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Aquaculture ponds are of increasing worldwide concerns as critical sources of atmospheric methane (CH 4 ) and nitrous oxide (N 2 O), but little is known about these gases emissions as affected by aquaculture species, stocking and water management in aquaculture ponds. Here, a two-year study was carried out to quantify CH 4 and N 2 O emissions from freshwater crab and fish aquaculture ponds in subtropical China. We further explored how the microbial functional genes [CH 4 : mcrA and pmoA; N 2 O: archaeal and bacterial amoA (AOA + AOB), nirS, nirK, nosZ] may drive CH 4 and N 2 O release in the crab aquaculture pond typically undergoing flooding-to-drainage alteration. Over the two-year period, annual CH 4 and N 2 O fluxes averaged 0.95 mg m -2  h -1 and 20.94 μg m -2  h -1 in the fish aquaculture, and 0.78 mg m -2  h -1 and 28.48 μg m -2  h -1 in the crab aquaculture, respectively. The direct N 2 O emission factors were estimated to be 0.77% and 0.36% of the total N input by feed or 1.59 g N 2 O-N kg -1 and 1.06 g N 2 O-N kg -1 aquaculture yield in the crab and fish ponds, respectively. Among three functional stocking areas, CH 4 and N 2 O emissions were consistently the highest at the feeding area (FA) in the both aquaculture ponds, followed by at the undisturbed area (UA) and aerated area (AA). The shift in sediment soil moisture from waterlogging to drainage conditions significantly increased the abundance of AOB relative to AOA and pmoA, decreased those of denitrifying functional genes (nirS, nirK, nosZ) and mcrA, while did not alter the functional group ratio of nirS + nirK relative to nosZ. Our results highlight that a better understanding of CH 4 and N 2 O emissions from aquaculture ponds requires taking into consideration of data sourced from more diverse aquaculture systems with different management patterns. In addition, a deep analysis of the microbial processes that drive CH 4 and N 2 O production and consumption from aquaculture ponds remains to be addressed in future studies., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The net balance of greenhouse gas (GHG) exchanges between terrestrial ecosystems and the atmosphere under elevated atmospheric carbon dioxide (CO 2 ) remains poorly understood. Here, we synthesise 1655 measurements from 169 published studies to assess GHGs budget of terrestrial ecosystems under elevated CO 2 . We show that elevated CO 2 significantly stimulates plant C pool (NPP) by 20%, soil CO 2 fluxes by 24%, and methane (CH 4 ) fluxes by 34% from rice paddies and by 12% from natural wetlands, while it slightly decreases CH 4 uptake of upland soils by 3.8%. Elevated CO 2 causes insignificant increases in soil nitrous oxide (N 2 O) fluxes (4.6%), soil organic C (4.3%) and N (3.6%) pools. The elevated CO 2 -induced increase in GHG emissions may decline with CO 2 enrichment levels. An elevated CO 2 -induced rise in soil CH 4 and N 2 O emissions (2.76 Pg CO 2 -equivalent year -1 ) could negate soil C enrichment (2.42 Pg CO 2 year -1 ) or reduce mitigation potential of terrestrial net ecosystem production by as much as 69% (NEP, 3.99 Pg CO 2 year -1 ) under elevated CO 2 . Our analysis highlights that the capacity of terrestrial ecosystems to act as a sink to slow climate warming under elevated CO 2 might have been largely offset by its induced increases in soil GHGs source strength., (© 2018 John Wiley & Sons Ltd/CNRS.)

