Abstract: Aims Priming effect (PE), as the short-term change in the decomposition rate of soil organic carbon (SOC) caused by exogenous carbon input, plays important roles in carbon cycling, and has important influences on the storage and dynamics of SOC in global terrestrial ecosystems. Leaf litter and fine root are the main sources of SOC in forest ecosystems and regulate PE. Increased soil nitrogen (N) availability caused by atmospheric N deposition also impacts PE. However, how PE induced by leaf litter and fine root differed and its response to increased soil N availability was still unclear. Methods Therefore, we conducted an incubation experiment by adding 13C-labelled leaf litter and fine root into Chinese fir soil. Leaf litter was covered on soil surface and fine root was mixed in the soils to simulate their existence in the natural field conditions in forests. During 35-days incubation, the amount and δ13C value of soil CO2 were measured, and soil nutrients and microbial community composition after incubation were also measured. Important findings 1) leaf litter addition promoted the SOC decomposition, that is, leaf litter induced the positive PE, with the magnitude of 1.69 mg C?kg–1?d–1, while fine root addition induced the negative PE with the magnitude of –1.26 mg C?kg–1?d–1. 2) Nitrogen addition reduced the magnitude of positive PE caused by leaf litter addition by 38.7%, while it increased the magnitude of negative PE caused by fine root addition by 16.6%. 3) Leaf litter addition reduced the ratio of fungi to bacteria by 22.9%, and adding fine root increased the fungal biomass by 30.8%. Furthermore, N addition increased the fungal: bacterial ratio and the fungal biomass. Our results highlighted the differences in the PE induced by leaf litter and fine root, and provided theoretical support for the prediction and management of SOC in forests under increasing atmospheric N deposition.

Key words: soil organic carbon decomposition, priming effect, nitrogen deposition, forest litter, soil microbial community composition