Responses of soil respiration and its temperature sensitivity to nitrogen and phosphorus depositions in a riparian zone.

Nitrogen and phosphorus depositions and global warming have continuously intensified, impacting soil respiration. However, the response mechanisms of soil respiration rate (R) and its temperature sensitivity (Q) to nitrogen and phosphorus depositions are still unclear, especially for riparian zones. Intact Fluvisols were collected at different water-level elevations (150, 160, 170, and 180 m) of the riparian zone of the Three Gorges Reservoir, China and incubated under 20 and 30 °C with additions of nitrogen (36 kg N ha yr), phosphorus (0.24 kg P ha yr), and the co-addition (36 kg N ha yr+0.24 kg P ha yr). Nitrogen addition and the co-addition reduced R by 29.67% and 26.67%, but phosphorus addition did not. Nitrogen and phosphorus additions did not change Q. Nitrogen addition increased nitrate, microbial biomass nitrogen, and dissolved organic nitrogen, and decreased metabolic quotient (P < 0.05). Distributions of R and Q were 160 m > 180 m > 150, 170 m, and 160 m > 150, 180 m > 170 m, respectively (P < 0.05). Controlling factors of R were soil organic carbon (SOC), dissolved organic carbon, and microbial biomass carbon, and that of Q were pH and SOC. Nitrogen additions and the co-additions reduced R by increasing microbial nitrogen assimilation and carbon utilization efficiency. R and Q exhibited spatial heterogeneity due to soil property differences among the water-level elevations. The results indicated that the effects of nitrogen and phosphorus depositions and water-level elevations should be emphasized for evaluating, preventing, and controlling soil CO release from the riparian zone.