Effects of N addition and precipitation reduction on soil respiration and its components in a temperate forest

As the important pathway for soil carbon (C) fluxes from terrestrial ecosystems to the atmosphere, soil respiration (Rs) is strongly regulated by soil nitrogen (N) and water availability. Knowledge of the effect of soil N and moisture availability on Rs and its components (heterotrophic respiration (Rh) and autotrophic respiration (Ra)) is important understand soil C responses to climate change. Therefore, an 8-year field manipulation experiment to investigate the responses of Rs and its components to a reduction in the growing season precipitation amount with or without N addition was conducted in northern China. The results show that the average annual Rs, Rh, and Ra rates increased on average by 47%, 94%, and 38%, respectively, under the N addition treatments; increased on average by 18%, 53%, and 24%, respectively, under the precipitation reduction treatments; and increased on average by 5%, 4%, and 6%, respectively, under their interaction. The Rs and Rh were positively correlated with fine root biomass and microbial biomass, whereas the Ra was only related to the fine root biomass. Nitrogen addition and precipitation reduction significantly increased the Rs, Rh and Ra, possibly due to increased root biomass and microbial biomass. The Rs and Rh were not significantly affected by the interaction of N addition and precipitation reduction, whereas their combined effects significantly decreased the Ra. The Rs and Ra were also negatively correlated with soil total P concentration. The N addition significantly decreased the contribution of Rh to Rs compared with that of the control and precipitation reduction treatments, and precipitation reduction only significantly decreased the contribution of Rh to Rs compared with that of the control treatment in 2017. The results suggest that the N addition played a more important role than precipitation reduction in changing the contribution of Rh and Ra to Rs. Moreover, the N addition and precipitation reduction significantly increased the Q(10) value of the Rs and Rh, suggesting that the N addition and precipitation reduction may alter the response of soil C emission to global climate warming to a certain degree. However, the study highlights the crucial importance of the impacts of N addition, precipitation reduction, and their interaction on Rs and its components and could improve predictions of the future states of the soil C cycle in response to global climate change.