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Nitrogen (N) inputs and land-use conversion are management practices that affect soil greenhouse gas (GHG) and nitric oxide (NO) emissions. Here, we measured soil methane (CH4), nitrous oxide (N2O), and NO fluxes from rice fields and a peach orchard that converted from paddies to assess the impacts of nitrogen (N) inputs and land-use conversion on their emissions. Treatments included four paddy field treatments (PN0, PN160, PN220, and PN280) and one peach orchard treatment (ON280) with number indicating the N-input rate of kg N ha−1. The results showed that cumulative emissions of CH4, N2O and NO ranged from 28.6 to 85.3 kg C ha−1, 0.5 to 4.0 kg N ha−1 and 0.2 to 0.3 kg N ha−1 during the rice-growing season, respectively. In terms of greenhouse gas intensity, the PN280 treatment is the recommended N application rate. Land-use conversion significantly reduced the global warming potential from croplands. The conversion shifted soils from an essential source of CH4 to a small net sink. In addition, N2O emissions from the rice–wheat rotation system were 1.8 times higher than from the orchard, mainly due to the difference in the N application rate. In summary, to reduce agriculture-induced GHG emissions, future research needs to focus on the effects of N inputs on rice-upland crop rotation systems. [ABSTRACT FROM AUTHOR]

Agricultural soils play an important role in the atmospheric methane (CH4) budget, where paddy soils can contribute significant CH4 to atmosphere whereas upland soils may act as a source or sink of atmospheric CH4, dependent on soil water conditions. Biochar amendments have effects on soil CH4 production or oxidation processes in individual experiments, but the causative mechanisms are yet to be fully elucidated. To synthesize the response of soil CH4 release or uptake to biochar amendment, we performed a meta-analysis using data from 61 peer-reviewed papers with 222 updated paired measurements. When averaged across all studies, biochar amendment significantly decreased CH4 release rates by 12% for paddy soils and 72% for upland soils, and CH4 uptake rates by 84% for upland soils. Neither soil CH4 release nor uptake responses to biochar amendment were significant in field soils. Nitrogen (N) fertilizer application would weaken the response of soil CH4 release or uptake to biochar amendment. Biochar-incurred decreases in soil CH4 release and uptake rates were the largest in medium-textured soils or neutral-pH soils. Soil CH4 release or uptake responses to biochar were also significantly altered by biochar characteristics, such as feedstock source, C/N ratio, pH, and pyrolysis temperature. The results of this synthesis suggest that the role of biochar in soil CH4 mitigation potential might have been exaggerated, particularly in fields when biochar is applied in combination with N fertilizer. [ABSTRACT FROM AUTHOR]

Abstract: The net balance of greenhouse gas (GHG) exchanges between terrestrial ecosystems and the atmosphere under elevated atmospheric carbon dioxide (CO2) remains poorly understood. Here, we synthesise 1655 measurements from 169 published studies to assess GHGs budget of terrestrial ecosystems under elevated CO2. We show that elevated CO2 significantly stimulates plant C pool (NPP) by 20%, soil CO2 fluxes by 24%, and methane (CH4) fluxes by 34% from rice paddies and by 12% from natural wetlands, while it slightly decreases CH4 uptake of upland soils by 3.8%. Elevated CO2 causes insignificant increases in soil nitrous oxide (N2O) fluxes (4.6%), soil organic C (4.3%) and N (3.6%) pools. The elevated CO2‐induced increase in GHG emissions may decline with CO2 enrichment levels. An elevated CO2‐induced rise in soil CH4 and N2O emissions (2.76 Pg CO2‐equivalent year−1) could negate soil C enrichment (2.42 Pg CO2 year−1) or reduce mitigation potential of terrestrial net ecosystem production by as much as 69% (NEP, 3.99 Pg CO2 year−1) under elevated CO2. Our analysis highlights that the capacity of terrestrial ecosystems to act as a sink to slow climate warming under elevated CO2 might have been largely offset by its induced increases in soil GHGs source strength. [ABSTRACT FROM AUTHOR]

