Can cropland management practices lower net greenhouse emissions without compromising yield?

Carbon footprint of maize-wheat cropping system after 40-year fertilization

Two main challenges which human society faces for sustainable development goals are the maintenance of food security and mitigation of greenhouse gas (GHG) emissions. Here, we examined the impacts of six fertilization treatments including unfertilized control (CK), mineral nitrogen (N, 90 kg N ha-1), mineral N plus 30 kg P ha-1 phosphorus (NP), NP combined with 3.75 Mg ha-1 straw (NP + Str), farmyard manure (Man, 75 Mg ha-1), and NP combined with manure (NP + Man) on crop productivity and carbon emissions (soil GHG emission; GHGI, yield-based GHG intensity; NGHGB, net GHG balance; carbon footprint, CF) in a maize-wheat cropping system during two years (April 2018-June 2020) in a semi-arid continental climate after 40 years of fertilization in the Northwest China. Manure and straw increased total GHG by 38-60 % compared to the mineral fertilizers alone, which was mainly due to the 49-80 % higher direct emissions of carbon dioxide (CO2) rather than nitrous oxide (N2O). Compared to the N fertilizer alone, organic amendments and NP increased cumulative energy yield by 134-202 % but decreased GHGI by 38-55 %, indicating that organic fertilizers increased crop productivity at the cost of higher GHG emissions. When the soil organic carbon changes (ΔSOC) were accounted for in the C emission balance, manure application acted as a net C sink due to the NGHGB recorded with -123 kg CO2-eq ha-1 year-1. When producing the same yield and economic benefits, the manure and straw addition decreased the CF by 59-85 % compared to N fertilization alone. Overall, the transition from mineral to organic fertilization in the semi-arid regions is a two-way independent solution to increase agricultural productivity along with the reduction of C emissions.

Global synthesis on the response of soil microbial necromass carbon to climate-smart agriculture

Climate-smart agriculture (CSA) supports the sustainability of crop production and food security, and benefiting soil carbon storage. Despite the critical importance of microorganisms in the carbon cycle, systematic investigations on the influence of CSA on soil microbial necromass carbon and its driving factors are still limited. We evaluated 472 observations from 73 peer-reviewed articles to show that, compared to conventional practice, CSA generally increased soil microbial necromass carbon concentrations by 18.24%. These benefits to soil microbial necromass carbon, as assessed by amino sugar biomarkers, are complex and influenced by a variety of soil, climatic, spatial, and biological factors. Changes in living microbial biomass are the most significant predictor of total, fungal, and bacterial necromass carbon affected by CSA; in 61.9%-67.3% of paired observations, the CSA measures simultaneously increased living microbial biomass and microbial necromass carbon. Land restoration and nutrient management therein largely promoted microbial necromass carbon storage, while cover crop has a minor effect. Additionally, the effects were directly influenced by elevation and mean annual temperature, and indirectly by soil texture and initial organic carbon content. In the optimal scenario, the potential global carbon accrual rate of CSA through microbial necromass is approximately 980 Mt C year-1, assuming organic amendment is included following conservation tillage and appropriate land restoration. In conclusion, our study suggests that increasing soil microbial necromass carbon through CSA provides a vital way of mitigating carbon loss. This emphasizes the invisible yet significant influence of soil microbial anabolic activity on global carbon dynamics.

Amendment of straw with decomposing inoculants benefits the ecosystem carbon budget and carbon footprint in a subtropical wheat cropping field

The incorporation of straw with decomposing inoculants into soils has been widely recommended to sustain agricultural productivity. However, comprehensive analyses assessing the effects of straw combined with decomposing inoculants on greenhouse gas (GHG) emissions, net primary production (NPP), the net ecosystem carbon budget (NECB), and the carbon footprint (CF) in farmland ecosystems are scant. Here, we carried out a 2-year field study in a wheat cropping system with six treatments: rice straw (S), a straw-decomposing Bacillus subtilis inoculant (K), a straw-decomposing Aspergillus oryzae inoculant (Q), a combination of straw and Bacillus subtilis inoculant (SK), a combination of straw and Aspergillus oryzae inoculant (SQ), and a control with no rice straw or decomposing inoculant (Control). We found that all the treatments resulted in a positive NECB ranging between 838 and 5065 kg C ha-1. Relative to the Control, the S treatment increased CO2 emissions by 16%, while considerably enhancing the NECB by 349%. This difference might be attributed to the straw C input and an increase in plant productivity (NPP, 30%). More importantly, in comparison to that in S, the NECB in SK and SQ significantly increased by 27-35% due to the positive response of NPP to the decomposing inoculants. Although the combination of straw and decomposing inoculants yielded a 3% increase in indirect GHG emissions, it also exhibited the lowest CF (0.18 kg CO2-eq kg-1 of grain). This result was attributed to the synergistic effects of straw and decomposing inoculants, which reduced direct N2O emissions and increased wheat productivity. Overall, the findings of the present study suggested that the combined amendment of straw and decomposing inoculants is an environmentally sustainable management practice in wheat cropping systems that can generate win-win scenarios through improvements in soil C stock, crop productivity, and GHG mitigation.