Aquaculture is an important source of atmospheric methane (CH4) and nitrous oxide (N2O), while few direct flux measurements are available for their regional and global source strength estimates. A parallel field experiment was performed to measure annual CH4 and N2O fluxes from rice paddies and rice paddy-converted inland crab-fish aquaculture wetlands in southeast China. Besides N2O fluxes dependent on water/sediment mineral N and CH4 fluxes related to water chemical oxygen demand, both CH4 and N2O fluxes from aquaculture were related to water/sediment temperature, sediment dissolved organic carbon, and water dissolved oxygen concentration. Annual CH4 and N2O fluxes from inland aquaculture averaged 0.37 mg m(-2) h(-1) and 48.1 μg m(-2) h(-1), yielding 32.57 kg ha(-1) and 2.69 kg N2O-N ha(-1), respectively. The conversion of rice paddies to aquaculture significantly reduced CH4 and N2O emissions by 48% and 56%, respectively. The emission factor for N2O was estimated to be 0.66% of total N input in the feed or 1.64 g N2O-N kg(-1) aquaculture production in aquaculture. The conversion of rice paddies to inland aquaculture would benefit for reconciling greenhouse gas mitigation and agricultural income increase as far as global warming potentials and net ecosystem economic profits are of concomitant concern. Some agricultural practices such as better aeration and feeding, and fallow season dredging would help to lower CH4 and N2O emissions from inland aquaculture. More field measurements from inland aquaculture are highly needed to gain an insight into national and global accounting of CH4 and N2O emissions.

Rice paddies and aquaculture wetlands are typical agricultural wetlands that constitute one of the important sources of atmospheric methane (CH4). 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 CH4 emissions from a transplanted rice paddy and its converted crab-fish farming wetland in southeast China. Over the rice-growing season, CH4 fluxes averaged 1.86 mg m(-2) h(-1) from rice paddies, and 1.14 and 0.50 mg m(-2) h(-1) for the treatments with or without aquatic vegetation present in the crab-fish farming wetlands, respectively. When averaged across the treatments, seasonal CH4 emissions from crab-fish framing wetlands were 52% lower than those from rice paddies. The CH4 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 CH4 fluxes on DO or DOC was intensified by the aquatic vegetation presence. By extrapolating the present CH4 emission rate with the current rice paddy-converted aquaculture cultivation area, the seasonal CH4 emissions from inland aquaculture wetlands during the critical farming stage (20 June to 18 October) were estimated to be 33.6 Gg ha(-1) in southeast China in 2012. Rice paddies conversion to crab-fish farming wetlands might have reduced CH4 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 CH4 release rate.

The aim of this experiment was to determine the impacts of climate change on soil profile concentrations and diffusion effluxes of methane in a rice-wheat annual rotation ecosystem in Southeastern China. We initiated a field experiment with four treatments: ambient conditions (CKs), CO2 concentration elevated to ~500 μmol/mol (FACE), temperature elevated by ca. 2°C (T) and combined elevation of CO2 concentration and temperature (FACE+T). A multilevel sampling probe was designed to collect the soil gas at four different depths, namely, 7 cm, 15 cm, 30 cm and 50 cm. Methane concentrations were higher during the rice season and decreased with depth, while lower during the wheat season and increased with depth. Compared to CK, mean methane concentration was increased by 42%, 57% and 71% under the FACE, FACE+T and T treatments, respectively, at the 7 cm depth during the rice season (p<0.05). Mean methane diffusion effluxes to the 7 cm depth were positive in the rice season and negative in the wheat season, resulting in the paddy field being a source and weak sink, respectively. Moreover, mean methane diffusion effluxes in the rice season were 0.94, 1.19 and 1.42 mg C/(m2·hr) in the FACE, FACE+T and T treatments, respectively, being clearly higher than that in the CK. The results indicated that elevated atmospheric CO2 concentration and temperature could significantly increase soil profile methane concentrations and their effluxes from a rice-wheat field annual rotation ecosystem (p<0.05)., (Copyright © 2015. Published by Elsevier B.V.)