Rice paddies and aquaculture wetlands are typical agricultural wetlands that constitute one of the important sources of atmospheric methane (CH). Traditional transplanted rice paddies have been experiencing conversion to pond aquaculture wetlands for pursuing higher economic benefits over the past decades in southeast China. A parallel field experiment was carried out to compare CH emissions from a transplanted rice paddy and its converted crab-fish farming wetland in southeast China. Over the rice-growing season, CH fluxes averaged 1.86 mg m h from rice paddies, and 1.14 and 0.50 mg m h for the treatments with or without aquatic vegetation present in the crab-fish farming wetlands, respectively. When averaged across the treatments, seasonal CH emissions from crab-fish framing wetlands were 52 % lower than those from rice paddies. The CH fluxes were negatively related to water dissolved oxygen (DO) concentration but positively related to soil/sediment dissolved organic carbon (DOC) content in crab-fish farming wetlands. Dependence of CH fluxes on DO or DOC was intensified by the aquatic vegetation presence. By extrapolating the present CH emission rate with the current rice paddy-converted aquaculture cultivation area, the seasonal CH emissions from inland aquaculture wetlands during the critical farming stage (20 June to 18 October) were estimated to be 33.6 Gg ha in southeast China in 2012. Rice paddies conversion to crab-fish farming wetlands might have reduced CH emissions by 22-54 % in mainland China. Results of this study suggest that the conversion of transplanted rice paddies to crab-fish aquaculture wetlands for higher economic benefits would also lead to a lower ecosystem CH release rate. [ABSTRACT FROM AUTHOR]

Background and aims: The rice production is experiencing a shift from conventionally seedling-transplanted (TPR) to direct-seeded (DSR) cropping systems in Southeast Asia. Besides the difference in rice crop establishment, water regime is typically characterized as water-saving moist irrigation for DSR and flooding-midseason drainage-reflooding and moist irrigation for TPR fields, respectively. A field experiment was conducted to quantify methane (CH) and nitrous oxide (NO) emissions from the DSR and TPR rice paddies in southeast China. Methods: Seasonal measurements of CH and NO fluxes from the DSR and TPR plots were simultaneously taken by static chamber-GC technique. Results: Seasonal fluxes of CH averaged 1.58 mg m h and 1.02 mg m h across treatments in TPR and DSR rice paddies, respectively. Compared with TPR cropping systems, seasonal NO emissions from DSR cropping systems were increased by 49 % and 46 % for the plots with or without N application, respectively. The emission factors of NO were estimated to be 0.45 % and 0.69 % of N application, with a background emission of 0.65 and 0.95 kg NO-N ha under the TPR and DSR cropping regimes, respectively. Rice biomass and grain yield were significantly greater in the DSR than in the TPR cropping systems. The net global warming potential (GWP) of CH and NO emissions were comparable between the two cropping systems, while the greenhouse gas intensity (GHGI) was significantly lower in the DSR than in the TPR cropping systems. Conclusions: Higher grain yield, comparable GWP, and lower GHGI suggest that the DSR instead of conventional TPR rice cropping regime would weaken the radiative forcing of rice production in terms of per unit of rice grain yield in China, and DSR rice cropping regime could be a promising rice development alternative in mainland China. [ABSTRACT FROM AUTHOR]

How to achieve high soil organic carbon (SOC) sequestration and low greenhouse gases emissions but without compromising yields is the challenge of rice cultivation in China. This study explored the potential to simultaneously increase SOC stocks and reduce methane (CH 4) emissions by optimizing rice cropping rotation and fertilization based on a 2-year field experiment in southeast China. We tested five optimized N management practices including directly reducing chemical N fertilizer input and replacing chemical N fertilizers with organic and controlled-release fertilizers in double-rice cropping and Chinese milk vetch-rice cropping rotation systems. Results showed higher seasonal rice yields, nitrogen use efficiency (NUE) and SOC stocks for Chinese milk vetch-rice rotation systems (CR) as compared to double-rice cropping systems (DR). Seasonal rice yields were the highest in the 2019 single-rice season (2019-SR), followed by the 2018 late (2018-LR) and early rice seasons (2018-ER). SOC stocks of CR for the treatment receiving organic N fertilizer (OF) were the largest among all rice seasons and fertilization treatments. Seasonal CH 4 emissions were 8–12% greater in the OF than in the chemical N fertilizer treatments across all rice seasons, with the greatest in 2019-SR and lowest in 2018-ER. Fertilization stimulated CH 4 emissions through enhancing mcrA gene activity, but regulated by soil properties, while rice rotation models indirectly influenced CH 4 emissions through direct effects on soil properties. However, yield-scaled CH 4 emissions remained unaffected by N fertilization in the two-year rice rotation systems. The NUE was significantly enhanced in OF than in other fertilizer treatments over all seasons. Seasonal net ecosystem C budget (NECB) was 24–41% greater in OF than in chemical N fertilizer treatments across all rice seasons, with the greatest in 2019-SR and lowest in 2018-LR. The NECB was increased by 46% in CR as compared to DR. Our results revealed that the optimized rice cropping rotation by incorporation of Chinese milk vetch with organic N fertilization is beneficial for enhancing soil C sequestration and maintaining rice yields in southeast China. [Display omitted] • Green manure rotation benefits for the enhancement of paddy soil carbon sinks. • Green manure rotation stimulates CH 4 emissions during the rice growing season. • Yield-scaled CH 4 emissions remain unaffected by N fertilization and crop rotation. • Green manure rotation can reconcile low climatic impacts with high yield of rice. [ABSTRACT FROM AUTHOR]

