Rice rhizodeposits affect organic matter priming in paddy soil: The role of N fertilization and plant growth for enzyme activities, CO2 and CH4 emissions

Carbon dioxide (CO2) and methane (CH4) production in paddy soils play a crucial role in the global carbon (C) cycle and greenhouse gas emissions. A rhizosphere priming effect (RPE) may change these emissions, but the relationships between RPE, CH4 emission, and the effect of N fertilization are unknown. We investigated the RPE on CO2 and CH4 emissions and their dependence from N fertilization in a(13)CO(2) continuous labelling experiment by partitioning total CO2 and CH4 derived from roots and soil organic matter (SOM). Because of plant-derived CO2, rice plants strongly increased total CO2 emission compared to that from unplanted soil. SOM-derived CO2 and CH4 increased in the presence of roots but decreased after N fertilization. The RPE for CO2 at an early growth stage (40 days) was negative: -1.3 and -1.9 mg C day(-1) kg(-1) soil without and with N fertilization, respectively. However, 52 days after transplanting, RPE for CO2 got to positive. The FtPE for CH4 increased gradually up to 1.6 and 0.5 mg C day(-1) kg(-1) soil at the end of the experiment without and with N fertilization, respectively. Moreover, the RPE for CH4 got half of the RPE for CO2 after 64 days showing the relevance of CH4 emissions for greenhouse gases balance and C cycling in paddy ecosystems. The RPE for CO2 and CH4 emissions increased with microbial biomass content and activities of xylanase and N-acetylglucosaminidase. Supporting the results to RPE, the enzyme activities decreased with N fertilization, suggesting that reduced N limitation decreased microbial potential to mine N from SOM. In conclusion, for the first time we showed that root microbial interactions stimulated SOM mineralization in rice paddies through rhizosphere priming effects not only for CO2 but also for CH4, but the RPE decreased with N fertilization.