly reduced bacterial 16S rRNA gene abundance and the biomass of major bacterial phyla. It also stimulated pathways linked to human diseases. However, DCD application alleviated the negative effects of GO exposure on soil bacterial biomass. While DCD application significantly reduced soil N2O emission, the GO application tended to hinder the inhibiting performance of DCD. Our findings highlight the hazards of GO exposure to soil microbes and the potential mitigation strategy with soil N management.

Microplastic contamination accelerates soil carbon loss through positive priming

The priming effect, i.e., the changes in soil organic matter (SOM) decomposition following fresh organic carbon (C) inputs is known to influence C storage in terrestrial ecosystems. Microplastics (particle size <5 mm) are ubiquitous in soils due to the increasing use and often inadequate end-of-life management of plastics. Conventional polyethylene and bio-degradable (PHBV) plastics contain large amounts of C within their molecular structure, which can be assimilated by microorganisms. However, the extent and direction of the potential priming effect induced by microplastics is unclear. As such, we added 14C-labeled glucose to investigate how background polyethylene and PHBV microplastics (1 %, w/w) affect SOM decomposition and its potential microbial mechanisms in a short-term. The cumulative CO2 emission in soil contaminated with PHBV was 42–53 % higher than under Polyethylene contaminated soil after 60-day incubation. Addition of glucose increased SOM decomposition and induced a positive priming effect, as a consequence, caused a negative net soil C balance (−59 to −132 μg C g−1 soil) regardless of microplastic types. K-strategists dominated in the PHBV-contaminated soils and induced 72 % higher positive priming effects as compared to Polyethylene-contaminated soils (160 vs. 92 μg C g−1 soil). This was attributed to the enhanced decomposition of recalcitrant SOM to acquire nitrogen. The stronger priming effect associated in PHBVs can be attributed to cooperative decomposition among fungi and bacteria, which metabolize more recalcitrant C in PHBV. Moreover, comparatively higher calorespirometric ratios, lower substrate use efficiency, and larger enzyme activity but shorter turnover time of enzymes indicated that soil contaminated with PHBV release more energy, and have a more efficient microbial catabolism and are more efficient in SOM decomposition and nutrient resource uptake. Overall, microplastics, (especially bio-degradable microplastics) can alter biogeochemical cycles with significant negative consequences for C sequestration via increasing SOM decomposition in agricultural soils and for regional and global C budgets.

Modified biochar affects CO2 and N2O emissions from coastal saline soil by altering soil pH and elemental stoichiometry

The application of biochar in degraded farmland improves soil productivity while achieving the recycling of agricultural waste. The collapse of the physical structure of coastal saline soils will greatly reduce the carbon sequestration potential of biochar. Phosphorus- and magnesium-modified biochar greatly improve the stability of biochar, which endows them with the potential to greatly improve the organic carbon pool of coastal saline soil. However, changes in the properties of modified biochar increase the uncertainty of microbial driven CO2 and N2O release by affecting soil chemistry properties. In this study, through laboratory soil microcosmic experiment, we investigated the effects of magnesium-modified biochar (BCMg) and phosphorus-modified biochar (BCP) on CO2 and N2O releases from coastal saline soils, and further uncovered their potential mechanisms. Compared with unapplied biochar (CK) and unmodified biochar (BC) treatment, BCMg reduced both the releases of CO2 and N2O, and BCP decreased N2O release but enhanced CO2 release. pH is the medium through which BCMg affects the release of CO2 and N2O. Specifically, BCMg increased soil pH above 8.5, which reduced the metabolic activity of the microbial community, and the abundance of bacteria directly or indirectly involved in N2O production, thereby decreasing the releases of CO2 and N2O. The amendment of BCP changed soil elemental stoichiometry causing microbial N-limitation. Increasing CO2 release and decreasing N2O release were strategies for microorganisms to cope with N-limitation. These findings suggested that BCMg is superior to BCP in mitigating greenhouse gas emissions, providing a basis for the application of modified biochar to improve the carbon pool and reduce greenhouse gas emissions of coastal saline soil.