Measurements of methane (CH(4)) and nitrous oxide (N(2)O) fluxes have been extensively taken following rice seedlings transplanted into paddy fields, while little is known about CH(4) and N(2)O fluxes from rice seedling nurseries. Fluxes of CH(4) and N(2)O were simultaneously measured in rice seedling nurseries under the water regimes of continuous flooding and moist irrigation without waterlogging in Southeast China in 2010. Fluxes of CH(4) and N(2)O from continuously flooded nurseries averaged 10.33-14.84 mg m(-2) h(-1) and 28.64-34.35 μg N(2)O-Nm(-2) h(-1) for the different fertilizer applied plots, respectively. Relative to continuous flooding, moist irrigation decreased total CH(4) by 14-50% but increased N(2)O by 72-186%, dependent on the fertilizer types. Compared with inorganic N fertilizer, organic manure application increased CH(4) by 44% and 148% in the continuously flooded and moist irrigation nurseries, respectively. Rice seedling growth parameters were the greatest in moist irrigation nurseries with inorganic N fertilizer application. Moist irrigation instead of continuous waterlogging and shifts from organic manure to combined organic/inorganic N fertilizer inputs have been increasingly experienced in Chinese rice seedling nurseries, which would benefit for mitigating the combined global warming potentials of CH(4) and N(2)O from rice seedling nurseries in China., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Field experiment was conducted in 2001 rice growing season to understand the effect of organic manure application on CH4 and N2O emissions. Five treatments of fertilizer including urea and NH4HCO3 used as chemical fertilizer (CF), chemical fertilizer + pig manure (CF + PM), chemical fertilizer + cow manure (CF + CM), chemical fertilizer + rapeseed cake (CF + RC) and chemical fertilizer + wheat straw (CF + WS) were dedicated to this experiment. Results from the field observations indicated that application of the organic manure did not change the seasonal pattern of CH4 and N2O emissions but significantly affected the seasonal amounts. Seasonal amount of CH4 emission from the treatments of CF + CM, CF + RC and CF + WS was respectively 45%, 252% and 250% higher than that from the treatment of CF, but the application of pig manure (CF + PM) did not significantly influence the CH4 emission. The N2O emissions, on the other hand, were reduced by 18%, 21% and 18% for the treatments of CF + PM, CF + CM and CF + WS, respectively. Incorporation of rapeseed cake enhanced the N2O emission, yielding an increment of 22%. On a time scale of 20-year period, the integrated global warming potential (GWP) of CH4 + N2O released during the rice growing season for the treatments of CF + RC, CF + WS and CF + CM was respectively 154%, 136% and 17% higher than that for the CF treatment, while a reduction of 10% was obtained for the CF + PM treatment. The calculation of the GWP per unit rice grain yield suggested that the application of pig manure took the slight advantages not only in reducing the GWP but also in improving the grain yield.

Inland waters (rivers, reservoirs, lakes, ponds, streams) and estuaries are significant emitters of methane (CH4) and nitrous oxide (N2O) to the atmosphere, while global estimates of these emissions have been hampered due to the lack of a worldwide comprehensive data set of CH4 and N2O flux components. Here, we synthesize 2997 in‐situ flux or concentration measurements of CH4 and N2O from 277 peer‐reviewed publications to estimate global CH4 and N2O emissions from inland waters and estuaries. Inland waters including rivers, reservoirs, lakes, and streams together release 95.18 Tg CH4 year−1 (ebullition plus diffusion) and 1.48 Tg N2O year−1 (diffusion) to the atmosphere, yielding an overall CO2‐equivalent emission total of 3.06 Pg CO2 year−1. The estimate of CH4 and N2O emissions represents roughly 60% of CO2 emissions (5.13 Pg CO2 year−1) from these four inland aquatic systems, among which lakes act as the largest emitter for both CH4 and N2O. Ebullition showed as a dominant flux component of CH4, contributing up to 62%–84% of total CH4 fluxes across all inland waters. Chamber‐derived CH4 emission rates are significantly greater than those determined by diffusion model‐based methods for commonly capturing of both diffusive and ebullitive fluxes. Water dissolved oxygen (DO) showed as a dominant factor among all variables to influence both CH4 (diffusive and ebullitive) and N2O fluxes from inland waters. Our study reveals a major oversight in regional and global CH4 budgets from inland waters, caused by neglecting the dominant role of ebullition pathways in those emissions. The estimated indirect N2O EF5 values suggest that a downward refinement is required in current IPCC default EF5 values for inland waters and estuaries. Our findings further indicate that a comprehensive understanding of the magnitude and patterns of CH4 and N2O emissions from inland waters and estuaries is essential in defining the way of how these aquatic systems will shape our climate. [ABSTRACT FROM AUTHOR]