Does no-till crop management mitigate gaseous emissions and reduce yield disparities: An empirical US-China evaluation

Global agricultural systems face one of the greatest sustainability challenges: meeting the growing demand for food without leaving a negative environmental footprint. United States (US) and China are the two largest economies and account for 39 % of total global greenhouse gases (GHG) emissions into the atmosphere. No-till is a promising land management option that allows agriculture to better adapt and mitigate climate change effects compared to traditional tillage. However, the efficacy of no-till for mitigating GHG is still debatable. In this meta-analysis, we comprehensively assess the impact of no-till (relative to traditional tillage) on GHG mitigation potential and crop productivity in different agroecological systems and management regimes in the US and China. Overall, no-till in China did not change crop yields, although soil CO2 (8 %) and N2O (12 %) emissions decreased significantly, while soil CH4 emissions increased by 12 %. In contrast to Chinese no-till, a significant improvement in crop yields (up to 12 %) was recorded on US cropland under no-till. Moreover, significant decreases in soil N2O (21 %) and CH4 (12 %) emissions were observed. Of the three cropping systems, only wheat showed significant reduction in CO2, N2O and CH4 emissions in the Chinese no-till system. In the case of US, no-till soybean-rice and maize cropping systems demonstrated significant emission reductions for N2O and CO2, respectively. Interestingly, yields of no-till maize in China and rice in US exceeded those of other no-till cereals. In China, no-till on medium-texture soils resulted in significant reductions in GHG emissions and higher crop yields compared to other soil types. In both countries, the relatively higher crop yields under irrigated versus non-irrigated no-till and the significant yield differences on fine textured soils under US no-till are likely due to the substantial N2O reductions. In summary, crop yield disparities from no-till between China and the US were related to the insignificant effects on controlling CH4 emissions and successfully mitigating N2O, respectively. This study comprehensively demonstrates how cropping system and pedoclimatic conditions influence the relative effectiveness of no-till in both countries.

Co-benefits for net carbon emissions and rice yields through improved management of organic nitrogen and water

Returning organic nutrient sources (for example, straw and manure) to rice fields is inevitable for coupling crop-livestock production. However, an accurate estimate of net carbon (C) emissions and strategies to mitigate the abundant methane (CH4) emission from rice fields supplied with organic sources remain unclear. Here, using machine learning and a global dataset, we scaled the field findings up to worldwide rice fields to reconcile rice yields and net C emissions. An optimal organic nitrogen (N) management was developed considering total N input, type of organic N source and organic N proportion. A combination of optimal organic N management with intermittent flooding achieved a 21% reduction in net global warming potential and a 9% rise in global rice production compared with the business-as-usual scenario. Our study provides a solution for recycling organic N sources towards a more productive, carbon-neutral and sustainable rice-livestock production system on a global scale.

A global meta-analysis of yield-scaled N2 O emissions and its mitigation efforts for maize, wheat, and rice

Maintaining or even increasing crop yields while reducing nitrous oxide (N2 O) emissions is necessary to reconcile food security and climate change, while the metric of yield-scaled N2 O emission (i.e., N2 O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta-analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield-scaled N2 O emissions for maize, wheat and rice and associated potential mitigation options. Our results showed that the average yield-scaled N2 O emissions across all available data followed the order wheat (322 g N Mg-1 , with the 95% confidence interval [CI]: 301-346) > maize (211 g N Mg-1 , CI: 198-225) > rice (153 g N Mg-1 , CI: 144-163). Yield-scaled N2 O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield-scaled N2 O emissions with increasing N use efficiency or in N2 O emissions for production systems with cereal yields >10 Mg ha-1 (maize), 6.6 Mg ha-1 (wheat) or 6.8 Mg ha-1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade-offs between crop production and N2 O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced-efficiency N fertilizers significantly reduced yield-scaled N2 O emissions at similar or even higher cereal yields. Our data-driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production.

