Nitrogen (N) availability regulates global carbon (C) balance. Anthropogenic N input, such as N deposition and fertilization, increases soil N availability in terrestrial ecosystems. Soil respiration (Rs), as the second large C flux between terrestrial ecosystems and the atmosphere, plays an important role in regulating terrestrial C dynamics. It is recognized that soil N availability exerts great controls on Rs. Previous studies usually evaluated the impact of increasing N availability on Rs and its components using only two discrete N levels, but how these processes respond to continuous N input remains unclear. Here we conducted a two-year field experiment to examine the changes of Rs and its autotrophic (Ra) and heterotrophic components (Rh) along a N addition gradient in an alpine steppe on the Tibetan Plateau, and used structural equation modeling (SEM) to explore the direct and indirect pathways of biotic and abiotic factors regulating Ra and Rh.
Results/Conclusions
Both Rs and Ra exhibited initial increasing and subsequent decreasing trend (unimodal response) during the two growing seasons. By contrast, Rh declined linearly with continuous increasing N addition. Single-factor regression analysis indicated that Rs, Ra and Rh were positively correlated with soil temperature, root and microbial biomass. SEM analysis further revealed that root and microbial biomass were the direct controllers of Ra and Rh, while soil temperature not only had a direct effect on the two components of Rs, but also indirectly regulated Ra and Rh via root and microbial biomass. Given that external N input to terrestrial biosphere is a continuous process, the nonlinear response patterns of Rs and the regulating pathways as revealed in our study should be incorporated into Earth System Models for better predicting terrestrial C balance.