Inland freshwater ecosystems are of increasing concerns in global methane (CH 4) budget in the atmosphere. Agricultural irrigation watersheds are a potential CH 4 emission hotspot owing to the anthropogenic carbon and nutrients loading. However, large-scale spatial variations of CH 4 concentrations and fluxes in agricultural catchments remain poorly understood, constraining an accurate regional estimate of CH 4 budgets. Here, we examined the spatiotemporal variations of dissolved CH 4 concentrations and fluxes from typical freshwater types (ditch, reservoir and river) within an agricultural irrigation watershed from Hongze catchment, which is subjected to intensive agricultural and rural activities in Eastern China. The dissolved CH 4 concentrations and fluxes showed similar temporal variations among the three freshwater types, with the highest rates in summer and the lowest rates in winter. The total CH 4 emission from this agricultural irrigation watershed was estimated to be 0.002 Gg CH 4 yr−1, with annual mean CH 4 concentration and flux of 0.12 μmol L−1 and 0.58 mg m−2 d−1, respectively. Diffusive CH 4 fluxes varied in samples taken from different freshwater types, the annual mean CH 4 fluxes for ditch, reservoir and river were 0.31 ± 0.06, 0.71 ± 0.13 and 0.72 ± 0.25 mg m−2 d−1, respectively. Among three freshwater types, the CH 4 fluxes were the lowest in ditch, which was associated with the lowest responses of CH 4 fluxes to water dissolved oxygen (DO), nitrate nitrogen (NO 3 −-N) and sediment dissolved organic carbon (DOC) concentrations in ditch. In addition, water velocity and wind speed were significantly lower in ditch than in reservoir and river, suggesting that they also played important roles in explaining the spatial variability of dissolved CH 4 concentrations and fluxes. These results highlighted a need for more field measurements with wider spatial coverage and finer frequency, which would further improve the reliability of flux estimates for assessing the contribution of agricultural watersheds to the regional and global CH 4 budgets. [Display omitted] • CH 4 concentrations and fluxes showed similar temporal variations among water types. • The CH 4 fluxes were significantly lower in ditches than in reservoirs and rivers. • The most sensitive responses of CH 4 fluxes to water parameters were found in rivers. • Water velocity and wind speed are responsible for spatial variability of CH 4 fluxes. [ABSTRACT FROM AUTHOR]

A full accounting of net greenhouse gas balance ( NGHGB) and greenhouse gas intensity ( GHGI) was examined in an annual coastal reclaimed saline Jerusalem artichoke-fallow cropping system under various soil practices including soil tillage, soil ameliorant, and crop residue amendments. Seasonal fluxes of soil carbon dioxide ( CO2), methane ( CH4), and nitrous oxide (N2O) were measured using static chamber method, and the net ecosystem exchange of CO2 ( NEE) was determined by the difference between soil heterotrophic respiration ( RH) and net primary production ( NPP). Relative to no-tillage, rotary tillage significantly decreased the NPP of Jerusalem artichoke while it had no significant effects on the annual RH. Rotary tillage increased CH4 emissions, while seasonal or annual soil N2O emissions did not statistically differ between the two tillage treatments. Compared with the control plots, soil ameliorant or straw amendment enhanced RH, soil CH4, and N2O emissions under the both tillage regimes. Annual NGHGB was negative for all the field treatments, as a consequence of net ecosystem CO2 sequestration exceeding the CO2-equivalents released as CH4 and N2O emissions, which indicates that Jerusalem artichoke-fallow cropping system served as a net sink of GHGs. The annual net NGHGB and GHGI were estimated to be 11-21% and 4-8% lower in the NT than in RT cropping systems, respectively. Soil ameliorant and straw amendments greatly increased NPP and thus significantly decreased the negative annual net NGHGB. Overall, higher NPP but lower climatic impacts of coastal saline bioenergy production would be simultaneously achieved by Jerusalem artichoke cultivation under no-tillage with improved saline soil conditions in southeast China. [ABSTRACT FROM AUTHOR]