Impacts of reduced synthetic fertiliser use under current and future climates: Exploration using integrated agroecosystem modelling in the upper River Taw observatory, UK

The intensification of farming and increased nitrogen fertiliser use, to satisfy the growing population demand, contributed to the extant climate change crisis. Use of synthetic fertilisers in agriculture is a significant source of anthropogenic Greenhouse Gas (GHG) emissions, especially potent nitrous oxide (N2O). To achieve the ambitious policy target for net zero by 2050 in the UK, it is crucial to understand the impacts of potential reductions in fertiliser use on multiple ecosystem services, including crop production, GHG emissions and soil organic carbon (SOC) storge. A novel integrated modelling approach using three established agroecosystem models (SPACSYS, CSM and RothC) was implemented to evaluate the associated impacts of fertiliser reduction (10%, 30% and 50%) under current and projected climate scenarios (RCP2.6, RCP4.5 and RCP8.5) in a study catchment in Southwest England. 48 unique combinations of soil types, climate conditions and fertiliser inputs were evaluated for five major arable crops plus improved grassland. With a 30% reduction in fertiliser inputs, the estimated yield loss under current climate ranged between 11% and 30% for arable crops compared with a 20-24% and 6-22% reduction in N2O and methane emissions, respectively. Biomass was reduced by 10-25% aboveground and by <12% for the root system. Relative to the baseline scenario, soil type dependent reductions in SOC sequestration rates are predicted under future climate with reductions in fertiliser inputs. Losses in SOC were more than doubled under the RCP4.5 scenario. The emissions from energy use, including embedded emissions from fertiliser manufacture, was a significant source (14-48%) for all arable crops and the associated GWP20.

Maximizing the carbon sink function of paddy systems in China with machine learning

Developing low-carbon agriculture and alleviating the "carbon crisis" requires optimizing strategies that fully leverage the carbon sink function of paddy systems. Accurate assessment of the effects of various agricultural management practices (AMPs) on the carbon sink function of paddy systems is crucial to this end. Here, we have presented a soil organic carbon sequestration rate (SOCSR) database of paddy systems in China based on 1388 groups of experimental data from 143 peer-reviewed publications. We analyzed the impact trend of different AMPs on SOCSR, compared two traditional regression models, four classic machine learning models and two deep learning models, and established a data-driven SOCSR prediction model to quantify the impact of AMPs on SOCSR and provide the optimal strategies. Our model (Random Forest) had the characteristics of high accuracy (R2 = 0.71, RMSE = 0.53 Mg ha-1), strong flexibility, low time cost with a certain degree of interpretability for the regional scale of China. We found that inorganic N fertilizer, inorganic K fertilizer, organic fertilizer, tillage and residue management are relatively important AMPs for improving SOCSR. The carbon sink function of paddy systems would reach saturation when the application rate of inorganic N fertilizer, inorganic K fertilizer and organic fertilizer reached around 80 kg N ha-1, 40 kg K ha-1 and 2200 kg C ha-1, respectively. Compared to half residue returning and conventional tillage, full residue returning and no-tillage increased SOCSR by 39.8 % and 9.2 %, respectively. Our optimal combination of strategies could achieve SOCSR of 1.179 Mg ha-1 in paddy systems of China. Our work enables swift and precise evaluation of SOCSR in paddy systems, provides a new idea for assessing SOCSR of paddy systems on a regional scale, and serves as an essential part for the accurate assessment of the carbon footprint of rice production.

Modification of the RothC model to evaluate the inconsistent effect of conservation tillage on SOC stock and a suggestion of a national-scale assessment framework

Simulation of conservation tillage effect on soil organic carbon (SOC) stock on the national scale is essential for Tier 3 level greenhouse gas inventory in the agricultural sector. However, the conservation tillage effects varied depending on different soil conditions, potentially leading to inaccurate national assessments. This study aimed to propose a framework for estimating the national scale impact of conservation tillage on SOC. As even in the most commonly used SOC dynamic model, the Rothamsted Carbon Model (RothC), does not reflect the conservation tillage effect in an explicit way, we modified it by developing the tillage rate modifiers (TRMs). First, we investigated the conditions for the inconsistent conservation tillage effects using the decision tree analysis based on 210 field experiment data from the mid-latitude region. The results highlighted that soil sand content and the existing SOC stock were the main factors driving the inconsistencies. After we categorized into four distinctive conditions, the TRMs for each condition were parameterized using a genetic algorithm. The average TRMs were 0.88 in the soils with sand content >37.6 % and 1.58 in the soils with sand content ≤37.6 %, indicating that conservation tillage is more effective in coarse-textured soil, and there is a risk of decreasing SOC stock in the latter condition. Using the modified RothC model, a three-step national-scale simulation framework was suggested: compiling country-specific data, establishing baseline and conservation tillage scenarios, and modeling conservation tillage effects with uncertainty analysis. Our approach also defined the maximum conservation tillage area, factoring in local cropping systems and soil conditions. Our refined RothC model with TRMs provides a nuanced understanding of conservation tillage effects, emphasizing the role of soil characteristics. The proposed national-scale simulation framework offers a reliable tool for evaluating conservation tillage impact on SOC, ensuring more accurate greenhouse gas inventories in agriculture.

