Impacts of combined water-saving irrigation and controlled-release urea on CH4 emission and its associated microbial communities and function

Water-saving irrigation and controlled-release nitrogen fertilizer are used in rice farming. The aim of this study was to understand the effects of water-saving irrigation and controlled-release urea on methane (CH4) emission and its associated microbial communities and function. A field experiment was conducted with two nitrogen treatments (NU 100% normal urea, CU 60% normal urea and 40% controlled-release urea, total N amount was the same) and three irrigation modes (CI continuous flooding irrigation, AI alternate wetting and drying irrigation, RI ridge irrigation). CH4 fluxes, organic acid contents and enzyme activities were measured, and soil microbial communities and function were investigated by whole-genome shotgun sequencing analysis, and then their relationships were analyzed by Spearman correlation analysis, redundancy analysis and mantel test. Compared to CI, AI and RI decreased cumulative CH4 emissions by 43.5% and 25.8% in NU, and 64.9% and 13.3% in CU, respectively. Among all treatments, AICU had the lowest CH4 emission and reduced it by 72.2% compared to CINU. AI and RI had higher contents of some organic acids than CI. Compared to CINU, AICU decreased the relative abundance of Methanosarcina barkeri and associated genes in the CO2-reduction methanogenesis pathway by 83.4% and 91.0%. Both abundance of methanogens and associated genes in the CO2-reduction methanogenesis pathway were positively correlated with cumulative CH4 emission, but negatively correlated with most soil organic acids. Thus AICU can mitigate CH4 emission by decreasing the abundance of methanogens and associated genes in the CO2-reduction methanogenesis pathway.