Co–elevation of CO2 and temperature enhances nitrogen mineralization in the rhizosphere of rice

It is unclear how elevated CO2 (eCO2) and warming interactively influence soil N mineralization in the rhizosphere of rice (Oryza sativa L.), given that the N mineralization process in anaerobic paddy soils differs from that of aerobic upland soils. In this study, we conducted a rhizobox experiment in open top chambers and used 15N-13C dual-labeling to examine the impacts of eCO2 (700 ppm) and warming (2 °C above the ambient) on N mineralization and associated microbial processes in the rhizosphere of rice plants under anaerobic conditions. Compared to the control, the combination of eCO2 and warming increased rice N uptake by 50% in a no-added-N treatment and 32% under an N-added treatment, with the additional uptake mainly consisting of soil-derived N. Co-elevation of CO2 and temperature increased microbial biomass C and N and increased N mineralization by 41% and 23% in the no-added-N and N-added treatments, respectively. The absolute abundances of the N-mineralization genes chiA, pepA, pepN, and urea hydrolysis gene ureC in the rhizosphere increased by 22–30% under eCO2 and warming, corresponding to the additional N mineralization and photosynthetic C allocation into the soil. However, eCO2 plus warming did not increase the metabolic efficiency of N mineralization (mineralized N per unit microbial N). Our results suggest that the co-elevation of CO2 and temperature stimulated microbially mediated soil N mineralization in the rhizosphere of rice, posing a risk on the acceleration of soil organic matter decomposition.