A complete accounting of net greenhouse gas balance (NGHGB) and greenhouse gas intensity (GHGI) affected by Fe(III) fertilizer application was examined in typical annual paddy rice-winter wheat rotation cropping systems in southeast China. Annual fluxes of soil carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) were measured using static chamber method, and the net ecosystem exchange of CO2 (NEE) was determined by the difference between soil CO2 emissions (R H) and net primary production (NPP). Fe(III) fertilizer application significantly decreased R H without adverse effects on NPP of rice and winter wheat. Fe(III) fertilizer application decreased seasonal CH4 by 27–44%, but increased annual N2O by 65–100%. Overall, Fe(III) fertilizer application decreased the annual NGHGB and GHGI by 35–47% and 30–36%, respectively. High grain yield and low greenhouse gas intensity can be reconciled by Fe(III) fertilizer applied at the local recommendation rate in rice-based cropping systems. [Copyright &y& Elsevier]

Greenhouse gas emissions from rice paddies under sewage irrigation deserve much attention since domestic sewage effluents are increasingly used for agriculture in developing countries. A field experiment was conducted to simultaneously measure methane (CH4) and nitrous oxide (N2O) emissions from rice (Oryza sativa L.) paddies under sewage and unpolluted river water irrigation in southeast China. The rice paddies were under a local typical water regime, which was characterized by flooding–midseason drainage–reflooding–moist intermittent irrigation but without water logging. Relative to unpolluted river water irrigation, sewage irrigation significantly increased CH4 and N2O emissions from rice paddies. Seasonal fluxes of CH4 averaged 1.51mgm−2 h−1 for the plots irrigated by river water and 1.92mgm−2 h−1 for the plots irrigated by sewage. In contrast with river water irrigation, sewage irrigation increased CH4 by 27% and 33% for paddy plots with and without chemical N addition, respectively. Under sewage irrigation, seasonal fluxes of N2O-N averaged 26.79μgm−2 h−1 for the plots without N application and 74.07μgm−2 h−1 for the plots applied at the rate of 200kgNha−1. Relative to river water irrigation, sewage irrigation increased N2O by 68% and 170% for the plots with and without N application, respectively. The direct emission factor of fertilizer N for N2O was estimated to be 0.71% for the rice paddies under sewage irrigation and 0.52% for the plots irrigated by river water. Besides direct N2O emissions, N input by sewage irrigation induced substantial indirect N2O emission from rice paddies. The results of the net GWPs from CH4 and N2O indicate that sewage irrigation would intensify the radiative forcing of rice paddies with midseason drainage and moist irrigation... [Copyright &y& Elsevier]

Efforts to advance our knowledge on the potential of bioenergy instead of fossil fuels in terms of mitigating climatic impact are in urgent need. No data is currently available on the use of urease and nitrification inhibitors in costal saline bioenergy cropping systems. An overall accounting of net greenhouse gas balance (NGHGB) and greenhouse gas intensity (GHGI) affected by combined effects of urease inhibitor hydroquinone (HQ) and nitrification inhibitor dicyandiamide (DCD) amendment was examined in a coastal saline Jerusalem artichoke bioenergy cropping system. The net ecosystem exchange of CO 2 (NEE) was determined by the difference between soil heterotrophic respiration ( R H ) and net primary production (NPP) using static chamber method. Urease and nitrification inhibitors amendment increased the NPP but exerted a suppression effect on soil R H over the Jerusalem artichoke cropping system. A trade-off relationship was observed by decreasing soil N 2 O but stimulating soil CH 4 emissions following HQ+DCD amendment. The plots combined urea with HQ+DCD application increased soil CH 4 by 167% while decreased N 2 O by 16% as compared to with urea only in the bioenergy cropping system. On average, the fertilizer N-induced emission factor of N 2 O was estimated to be 0.25% across the fertilized plots. Compared with urea, the plots with urea and HQ+DCD resulted in a further decrease by 37% and 15% in estimated NGHGB and GHGI over the Jerusalem artichoke cropping system, respectively. Overall, Jerusalem artichoke production would achieve higher biomass as source of biofuels but lower climatic impacts, particularly when together with urease and nitrification inhibitors amendment in coastal saline soils. [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]

