Interactive Impacts Of Air Warming And CO₂ Enrichment On Nitrogen Use In A Chinese Double Rice Cropping System

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,translocation 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℃)in Hubei Province,Central China from 2013 to 2016.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(NUEsys),in both early rice and late rice.Meanwhile,elevated[CO2]decreased N harvest index and N translocation efficiency from shoots to grains during heading to maturity at early rice,while increasing the efficiency at 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.Elevated temperature benefited N translocation for both early and late rice.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 NUEsys 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,and can have important implications for smart N management under future climate change.