Carbon footprint of global rice production and consumption

Rice paddies are not only sources of staple food for half of the global population but also account for nearly half of the anthropogenic greenhouse gas emissions (GHGs) of croplands. Carbon footprint (CF) is a key tool for identifying and weighting sources of the GHGs along the food supply chain, promoting the efforts to curb these emissions within the targets of the climatic change protocol of the Paris Agreement. We introduced a comprehensive global quantification of rice CF, including direct and indirect emissions and sinks of the GHGs of inputs production, packaging, transportation, and application, soil and plant systems, farm operations, and uses of the produced biomass until the end of life. Globally, the rice CF was 2430 kg CO2eq. Mg−1 grain in 2020, of which 46% and 42% were sourced from the Gate and Grave stages, respectively, after excluding 3265 kg CO2eq. Mg−1 grain, that is the assimilated C in plant biomass. Net GHGs emissions of soil, biomass mulching and burning, and farm operations accounted for 20, 17, and 63%, respectively of the Gate stage CF. Meanwhile, food consumption contributed to the Grave stage CF by 92%. The rice CF ranged between 14 and 4854 kg CO2eq. Mg−1 grain among countries, wherein, for example, the rice CF values in Indonesia, India, Vietnam, and Russia represented 9, 50, 97, and 122% of the global average. Southeast, South, and East Asia were the major contributors (35, 34 and 18%, respectively) to the atmospheric CO2 load (2.4 Pg CO2eq.) of global rice production and consumption. This CO2 load will increase to 3.1 Pg CO2eq. in 2100, driven by a 32% growth in rice consumption. Here, we suggested an optimistic strategy (green energy use, hybridization, improving use efficiency of the inputs, and reducing food losses) to reduce the CO2 load by 60%.

Evaluation of impacts of biosolids application and drainage water management on soil N2O and CH4 emissions using the flux gradient method

Existing studies have shown contradictory findings with respect to whether biosolids applications on agricultural lands lead to intensification of soil greenhouse gas (GHG) emissions. Here, we describe the results of deployment of the micrometeorological flux gradient method to quantify post-biosolid soil emissions of nitrous oxide (N2O) and methane (CH4) on a farm with drainage water management (DWM) on the Eastern Shore of Maryland. The fluxes following biosolid additions to cornfields in 2020 were compared with fluxes from the same farm in 2018, when no fertilizer was applied to soybeans, and in 2019, when urea ammonium nitrate (UAN) was applied to corn. Extractable soil nitrate was highest following biosolids application, contributing to the highest N2O emissions in the growing season of 2020 compared to 2018 (no fertilizer) and 2019 (UAN). Other contributing factors include the low C:N ratio of the biosolids and the above average precipitation in 2020. In contrast, different fertilization regimes did not generate distinct differences for CH4 fluxes, which were very low in all three years. No statistically significant treatment effect of DWM was found for either N2O or CH4 during the peak emission period after biosolids application, which aligns with the result of our earlier research. Annualized estimated N2O emission factors (EFs) for biosolids addition were 5–6 % in the DWM and 3–4 % in the non-DWM fields, although this includes uncertainties associated with gap filling. These biosolids EFs are 2–3 times the N2O EF for synthetic fertilizer application at this same farm in 2019 (1–2.5 %) and 2–4 times the IPCC Tier 1 EF (1.6 %) for synthetic fertilizer, demonstrating the intensification effect of biosolids addition on soil N2O emissions for the cropland studied here.

