LAUSR

Abstract Alternate drying–rewetting, as a type of irrigation regime, can influence carbon (C) and nitrogen (N) transformations and microbial activity in soils under the application of organic amendments, but such effects have seldom been explored in paddy soils amended with organic matters with various C:N ratio. The objective of the present study was to investigate the effect of composted azolla (azocompost) with a low C:N ratio (11) and rice straw with a high C:N ratio (42) on soil C and N changes, the enzymes effective at the transformations of these nutrients, soil respiration, and microbial biomass in paddy soil under drying–rewetting cycles during a 60–day incubation experiment. Experiment treatments included: organic amendment factor at three levels (i) unamended soil (control); (ii) soil amended with 1.5% azocompost; and (iii) soil amended with 1.5% rice straw and irrigation regimes factor at three levels (i) constant soil moisture in a waterlogged state (ConstM); (ii) mild drying–rewetting (MildM); and (iii) severe drying–rewetting (SevM). Drying–rewetting decreased respiration rate (by 40%), dissolved organic carbon (by 40%), microbial biomass carbon (by 75%), NH4+ concentration (by 21%), urease activity (by 55%) and C/N ratio (by 1%) of unamended soil under SevM conditions. However, the rice straw amendment increased the same factors under the same conditions. Drying–rewetting decreased NO3− concentration by 35% in control while azocompost increased it by 78%. Drying–rewetting events significantly increased cumulative CO2–C in soils amended with rice straw under all irrigation regimes at 30 days after incubation (DAI). The maximum and minimum soil respiration rate was observed in soil amended with rice straw under MildM conditions (83% increase compared to control) and unamended soil under SevM conditions, respectively. The highest microbial biomass carbon and microbial biomass nitrogen were registered in soil amended with rice straw under MildM conditions at 15 DAI and in soil amended with azocompost under ConstM conditions at 60 DAI, respectively. The C:N ratio had the highest correlation with other measured carbon and nitrogen indicators. It can be concluded that the effectiveness of organic amendments on biochemical changes of C and N in the short–term, which strongly depended on their C:N ratio, was greater than that of irrigation regimes.