Rice paddies are a significant source of atmospheric methane (CH 4) threatening global warming. Methane fluxes from rice soils are mediated by both methanogens and methanotrophs, while subject to fertilization regimes. A field experiment was conducted to investigate the response of CH 4 emissions, with a close link to methanogenic and methanotrophic communities, to different fertilization strategies over two consecutive rice growing seasons in a paddy field. Four fertilized treatments were established consisting of chemical phosphorous (P) and potassium (K) fertilizer (PK), chemical nitrogen (N) PK fertilizer (NPK), chemical NPK combined with organic manure (NPKM), and organic manure alone (M). Results showed that, relative to chemical PK treatment without N fertilizer input, the application of chemical NPK fertilizer, manure, and their combination significantly increased seasonal mean CH 4 emissions by 67.4, 20.4 and 101.2%, respectively. Methane fluxes were significantly and positively (r 2 = 0.75) related to mcrA gene copy numbers and the ratio of mcrA / pmoA , while negatively (r 2 = 0.40) related to pmoA gene copy numbers, which was mainly regulated by the dynamics of water irrigation regime. The abundance of mcrA and pmoA had trade-off relationship over the rice growing season. In comparison with PK fertilization, N fertilized treatments did not alter the seasonal trend of CH 4 fluxes, but enhanced the source strength of CH 4 emissions through increased mcrA abundances rather than reduced pmoA -dominated methanotrophic CH 4 oxidation in rice paddies. In contrast, N fertilization showed no effect on pmoA abundance and associated methanotrophic community in rice paddies. Results highlight that, given the fertilizer-driven balance between methanogenic and methanotrophic communities, fertilization options with either chemical or organic N input stimulated CH 4 emissions mainly through improving soil C substrate and methanogenic activities, especially under the combination of chemical and organic N fertilization in rice paddies. • Methane fluxes were most enhanced by combination of chemical and organic N fertilization. • Methane fluxes were positively related to abundance of mcrA and its ratio to those of pmoA. • The abundance of mcrA and pmoA had an obvious seasonal trade-off relationship. • Long-term N fertilization increased abundances of mcrA rather than those of pmoA. • The methanogenic community was largely altered under long-term organic N fertilization. [ABSTRACT FROM AUTHOR]

• CH 4 and N 2 O emissions from rice-aquaculture systems remain poorly constrained. • Rice-crayfish co-culture production is beneficial to reduce CH 4 and N 2 O emissions. • Ditched culture areas release more CH 4 but less N 2 O relative to rice planted areas. • Lower emission factors of N 2 O in rice-crayfish co-culture than monoculture systems. Rice paddies and rice-aquaculture co-culture systems are important sources of methane (CH 4) and nitrous oxide (N 2 O) to the atmosphere. Due to the growing human demand of aquaculture protein and the rapid development of green agriculture, rice-aquaculture co-culture systems have been increasingly developed from rice paddies in southeast China. However, the strength of CH 4 and N 2 O emissions from rice-aquaculture co-culture systems remains poorly quantified, in particular the contribution of these emissions from different functional cultivation areas. Here, a two-year parallel field experiment was conducted to examine the changes in CH 4 and N 2 O fluxes following the conversion from rice paddies to rice-crayfish co-culture systems in southeast China. Over the two-year period, annual CH 4 and N 2 O fluxes from rice-crayfish co-culture systems averaged 4.59 mg m-2 h-1 and 39.50 μg m -2 h -1, amounting to 391.9 kg ha−1 and 3.46 kg ha−1, respectively. The conversion from rice paddies to rice-crayfish co-culture systems significantly reduced CH 4 and N 2 O emissions by 14% and 31%, respectively. The emission factors of N 2 O were estimated to be 1.25–1.61% in the rice-crayfish co-culture system. Relative to rice planted areas, CH 4 and N 2 O fluxes from crayfish ditched culture areas were three times higher and 50% lower, which contributed 26% and 5% to the total CH 4 and N 2 O emissions from rice-crayfish co-culture systems, respectively. Our results highlight that the conversion of paddy rice monoculture to rice-crayfish co-culture systems can benefit low greenhouse gas emissions and high ecosystem economic profits as a potential option to sustain agricultural development and achieve carbon neutrality. [Display omitted] [ABSTRACT FROM AUTHOR]

