Nonlinear Coupling of Carbon and Nitrogen Release During Litter Decomposition and its Responses to Nitrogen Addition

During decomposition of plant litter, it remains unclear whether carbon (C) and nitrogen (N) are released at similar proportional rates, which hinders our prediction of soil C and N dynamics under the context of global changes. Here, we investigated the relationships between C and N release and their responses to N addition (5 g N m −2  yr −1 ) to better understand this question. Litter gross N release and immobilization were differentiated by the 15 N‐labeling technique. Results showed that the linear relationship between gross N release and C release followed the 1:1 line during the first year but then deviated above the 1:1 line for three types of the four studied litters. Nitrogen addition did not change this relationship, showing an inhibition effect on both C and N releases from the N‐rich foliar litter at the later stage. The inhibition effect of N addition was not found in the N‐poor twig litter, indicating that initial litter N interacted with deposited N to affect litter N release. We propose a novel conceptual model to describe that under N addition, the decrease in gross N release caused a decline in net N release from N‐rich litter, while a decline in net N release from N‐poor litter was due to the increase in gross N immobilization. These findings point to the importance of differentiating the gross N release and immobilization processes for understanding the response of litter N cycling to increasing N deposition and the relationships between C and N release during different stages of litter decomposition.