Air warming and CO₂ enrichment increase N use efficiency and decrease N surplus in a Chinese double rice cropping...

Effectiveness of N might be modified in rice cultivation under future climate change with elevated atmospheric CO₂ concentration ([CO₂]). At present, limited information is available to understand how plant N uptake and N use efficiency respond to elevated [CO₂] and/or temperature in Chinese double rice cropping systems. A four-year field experiment was therefore conducted using open-top chambers with varying [CO₂] (ambient, ambient +60 μmol mol^-1) and varying temperature (ambient, ambient +2 °C) in Hubei Province, Central China. Compared with ambient conditions, elevated [CO₂] increased plant N uptake and N use efficiency, as measured by fertilizer N recovery efficiency (NRE), N agronomic efficiency (NAE), N physiological efficiency (NPE) and apparent system N use efficiency (NUE_sys), in both early rice and late rice. CO₂ enrichment tended to decrease soil mineral N concentration since more N was assimilated by plants. Elevated temperature led to lower plant N uptake and decreased NRE and NAE in early rice, due to a reduction in grain yield induced by heat injury. In contrast, warming increased plant N uptake and N use efficiency in late rice as no heat stress existed. Warming tended to increase soil mineral N concentration in early rice but had negligible effects in late rice. When elevated [CO₂] and temperature were combined, the positive effects of CO₂ enrichment for N utilization were able to compensate for the negative effects of warming in early rice, while the interaction was synergetic in late rice. Hence, co-elevation of [CO₂] and temperature led to higher N use efficiency (64.6% for NUE_sys across four years) and decreased annual N surplus by 28.6-36.5 kg N ha^-1 compared with ambient conditions. Our findings confirm that CO₂ enrichment and air warming can improve N use efficiency at both crop level and system level in Chinese double rice cultivation.