Soil pH-dependent efficacy of DMPP in mitigating nitrous oxide under different land uses

The emission pathways and intensities of nitrous oxide (N2O) vary across land uses, and the efficacy of mitigation measures may also differ. To investigate the key factors influencing the effectiveness of the nitrification inhibitor 3,4-Dimethylpyrazole phosphate (DMPP) in mitigating N2O under various land use conditions, we collected ten pairs of soils from tea plantations and adjacent cultivated land for laboratory incubation. As a complementary study, we conducted a meta-analysis based on 261 pairs of observations from 40 incubation studies to investigate the determinants of soil N2O mitigation by DMPP and to validate our experimental results. Results of the incubation experiment showed that DMPP was significantly less effective in mitigating N2O in tea plantation soils (73%) than in cropland soils (82%). Soil pH is a key factor influencing DMPP efficiency under different land use conditions. The lower pH of acidic tea plantation soils resulted in lower abundance and activity of ammonia-oxidizing bacteria (AOB). Under these low pH conditions, heterotrophic nitrification became an important source of N2O emissions in acidic tea plantation soils. As DMPP primarily inhibits AOB, it was less effective against heterotrophic nitrification, which likely contributed to its reduced efficacy in tea plantation soils compared to cropland soils. Our meta-analysis further confirmed the critical role of soil pH in determining DMPP effectiveness. The effect of soil pH on DMPP efficacy was mainly attributed to its regulation of heterotrophic and AOB-derived nitrification. These findings suggest that the field efficacy of DMPP in N2O abatement in tea plantations requires further investigation. Given the significant contribution of heterotrophic nitrification to N2O emissions in strongly acidic tea plantation soils, alternative N2O mitigation strategies should be explored.

Responses of Methane Emission and Bacterial Community to Fertilizer Reduction Plus Organic Materials over the Course of an 85-Day Leaching Experiment

Methane produced from paddy fields has a negative impact on global climate change. However, the role of soil bacterial community composition in mediating methane (CH4) emission from waterlogged paddy soil using the column experiment is poorly known. In the present study, various fertilization treatments were adopted to investigate the effects of fertilizer reduction combined with organic materials (CK: control; CF: conventional fertilization; RF: 20% fertilizer reduction; RFWS: RF plus wheat straw amendment; RFRS: RF plus rapeseed shell amendment; RFAS: RF plus astragalus smicus amendment) on CH4 emission and soil bacterial community during an 85-day leaching experiment. We hypothesized that the fertilizer reduction plus the organic materials could enrich the bacterial communities and increase CH4 emission. The average CH4 flux varied from 0.03 μg m−2 h−1 to 76.19 μg m−2 h−1 among all treatments in the nine sampling times, which may account for the experimental conditions such as air temperature, moisture, and anthropogenic factors. In addition, high-throughput sequencing was utilized to investigate the alteration of the soil bacterial community structure. It was revealed that the diversity and composition of the bacterial community in the topsoil amended with organic materials underwent significant shifts after the 85-day leaching experiment. Proteobacteria was identified as the dominant phylum of the soil bacteria, with an average proportion of 35.2%. For Firmicutes, the proportion of RFRS (11%) was higher than that in the CK (8%), RF (8%), RFWS (7%), RFAS (6%), and CF (5%) treatments. Additionally, Gammaproteobacteria and Alphaproteobateria were supposed to be the major class bacterial communities, with average proportions of 12.8% and 12.2%, respectively. For the RFWS treatment, the contribution of Alphaproteobateria was the highest among all the bacterial relative abundance. According to the correlation heatmap analysis, the top ten bacterial communities were positively related to soil microbial biomass carbon (MBC) and ammonia nitrogen (NH4+-N) (p &lt; 0.01). The findings also indicated that the RFRS treatment was the favorable management to alleviate CH4 emission during an 85-day leaching experiment or possibly in paddy production. Collectively, these results predict that the impacts of different treatments on CH4 production are strongly driven by soil microbial communities and soil properties, with soil bacteria being more prone to the crop residue degradation stage and more sensitive to soil properties. The discoveries presented in this study will be useful for assessing the efficacy and mechanisms of organic material amendments on CH4 emissions in paddy soil.