Biomass, chemical composition, and microbial decomposability of rice root and straw produced under co-elevated CO₂ and temperature

Rice residue including root and straw are unique carbon (C) source in paddy soils. However, the potential changes in quantity and chemical composition of rice residue under co-elevated atmospheric CO₂ concentration ([CO₂]) and air temperature (T_air) and the legacy effect of the changed chemical composition on residue decomposition have not been investigated. This study was conducted to investigate biomass, chemical composition, and decomposability of rice root and straw produced under elevated [CO₂] and T_air. Root and straw biomass increased by elevated [CO₂] and elevated T_air, respectively, and the greatest biomass was achieved under co-elevated [CO₂]-T_air for both root and straw. The concentration of lignin (recalcitrant) decreased while that of nonstructural carbohydrates (less recalcitrant) increased by co-elevated [CO₂]-T_air. The ratio of lignin-to-nitrogen (lignin/N) decreased by co-elevated [CO₂]-T_air compared to ambient [CO₂]-T_air due to increased N and decreased lignin concentrations. Decomposability of root (lignin/N, 36.4) produced under co-elevated [CO₂]-T_air was greater than that under ambient co-elevated [CO₂]-T_air (lignin/N, 53.7); however, there was no difference in decomposability for straw, which had relatively narrow range of lignin/N (27.3–36.5) regardless of [CO₂]-T_air conditions. The results of this study provide a novel insight into the changes in quantity and quality of rice residue under elevated [CO₂]-T_air that are necessary to predict changes in paddy soil C sequestration under global warming.