Exploring low-carbon mulching strategies for maize and wheat on-farm: Spatial responses, factors and mitigation potential

Mulching strategies - including plastic film mulching (FM) and straw mulching (SM) - can enhance crop yields while affecting multiple greenhouse gas (GHG) fluxes. However, most of currently published site-based studies only focus on a certain gas, resulting in an inability to spatially integrated understanding of changes in agricultural global warming potential (GWP) and greenhouse gas intensity (GHGI) caused by mulching across China. Thus, we developed an optimal model considering crop type, meteorology, soil and management variables by four machine learning methods, namely support vector machine, multilayer perceptron, random forest, and gradient boosting machine (GBM). Then we mapped the relative changes in yield and GHG fluxes caused by mulching strategies. The GBM model had the best simulation capability for yield and GHGs in China. Our result showed that FM increased yield in maize (25 %) and wheat (19 %), while SM respectively increased by 14 % and 11 %. Among the relative changes due to mulching strategies, yield and N2O emissions were mainly influenced by soil fertility and soil properties, CH4 uptakes and CO2 emissions were more affected by environmental factors. GWP in maize and wheat average increased by 40 % under FM, while SM decreased GWP by 14 % and 2 %, respectively. Besides, FM average increased GHGI in maize and wheat by 17 % and 9 %, and SM decreased GHGI by 22 % and 12 %, respectively. Spatially, FM reduced maize GWP on 19 % of cropland, while SM reduced maize and wheat GWP on 71 % and 64 % of cropland, respectively. Soil pH was significantly correlated with ΔGHGI in maize and wheat. Our analysis not only estimated for the first time the spatial effects of mulching strategies across China, but also systematically analyzes the agricultural carbon emission mitigation potential of mulching strategies, which promote the development of low-carbon agriculture based on locally appropriate mulching strategies.

Spatiotemporal co-optimization of agricultural management practices towards climate-smart crop production

Co-optimization of multiple management practices may facilitate climate-smart agriculture, but is challenged by complex climate-crop-soil management interconnections across space and over time. Here we develop a hybrid approach combining agricultural system modelling, machine learning and life cycle assessment to spatiotemporally co-optimize fertilizer application, irrigation and residue management to achieve yield potential of wheat and maize and minimize greenhouse gas emissions in the North China Plain. We found that the optimal fertilizer application rate and irrigation for the historical period (1995-2014) are lower than local farmers' practices as well as trial-derived recommendations. With the optimized practices, the projected annual requirement of fertilizer, irrigation water and residue inputs across the North China Plain in the period 2051-2070 is reduced by 16% (14-21%) (mean with 95% confidence interval), 19% (7-32%) and 20% (16-26%), respectively, compared with the current supposed optimal management in the historical reference period, with substantial greenhouse gas emission reductions. We demonstrate the potential of spatiotemporal co-optimization of multiple management practices and present digital mapping of management practices as a benchmark for site-specific management across the region.

Net greenhouse gas balance in U.S. croplands: How can soils be part of the climate solution?

Agricultural soils play a dual role in regulating the Earth's climate by releasing or sequestering carbon dioxide (CO2 ) in soil organic carbon (SOC) and emitting non-CO2 greenhouse gases (GHGs) such as nitrous oxide (N2 O) and methane (CH4 ). To understand how agricultural soils can play a role in climate solutions requires a comprehensive assessment of net soil GHG balance (i.e., sum of SOC-sequestered CO2 and non-CO2 GHG emissions) and the underlying controls. Herein, we used a model-data integration approach to understand and quantify how natural and anthropogenic factors have affected the magnitude and spatiotemporal variations of the net soil GHG balance in U.S. croplands during 1960-2018. Specifically, we used the dynamic land ecosystem model for regional simulations and used field observations of SOC sequestration rates and N2 O and CH4 emissions to calibrate, validate, and corroborate model simulations. Results show that U.S. agricultural soils sequestered13.2 ± 1.16 $$ 13.2\pm 1.16 $$ Tg CO2 -C year-1 in SOC (at a depth of 3.5 m) during 1960-2018 and emitted0.39 ± 0.02 $$ 0.39\pm 0.02 $$ Tg N2 O-N year-1 and0.21 ± 0.01 $$ 0.21\pm 0.01 $$ Tg CH4 -C year-1 , respectively. Based on the GWP100 metric (global warming potential on a 100-year time horizon), the estimated national net GHG emission rate from agricultural soils was122.3 ± 11.46 $$ 122.3\pm 11.46 $$ Tg CO2 -eq year-1 , with the largest contribution from N2 O emissions. The sequestered SOC offset ~28% of the climate-warming effects resulting from non-CO2 GHG emissions, and this offsetting effect increased over time. Increased nitrogen fertilizer use was the dominant factor contributing to the increase in net GHG emissions during 1960-2018, explaining ~47% of total changes. In contrast, reduced cropland area, the adoption of agricultural conservation practices (e.g., reduced tillage), and rising atmospheric CO2 levels attenuated net GHG emissions from U.S. croplands. Improving management practices to mitigate N2 O emissions represents the biggest opportunity for achieving net-zero emissions in U.S. croplands. Our study highlights the importance of concurrently quantifying SOC-sequestered CO2 and non-CO2 GHG emissions for developing effective agricultural climate change mitigation measures.

