[Effects of Soil Water and Availability of Carbon and Nitrogen on CH₄ and CO₂ Emissions in Paddy Soil].

In recent years， the rapid socio-economic development and the improvement of people's diets have driven the conversion of paddy soil to upland crop cultivation， leading to changes in soil water content， carbon and nitrogen availability， and the intensity of greenhouse gas emission. Therefore， it is crucial to study the effects of changes in soil water content and carbon and nitrogen availability on greenhouse gas CH₄ and CO₂ emissions and identify the key controlling factors upon rice paddy conversion into upland field， especially during the initial stage of conversion. Soil samples used in the present study were collected from a long-term rice paddy field and an adjacent upland field previously converted from rice paddy. The paddy soil was set into submerged （water to soil ratio of 2∶1） and from submerged to a slowly draining treatment （water to soil ratio of 2∶1 slowly decreased to 70% field water capacity and then remained stable） and compared with the upland soil （soil water content remained at 70% field water capacity）. Under each water gradient， the soil was supplied with labile C and N to change substrate availability： ① control （no substrate addition）， ② C addition （glucose）， ③ N addition （NH₄Cl）， and ④ C and N additions （glucose+NH₄Cl）. CH₄ and CO₂ emissions and soil biochemical properties were measured regularly during the incubation period so as to investigate the effects of soil water content， carbon and nitrogen availability， and their interaction on CH₄ and CO₂ emissions in paddy soil. The changes in contents of soil microbial biomass carbon （ΔMBC）， dissolved organic carbon （ΔDOC）， and soil mineral N （ΔMineral-N， containing ΔNH₄⁺-N and ΔNO₃^--N） over the incubation period were calculated by subtracting the initial values from the final values at the end of the incubation period. The results showed that as compared to the submerged condition， the drainage of submerged paddy soil significantly reduced CH₄ emission by 95% on average and increased CO₂ emission by 46% on average. The cumulative emissions of CH₄ and CO₂ were significantly higher in drained paddy soil （1.36 mg·kg^-1 and 584.13 mg·kg^-1 for CH₄ and CO₂， respectively） relative to those in upland soil （0.01 mg·kg^-1 and 407.70 mg·kg^-1）. CH₄ emissions from the submerged paddy soil significantly increased by 40% after carbon addition and decreased by 63% after nitrogen addition. The simultaneous additions of carbon and nitrogen had little effect on the CH₄ emissions from submerged paddy soil. CH₄ emissions from the drained paddy soil increased significantly by 48% after carbon addition， but there was no significant difference among other substrate addition treatments. In upland soil， the additions of carbon and nitrogen had no significant effect on CH₄ emissions but significantly increased CO₂ emissions by 45%-109%. The additions of carbon and nitrogen had little effect on CO₂ emissions in submerged paddy soil. The concurrent addition of carbon and nitrogen significantly increased CO₂ emissions by 36% in drained paddy soil. The interactions between soil water change and N addition had no significant effect on CH₄ emissions， while the interactions between soil water change and C and CN additions significantly affected CH₄ emissions. No significant interactions between soil water change and C and N availability were observed for CO₂ emissions. The conversion of submerged paddy to upland soil decreased soil pH， DOC， MBC， and NH₄⁺-N contents but increased NO₃^--N content. The additions of carbon and nitrogen significantly affected soil biochemical properties. The results of correlation analysis showed that CH₄ emissions were significantly positively correlated with soil pH， ΔMBC， and ΔNH₄⁺-N and negatively correlated with ΔNO₃^--N among treatments. Conversely， CO₂ emissions were significantly positively correlated with ΔNO₃^--N but negatively correlated with pH， ΔDOC， ΔMBC， and ΔNH₄⁺-N. The changes of soil chemical and biological properties induced by soil water change and carbon and nitrogen availability were the main factors influencing CH₄ and CO₂ emissions from paddy soil. In summary， changes in soil water content and carbon and nitrogen availability affect CH₄ and CO₂ emissions by altering soil biochemical properties. Drainage of paddy soil is an effective measure to reduce CH₄ emissions， but the risk of increased CO₂ emissions during the short-term period upon drainage should be considered. Therefore， when developing strategies for rice paddy management， it is crucial to consider the combined effects of water and C and N management so as to achieve effective greenhouse gas mitigation and green and sustainable agricultural production.