Both elevated atmospheric carbon dioxide (CO 2 ) and rising temperature can alter soil methane (CH 4 ) fluxes, leading to a feedback to climate change. However, predicting this feedback needs to understand the microbial mechanisms involved in CH 4 emissions driven by climate change. A 3-year field measurement of CH 4 fluxes from rice paddies was taken in 2012–2014 to examine their responses to elevated CO 2 (enriched up to 500 μmol mol −1 ) and rising canopy air temperature (above ambient 1.5–2.0 °C) using a free-air CO 2 enrichment (FACE) system. Using real-time PCR and Illumina MiSeq sequencing of 16S rRNA genes, we measured the abundance and composition of methanogenic community in rhizosphere soil of rice paddies in 2014. Elevated CO 2 and rising temperature showed additive effects on CH 4 fluxes and methanogen abundances, where CH 4 fluxes were correlated with methanogen abundances. Elevated CO 2 , rising temperature and their combination increased seasonal CH 4 emissions by 28–120%, 38–74% and 82–143%, respectively. Either elevated CO 2 or rising temperature did not significantly alter the diversity of methanogenic community, and methanogenic genera Methanosaeta , Methanosarcina , Methanobacterium , Methanocella and Methanoregula dominated in rhizosphere soils for all treatments. However, elevated CO 2 induced a shift from acetoclastic to hydrogenotrophic methanogens in their relative abundances. Rising temperature stimulated CH 4 emissions by increasing CH 4 production per individual predominant methanogen genus. Besides the enhancement of soil C substrates and rhizosphere methanogen abundances as previously reported, an additive effect of elevated CO 2 and canopy warming on CH 4 emissions is also associated with elevated CO 2 -induced changes in the composition of methanogenic archaea and warming-stimulated the activity of methanogenic archaea in rice paddies. [ABSTRACT FROM AUTHOR]

Rice paddies and rivers draining agricultural watersheds have been documented as two major sources of atmospheric methane (CH 4), while parallel flux measurements have been rarely taken on CH 4 from rice paddies against the drainage rivers in agricultural watersheds. Moreover, the drainage events during the rice growing season deliver sufficient organic and inorganic nutrients to rivers, which makes there an increasing concerned source of atmospheric CH 4. Here, we compared CH 4 emissions from rice paddies and the associated rivers draining agricultural watersheds by a parallel field experiment in southeast China. Over the whole rice–growing season (June–November), CH 4 fluxes from drainage rivers averaged 25 mg m–2 h–1, which were 82 % higher than those from rice paddies (13.7 mg m–2 h–1). Besides the dependence of riverine CH 4 fluxes on water dissolved oxygen (DO), both ecosystem CH 4 fluxes were significantly correlated with water/soil dissolved organic carbon (DOC), oxidation-reduction potential (Eh) and the microbial functional genes that drive CH 4 production or uptake [methanogens (mcrA and methanogenic archaeal 16S rRNA) and methanotrophs (pmoA)]. Our results highlighted that the CH 4 mitigation potential benefited from the drainage practice in rice paddies might have been totally offset by its induced increases in CH 4 emissions from rivers draining agricultural watersheds at the regional scale. ● Rivers draining agricultural watersheds are an important source of atmospheric CH 4. ● CH 4 fluxes from drainage rivers have been rarely related to the abundance of microbes. ● CH 4 fluxes from drainage rivers were 82 % greater than those from rice paddies. ● The increase in CH 4 emissions in rivers may offset the mitigation benefit of CH 4 in rice paddies. [ABSTRACT FROM AUTHOR]