Agricultural transformation towards delivering deep carbon cuts in China's arid inland areas

Since agriculture is a main source of global greenhouse gas (GHG) emissions, reducing agricultural GHG emissions is crucial for achieving global climate goals. Nevertheless, there has been a lack of thorough and systematic assessment of the spatiotemporal distribution of agricultural GHG emissions at the county level, considering many factors such as crop and livestock products, different processes and gases, and the impact of carbon fixation. Furthermore, the potential of comprehensive technical strategies to reduce GHG emissions remains uncertain. Considering the unique attributes of agricultural development in arid areas of northwest China, this study aimed to explore long-term changes in agricultural net GHG emissions by county, product group, process, and gas and quantify the future reduction potential based on the Agricultural System-induced GreenHouse Gases INVentory (ASGHG-INV) econometric model. The results showed increasing trends in carbon emissions (CE), carbon sequestration (CS), carbon footprint (CF), crop carbon footprint per unit area (CFCF), and crop carbon footprint per unit product (CPCF) in various regions from 1991 to 2019, while there was a decreasing trend in livestock carbon footprint per unit product (LPCF). Focus on reducing GHG emissions in the crop-sector should be in Shihezi, Alaer, and Liangzhou; those of the livestock-sector should be in Xinyuan, Yecheng, Liangzhou, and Gaotai. Scenario analysis indicated that agricultural transformation could substantially reduce GHG emissions in all regions. Reducing the loss of reactive nitrogen was shown to be the most effective single strategy for reducing crop emissions. A comprehensive scheme further integrating the optimization of nitrogen fertilizer management, increasing water-saving, manure application, and straw returning measures, and using biochar and inhibitors can decrease CE, CF, CFCF, and CPCF by 22.62-43.45%, 40.55-111.60%, 41.38-111.78%, and 43.33-111.32%, respectively, increase CS by 9.07-39.97%. Optimizing forage composition was the most influential strategy for reducing livestock GHG emissions. The integrated strategy of further using forage additives, breeding low-emission varieties, and optimizing fecal management can reduce CF and LPCF by 37.32-76.42% and 40.51-78.70%, respectively. This study's results can be a reference for developing more effective GHG emissions reduction and green transformation pathways for global dryland agriculture.

Mitigation of greenhouse gas emissions and improved yield by plastic mulching in rice production

Soil mulching technologies are effective practices which alleviate non-point source pollution and carbon emissions, while ensuring grain production security and increasing water productivity. However, the lack of comprehensive understanding of the impacts of mulching technologies on rice fields has hindered progress in global implementation due to the varying environments and application conditions under which they are implemented. This study conducted a meta-analysis based on 2412 groups of field experiment data from 313 studies to evaluate the effects of soil mulching methods on rice production, greenhouse gas (GHG) emissions and water use efficiency. The results show that plastic mulching, straw mulching and no mulching (PM, SM and NM) have reduced CH₄ emissions (68.8 %, 61.4 % and 57.2 %), increased N₂O emissions (84.8 %, 89.1 % and 96.6 %), reduced global warming potentials (50.7 %, 47.5 % and 46.8 %) and improved water use efficiency (50.2 %, 40.9 % and 34.0 %) compared with continuous flooding irrigation. However, PM increased rice yield (1.6 %), while SM and NM decreased yield (4.3 % and 9.2 %). Furthermore, analysis using random forest models revealed that rice yield, GHG emissions and WUE response to soil mulching were related to climate, soil properties, fertilizer and rice varieties. Our findings can guide the implementation of plastic mulching technology in priority areas, contribute to agricultural carbon neutrality and support the development of practical guidelines for farmers.

Management-induced changes in soil organic carbon and related crop yield dynamics in China's cropland

Enhancing soil organic carbon (SOC) sequestration and food supply are vital for human survival when facing climate change. Site-specific best management practices (BMPs) are being promoted for adoption globally as solutions. However, how SOC and crop yield are related to each other in responding to BMPs remains unknown. Here, path analysis based on meta-analysis and machine learning was conducted to identify the effects and potential mechanisms of how the relationship between SOC and crop yield responds to site-specific BMPs in China. The results showed that BMPs could significantly enhance SOC and maintain or increase crop yield. The maximum benefits in SOC (30.6%) and crop yield (79.8%) occurred in mineral fertilizer combined with organic inputs (MOF). Specifically, the optimal SOC and crop yield would be achieved when the areas were arid, soil pH was ≥7.3, initial SOC content was ≤10 g kg-1 , duration was >10 years, and the nitrogen (N) input level was 100-200 kg ha-1 . Further analysis revealed that the original SOC level and crop yield change showed an inverted V-shaped structure. The association between the changes in SOC and crop yield might be linked to the positive role of the nutrient-mediated effect. The results generally suggested that improving the SOC can strongly support better crop performance. Limitations in increasing crop yield still exist due to low original SOC level, and in regions where the excessive N inputs, inappropriate tillage or organic input is inadequate and could be diminished by optimizing BMPs in harmony with site-specific conditions.