Inland aquaculture ponds have been documented as important sources of atmospheric methane (CH 4 ) and nitrous oxide (N 2 O), while their regional or global source strength remains unclear due to lack of direct flux measurements by covering more typical habitat-specific aquaculture environments. In this study, we compared the CH 4 and N 2 O fluxes from rice paddies and nearby inland fish aquaculture wetlands that were converted from rice paddies in southeast China. Both CH 4 and N 2 O fluxes were positively related to water temperature and sediment dissolved organic carbon, but negatively related to water dissolved oxygen concentration. More robust response of N 2 O fluxes to water mineral N was observed than to sediment mineral N. Annual CH 4 and N 2 O fluxes from inland fish aquaculture averaged 0.51 mg m −2 h −1 and 54.78 μg m −2 h −1 , amounting to 42.31 kg CH 4 ha −1 and 2.99 kg N 2 O-N ha −1 , respectively. The conversion of rice paddies to conventional fish aquaculture significantly reduced CH 4 and N 2 O emissions by 23% and 66%, respectively. The emission factor for N 2 O was estimated to be 0.46% of total N input in the feed or 1.23 g N 2 O-N kg −1 aquaculture production. The estimate of sustained-flux global warming potential of annual CH 4 and N 2 O emissions and the net economic profit suggested that such conversion of rice paddies to inland fish aquaculture would help to reconcile the dilemma for simultaneously achieving both low climatic impacts and high economic benefits in China. More solid direct field measurements from inland aquaculture are in urgent need to direct the overall budget of national or global CH 4 and N 2 O fluxes. [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]

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]

The potential of biochar for mitigating climatic impacts of coastal saline bioenergy production is not well established. A full accounting of net greenhouse gas balance (NGHGB) and greenhouse gas intensity (GHGI) affected by biochar amendment combined with or without nitrogen (N) fertilizer application was examined in an annual coastal reclaimed Jerusalem artichoke bioenergy cropping system. The net ecosystem exchange of CO 2 (NEE) was determined by the difference between soil heterotrophic respiration ( R H ) and net primary production (NPP) using static chamber method. Biochar amendment raised the seasonal R H but without suppressing the NPP during the Jerusalem artichoke cropping season. Soil CH 4 emissions were 72% and 80% lower in the biochar amended than unamended plots when combined with N fertilizer application during the Jerusalem artichoke cropping and non-cropping seasons, respectively. The biochar-induced soil N 2 O mitigation efficiency was weakened by N fertilizer input over the annual cycle. Annual NGHGB and GHGI were negative for all the field treatments and were significantly lower in biochar amended than in unamended soils, suggesting that Jerusalem artichoke cropping system served as a net sink of GHGs due to net ecosystem CO 2 and biochar-induced C sequestration exceeding CO 2 -equivalents released as CH 4 and N 2 O emissions. On average, biochar amendment significantly enhanced GHGs sink capacity by resulting in almost 4–5 folds decrease in annual NGHGB or GHGI when combined with N fertilizer application or not. Therefore, higher biomass gain as potential alternative source of biofuels but lower climatic impacts of bioenergy production would be reconciled by biochar use in southeast coastal China. [ABSTRACT FROM AUTHOR]

