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_s) and its temperature sensitivity (Q₁₀) 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^-1 yr^-1), phosphorus (0.24 kg P ha^-1 yr^-1), and the co-addition (36 kg N ha^-1 yr^-1+0.24 kg P ha^-1 yr^-1). Nitrogen addition and the co-addition reduced R_s by 29.67% and 26.67%, but phosphorus addition did not. Nitrogen and phosphorus additions did not change Q₁₀. Nitrogen addition increased nitrate, microbial biomass nitrogen, and dissolved organic nitrogen, and decreased metabolic quotient (P < 0.05). Distributions of R_s and Q₁₀ were 160 m > 180 m > 150, 170 m, and 160 m > 150, 180 m > 170 m, respectively (P < 0.05). Controlling factors of R_s were soil organic carbon (SOC), dissolved organic carbon, and microbial biomass carbon, and that of Q₁₀ were pH and SOC. Nitrogen additions and the co-additions reduced R_s by increasing microbial nitrogen assimilation and carbon utilization efficiency. R_s and Q₁₀ 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 CO₂ release from the riparian zone.