Changes in cropland soil carbon through improved management practices in China: A meta-analysis

Recommended management practices (RMPs, e.g., manuring, no-tillage, crop residue return) can increase soil organic carbon (SOC), reduce greenhouse gas emissions, and maintain soil health in croplands. However, there is no consensus on how RMPs affect the SOC storage potential of cropland soils for climate change mitigation. Here, based on 2301 comparisons from 158 peer-reviewed papers, a meta-analysis was conducted to explore management-induced SOC stock changes and their variations under different conditions. The results show that SOC stocks in the 0-20 cm layer were increased by 31.8% when chemical fertilization combined with manure application was compared with no fertilizer; 9.98% when no-tillage was compared with plow tillage; and 10.84% when straw return was compared with removal. The RMPs favorably increased SOC stock in arid areas, and in alkaline and fine-textured soils. Initial SOC, carbon-nitrogen ratio, and experimental duration could also affect SOC storage. Compared with the initial SOC stock, RMPs increased the SOC sequestration potential by 2.6-4.5% in the 0-20 cm soil depth, indicating that these practices can help China achieve targets to increase SOC by 4.0‰. Hence, it is essential to implement RMPs for climate change mitigation and soil fertility improvement.

Integrated biochar solutions can achieve carbon-neutral staple crop production

Agricultural food production is a main driver of global greenhouse gas emissions, with unclear pathways towards carbon neutrality. Here, through a comprehensive life-cycle assessment using data from China, we show that an integrated biomass pyrolysis and electricity generation system coupled with commonly applied methane and nitrogen mitigation measures can help reduce staple crops' life-cycle greenhouse gas emissions from the current 666.5 to -37.9 Tg CO₂-equivalent yr^-1. Emission reductions would be achieved primarily through carbon sequestration from biochar application to the soil, and fossil fuel displacement by bio-energy produced from pyrolysis. We estimate that this integrated system can increase crop yield by 8.3%, decrease reactive nitrogen losses by 25.5%, lower air pollutant emissions by 125-2,483 Gg yr^-1 and enhance net environmental and economic benefits by 36.2%. These results indicate that integrated biochar solutions could contribute to China's 2060 carbon neutrality objective while enhancing food security and environmental sustainability.

Soil organic carbon regulates CH4 production through methanogenic evenness and available phosphorus under different straw managements

Methane (CH₄) is the main greenhouse gas emitted from rice paddy fields driven by methanogens, for which methanogenic abundance on CH₄ production has been intensively investigated. However, information is limited about the relationship between methanogenic diversity (e.g., richness and evenness) and CH₄ production. Three independent field experiments with different straw managements including returning method, burial depth, and burial amount were used to identify the effects of methanogenic diversity on CH₄ production, and its regulating factors from soil properties in a rice-wheat cropping system. The results showed that methanogenic evenness (dominance) can explain 23% of variations in CH₄ production potential. CH₄ production potential was positively related to methanogenic evenness (R² = 0.310, p < 0.001), which is driven by soil organic carbon (SOC), available phosphorus (AP), and nitrate (NO₃^-) through structure equation model (SEM). These findings indicate that methanogenic evenness has a critical role in evaluating the responses of CH₄ production to agricultural practices following changes in soil properties. The SEM also revealed that SOC concentration influenced CH₄ production potential indirectly via complementarity of methanogenic evenness (dominance) and available phosphorus (AP). Increasing SOC accumulation improved AP release and stimulated CH₄ production when SOC was at a low level, whereas decreased evenness and suppressed CH₄ production when SOC was at a high level. A nonlinear relationship was detected between SOC and CH₄ production potential, and CH₄ production potential decreased when SOC was ≥14.16 g kg^-1. Our results indicated that the higher SOC sequestration can not only mitigate CO₂ emissions directly but CH₄ emissions indirectly, highlighting the importance to enhance SOC sequestration using optimum agricultural practices in a rice-wheat cropping system.

