Background nitrogen deposition controls the effects of experimental nitrogen addition on soil gross N transformations in forest ecosystems

摘要

Nitrogen (N) deposition can profoundly alter soil N transformation processes and the long-term productivity of forest ecosystems. The response of soil gross N transformations to N deposition in forest ecosystems has been well studied through simulated N addition experiments. Simulated N addition experiments are conducted under a wide range of background N deposition rates. However, it remains unclear whether the response of soil gross N transformation rates to simulated N addition is dependent on background N deposition rates. Here, we collate results from the literature in forest ecosystems, and found, for the first time, that the responses of gross rates of N mineralization, nitrification, and NO3- immobilization to experimental N addition changed from positive to negative with increasing background N deposition rates with the thresholds for such changes were 3.23, 6.02, 1.90 kg N ha(- 1) yr(- 1), respectively. Our results suggest that background N deposition rates shall be incorporated into ecosystem models to better predict forest ecosystem N cycling under future N deposition scenarios.