• IndirectN2Oemissions from rivers draining agricultural watersheds are of increasing concerns. • Seasonal dissolved N2O concentrationsand N2O fluxes showed a similar variation pattern. • Annual mean model-based N2O fluxes werecomparable with chamber-based N2O fluxes. • The indirect riverine EFof N2O dependedlargely on surface water NO3–N concentrations. • Annual mean riverine indirect EF of N2O was significant lower than the IPCC default value. Indirect nitrous oxide (N 2 O) emissions from rivers draining agricultural watersheds are of increasing concerns due to riverine abundant sources of nitrogen loaded through leaching and runoff. However, the seasonal variation of N 2 O emissions from agricultural drainage rivers is poorly explored, especially the uncertainty in quantifying indirect N 2 O emission factors (EFs) from these aquatic environments. Here, a two-year study (2014-2016) was conducted to quantify indirect N 2 O emissions from a river draining a rice paddy watershed in subtropical China. Indirect N 2 O fluxes were simultaneously determined using the floating chamber method (chamber-based) and the gas exchange modeling approach (model-based) based on the measurement of dissolved N 2 O concentration. Results showed that seasonal dissolved N 2 O concentration and N 2 O fluxes had a similar variation pattern, with the highest and the lowest levels in summer and winter, respectively. The annual mean of model-based N 2 O fluxes (20.24 ± 3.34 μmol m−2 d−1) was generally in agreement with chamber-based N 2 O fluxes (18.70 ± 3.56 μmol m−2 d−1). The indirect emission factor of N 2 O was highly dependent on the surface water NO 3 −-N concentration. Annual mean indirect EF of N 2 O from the drainage river was estimated to be 0.00051, which was significantly lower than the default EF 5r value (0.0025) proposed by the Intergovernmental Panel on Climate Change (IPCC). These results suggest that the use of IPCC default value might have overestimated indirect N 2 O emissions from agricultural impacted riverine systems. Our study also highlights that more extensive in-situ measurements are required for monitoring indirect N 2 O emissions from agricultural impacted waters with different drainage characteristics, which would benefit for refining the IPCC EF 5r default value to further constrain global N 2 O budget. [ABSTRACT FROM AUTHOR]

• Relating CH 4 fluxes to methanogens and methanotrophs under GCRPS remains unclear. • GCRPS reduced CH 4 emissions and the yield-scaled CH 4 emissions. • GCRPS decreased the abundance of mcrA gene while stimulated that of pmoA gene. • Microbial parameters are useful to better predict CH 4 emissions from rice paddies. While water management has been documented to regulate CH 4 emissions from rice paddies, the mechanisms behind the association between CH 4 emissions and functional microbes remain unclear under water-saving ground cover rice production systems (GCRPS). In GCRPS, the soil is kept aerobic by maintaining 80 %–90 % of the water holding capacity but without standing water over the entire rice-growing season, and the soil surface is covered with rice straw or plastic film to reduce water evaporation. A split-plot field experiment was conducted to examine the effects of various GCRPS-related water-saving regimes on CH 4 emissions by linking to CH 4 -related microbes over the 2015 and 2016 rice growing seasons in a typical Chinese rice field. Methane fluxes and related functional genes [methanogens (mcrA and methanogenic archaeal 16S rRNA) and methanotrophs (pmoA)] abundances were simultaneously measured using the closed chamber method and molecular techniques, respectively. The results showed that relative to the conventional waterlogged control (WRPS), GCRPS-related water-saving practices consistently reduced CH 4 emissions, which was largely attributed to the decreased relative abundance of the methanogenic functional gene mcrA while the methanotrophic functional gene pmoA increased. When averaged over the two rice seasons, the grain yield was 17.38 % and 12.22 % greater under the GCRPS -straw and GCRPS -film water-saving systems relative to WRPS, respectively. Water-saving regimes decreased the yield-scaled CH 4 emissions as compared with the WRPS, especially under GCRPS -film. The CH 4 fluxes showed a positive correlation with the relative abundance of mcrA and its ratio to pmoA , but a negative correlation with the relative abundance of pmoA. The performance of CH 4 -simulated models could be improved by incorporating microbial parameters to predict CH 4 emissions from rice fields. Overall, this study updates our understanding of the microbial mechanisms underlying CH 4 emissions from water-saving GCRPSs. We proposed the GCRPS regime with biodegradable film mulching as a desirable water-saving strategy to reconcile high yields and low CH 4 emissions in rice production. [ABSTRACT FROM AUTHOR]