Reduced nitrogen fertilization under flooded conditions cut down soil N2O and CO2 efflux: An incubation experiment

Unreasonable water (W) and inorganic nitrogen (N) fertilization cause an intensification of soil greenhouse gas (GHGs) emissions. W-N interactions (W × N) patterns can maximise the regulation of soil GHGs efflux through the rational matching of W and N fertilization factors. However, the effects of W × N patterns on soil GHGs efflux and the underlying mechanism remain unclear. In this study, urea fertilizers were applied to paddy soils in a gradient of 100 (N100), 80 (N80), and 60 mg kg^-1 (N60) concentrations. Flooding (W1) and 60% field holding capacity (W2) was set for each N fertilizer application to observe the effects of W × N patterns on soil properties and GHGs efflux through incubation experiments. The results showed that W significantly affected soil electrical conductivity and different N forms (i.e., alkali hydrolyzed N, ammonium N, nitrate N and microbial biomass N) contents. Soil organic carbon (C) content was reduced by 14.40% in W1N60 relative to W1N100, whereas microbial biomass C content was increased by 26.87%. Moreover, soil methane (CH₄) fluxes were low in all treatments, with a range of 1.60-1.65 μg CH₄ kg^-1. Soil nitrous oxide (N₂O) and carbon dioxide (CO₂) fluxes were significantly influenced by W, N and W × N. Global warming potential was maintained at the lowest level in W1N60 treatment at 0.67 g CO₂-eq kg^-1, suggesting W1N60 as the preferred W × N pattern with high environmental impact. Our findings demonstrate that reduced N fertilization contributes to the effective mitigation of soil N₂O and CO₂ efflux by lowering the soil total N and organic C contents and regulating soil microbial biomass C and N.

Heavy metals in agricultural soil in China: A systematic review and meta-analysis

Research about farmland pollution by heavy metals/metalloids in China has drawn growing attention. However, there was rare information on spatiotemporal evolution and pollution levels of heavy metals in the major grain-producing areas. We extracted and examined data from 276 publications between 2010 and 2021 covering five major grain-producing regions in China from 2010 to 2021. Spatiotemporal evolution characteristics of main heavy metals/metalloids was obtained by meta-analysis. In addition, subgroup analyses were carried out to study preliminary correlations related to accumulation of the pollutants. Cadmium (Cd) was found to be the most prevailing pollutant in the regions in terms of both spatial distribution and temporal accumulation. The Huang-Huai-Hai Plain was the most severely polluted. Accumulation of Cd, mercury (Hg) and copper (Cu) increased from 2010 to 2015 when compared with the 1990 background data. Further, the levels of five key heavy metals (Cd, Cu, Hg, lead [Pb] and zinc [Zn]) showed increasing trends from 2016 to 2021 in all five regions. Soil pH and mean annual precipitation had variable influences on heavy metal accumulation. Alkaline soil and areas with less rainfall faced higher pollution levels. Farmlands cropped with mixed species showed smaller effect sizes of heavy metals than those with single upland crop, suggesting that mixed farmland use patterns could alleviate the levels of heavy metals in soil. Of various soil remediation efforts, farmland projects only held a small market share. The findings are important to support the research of risk assessment, regulatory development, pollution prevention, fund allocation and remediation actions.

Current status of carbon neutrality in Chinese rice fields (2002-2017) and strategies for its achievement

China has pledged to achieve carbon neutrality by 2060 to address global climate change, and achieving carbon neutrality in rice fields is a vital component of this commitment. However, the current status of carbon neutrality in rice fields in China is unclear, and there are few feasible strategies to achieve its successful implementation. Therefore, this study calculated the net carbon sequestration rate (NCSR, i.e., carbon sequestration minus carbon emissions) of rice fields in China from 2002 to 2017 to clarify the carbon neutrality status of Chinese rice fields. Furthermore, the effects of field management measures, rice sown area, and rice yield on NCSR were analyzed to identify suitable carbon neutralization pathways in Chinese rice fields. Our findings indicated that the annual carbon sequestration rate in rice fields was lower than the carbon emissions, resulting in continuous net emissions of 195.49 Tg CO₂-eq yr^-1. The NCSR of paddy fields increased first and then decreased with increases in rice sown area and yield. Meta-analysis indicated that management measures such as water conservation and biochar significantly increased NCSR by ~5766.50 kg CO₂-eq ha^-1 yr^-1 and 22,296.62 kg CO₂-eq ha^-1 yr^-1, respectively. Our findings suggests that proper control of rice sown area and the adoption of reasonable field management measures (water conservation and biochar) can promote carbon neutrality in Chinese rice fields.

Increased rice yield and reduced greenhouse gas emissions through alternate wetting and drying in a triple-cropped rice field in the Mekong Delta