Freshwater aquaculture ponds constitute one of the important anthropogenic sources of atmospheric methane (CH 4). Nevertheless, estimates of global CH 4 emissions from freshwater aquaculture have large uncertainties due to a lack of data from different aquaculture types. Furthermore, despite that ebullition is a major pathway of CH 4 in aquatic systems, the quantification of ebullitive CH 4 fluxes from typical freshwater aquaculture ponds has been poorly represented. Here, field measurements of CH 4 fluxes over two years were taken to quantify ebullitive CH 4 fluxes from inland freshwater fish and crab aquaculture ponds in subtropical China. Ebullitive CH 4 fluxes averaged 15.97 ± 1.57 and 11.22 ± 1.26 mg m−2 d−1 in the fish and crab ponds in the first experimental year, respectively, and were 22.86 ± 2.30 and 21.95 ± 2.19 mg m−2 d−1 in the second year. During aquaculture period, ebullition dominated the emission pathways of CH 4 , accounting for 83% and 98% of the total CH 4 emissions in the fish and crab ponds, respectively. Ebullitive CH 4 fluxes exhibited considerable spatial variations, with the lowest flux rates captured at the aeration area due to aerator-use in both the fish and crab ponds. Dissolved oxygen and dissolved organic carbon were the two primary factors that drove ebullitive CH 4 fluxes in both aquaculture ponds. By incorporating global measurement data, we further assessed the CH 4 mitigation potential of aerator use in freshwater aquaculture and revealed the dominant role of ebullition in this mitigation contribution. Together with the rice-based aquaculture, aerator use could reduce CH 4 emissions from freshwater aquaculture ponds globally by 71% and in China by 63%. • Ebullition dominates the pathways of CH 4 emissions from inland aquaculture. • Ebullitive CH 4 fluxes differ with bioturbation to sediment in aquaculture ponds. • Aerator use could reduce global inland aquaculture CH 4 emissions by 71%. • Specific area aerator use benefits the whole aquaculture pond CH 4 mitigation. [ABSTRACT FROM AUTHOR]

The optimization of fertilization management has the potential to improve crop yield, reduce greenhouse gas (GHG) emissions and enhance nitrogen agronomy efficiency (NAE). However, less is known about whether these benefits can be simultaneously achieved by optimizing fertilization regimes in Chinese croplands. We carried out a year-round field experiment to measure methane (CH 4) and nitrous oxide (N 2 O) fluxes, crop yield and NAE under different fertilization regimes in a subtropical double-rice cropping system. Relative to the conventional chemical N fertilizer application (F), alternative fertilization strategies significantly decreased N 2 O emissions by 26–78% when averaged across the options of chemical N fertilizer application with reduced rates (RF, 30% off), chemical N fertilizer fully replaced by organic N fertilizer with reduced rates (OF, 30% off) and chemical N fertilizer fully replaced by controlled-release N fertilizer with reduced rates (CF, 30% off), with the largest mitigation potential occurring in OF-treated plots, but comparable extents between the early- and late-rice seasons. Soil CH 4 emissions had no consistent response to alternative fertilization regimes, showing contrasting seasonal patterns between in the early- and late-rice seasons. Alternative fertilization options consistently increased NAE by 7–36% and 30–38% in the early- and late-rice seasons, respectively, and this benefit was maximized in OF-treated plots. Direct emission factors of N fertilizer for N 2 O and the combined greenhouse gas intensity (GHGI) of N 2 O and CH 4 were negatively related to NAE while rice yield was positively related to NAE. Our findings suggest that optimized fertilization strategies especially through the option of chemical N fertilizer fully substituted by organic N fertilizer can reconcile low climatic impact and high NAE, but without compromising yield in double-rice cropping systems. • Soil GHG emissions were linked to NAE in double-rice under different fertilization regimes. • Soil N 2 O emissions showed a significant decrease by 26–78% across optimized fertilization regimes. • Optimized fertilization alternatives consistently increased NAE in double-rice cropping seasons. • Direct EFs of N 2 O and annual combined GHGI of CH 4 and N 2 O were negatively related to NAE. • Organic N substitution stood out to reconcile low SGWP and high NAE of double-rice production. [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]