For sustainable food production in the Mekong Delta, greenhouse gas (GHG) emissions from rice cropping activities need to be reduced without sacrificing rice productivity. Each year, a substantial amount of straw is incorporated into paddy soils through triple rice cropping, which is characterized by a short cropping period and nearly year-round flooding, such that a large amount of methane is emitted. Exposing these soils to oxidative conditions by altering the cropping-period water regime might have the potential to reduce GHG emissions with increased rice yield. To test this potential, a split-plot experiment was conducted in a typical triple-cropped alluvial farmer's paddy in a central delta area over five years and 15 consecutive cropping seasons. The emissions observed from the continuously inundated paddies were 1.1-2.7 times greater than the reported emission factors for Vietnamese continuously inundated paddies. A significantly higher emission peak was detected at the beginning of the rice cropping and flooding fallow periods in continuously flooded (CF) paddies than in alternate wetting and drying (AWD) paddies, although the differences in field water level and soil moisture among the paddies were negligible. AWD reduced annual methane emissions (-51 %) and increased rice yield (+9 %), presumably through enhanced translocation of carbohydrates from leaves to panicles. The amount of GHGs emitted from straw use also decreased (11 %) under AWD management because the straw production rate was significantly lowered (9 %) by enhanced nutrient translocation. These results indicate that GHG emission reduction potentials in the Mekong Delta have been underestimated by previous studies, corroborate the necessity of additional long-term observations of triple rice cropping systems and demonstrate the need for a robust methodology for monitoring the permanence of AWD effects after policies promoting its widespread dissemination take effect.

Global benefits of non-continuous flooding to reduce greenhouse gases and irrigation water use without rice yield penalty

Non-continuous flooding is an effective practice for reducing greenhouse gas (GHG) emissions and irrigation water use (IRR) in rice fields. However, advancing global implementation is hampered by the lack of comprehensive understanding of GHGs and IRR reduction benefits without compromising rice yield. Here, we present the largest observational data set for such effects as of yet. By using Random Forest regression models based on 636 field trials at 105 globally georeferenced sites, we identified the key drivers of effects of non-continuous flooding practices and mapped maximum GHGs or IRR reduction benefits under optimal non-continuous flooding strategies. The results show that variation in effects of non-continuous flooding practices are primarily explained by the UnFlooded days Ratio (UFR, that is the ratio of the number of days without standing water in the field to total days of the growing period). Non-continuous flooding practices could be feasible to be adopted in 76% of global rice harvested areas. This would reduce the global warming potential (GWP) of CH₄ and N₂ O combined from rice production by 47% or the total GWP by 7% and alleviate IRR by 25%, while maintaining yield levels. The identified UFR targets far exceed currently observed levels particularly in South and Southeast Asia, suggesting large opportunities for climate mitigation and water use conservation, associated with the rigorous implementation of non-continuous flooding practices in global rice cultivation.

Unexpected Parabolic Temperature Dependency of CH4 Emissions from Rice Paddies

Global warming is expected to affect methane (CH₄) emissions from rice paddies, one of the largest human-induced sources of this potent greenhouse gas. However, the large variability in warming impacts on CH₄ emissions makes it difficult to extrapolate the experimental results over large regions. Here, we show, through meta-analysis and multi-site warming experiments using the free air temperature increase facility, that warming stimulates CH₄ emissions most strongly at background air temperatures during the flooded stage of ∼26 °C, with smaller responses of CH₄ emissions to warming at lower and higher temperatures. This pattern can be explained by divergent warming responses of plant growth, methanogens, and methanotrophs. The effects of warming on rice biomass decreased with the background air temperature. Warming increased the abundance of methanogens more strongly at the medium air temperature site than the low and high air temperature sites. In contrast, the effects of warming on the abundance of methanotrophs were similar across the three temperature sites. We estimate that 1 °C warming will increase CH₄ emissions from paddies in China by 12.6%─substantially higher than the estimates obtained from leading ecosystem models. Our findings challenge model assumptions and suggest that the estimates of future paddy CH₄ emissions need to consider both plant and microbial responses to warming.

Technologies and perspectives for achieving carbon neutrality

Global development has been heavily reliant on the overexploitation of natural resources since the Industrial Revolution. With the extensive use of fossil fuels, deforestation, and other forms of land-use change, anthropogenic activities have contributed to the ever-increasing concentrations of greenhouse gases (GHGs) in the atmosphere, causing global climate change. In response to the worsening global climate change, achieving carbon neutrality by 2050 is the most pressing task on the planet. To this end, it is of utmost importance and a significant challenge to reform the current production systems to reduce GHG emissions and promote the capture of CO₂ from the atmosphere. Herein, we review innovative technologies that offer solutions achieving carbon (C) neutrality and sustainable development, including those for renewable energy production, food system transformation, waste valorization, C sink conservation, and C-negative manufacturing. The wealth of knowledge disseminated in this review could inspire the global community and drive the further development of innovative technologies to mitigate climate change and sustainably support human activities.

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Carbon neutrality may be achieved by reforming current global development systems to minimize greenhouse gas emissions and increase CO₂ capture

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Harnessing the power of renewable and carbon-neutral resources to produce energy and other fossil-based alternatives may eliminate our dependence on fossil fuels

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Protecting natural carbon sinks and promoting CO₂ capture, utilization, and storage are conducive to mitigating climate change

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This review presents the current state, opportunities, challenges, and perspectives of technologies related to achieving carbon neutrality