Effects of nitrogen addition on DOM-induced soil priming effects in a subtropical plantation forest and a natural forest

Dissolved organic matter (DOM) plays a key role in soil organic matter (SOM) decomposition via the priming effect (PE). The DOM-induced soil PE is closely related to nutrient availability, especially nitrogen (N). Regardless of the widespread of chronic N addition, how elevated N deposition affects DOM-induced PEs remains poorly understood. To fill this knowledge gap, we studied the effects of N addition, ¹³C-labeled leaf-DOM (herein DOM) addition, and leaf-DOM plus N addition (DOM+N) on soil PEs in soils of a subtropical Chinese-fir (Cunninghamia lanceolata) plantation and a natural Castanopsis carlesii forest (hereafter referred to as Chinese-fir soil and Castanopsis soil, respectively). Soil properties (e.g., soil organic C, total N, available phosphorus, and ratio of C and N), dissolved organic C (DOC), soil microbial biomass C (MBC), phospholipid fatty acid (PLFA), and enzyme activities were also investigated, because these parameters predominantly affect the intensity and direction of soil priming. The addition of DOM induced positive PEs in the Castanopsis soil but negative PEs in the Chinese-fir soil. In addition, DOM addition increased MBC and fungal abundance and the activities of phenol oxidase (PhOx) and peroxidase (Perox) in the Castanopsis soil but not in the Chinese-fir soil. Compared with DOM-only addition, DOM+N addition significantly enhanced PEs in the Chinese-fir soil but not in the Castanopsis soil. Furthermore, compared with DOM-only addition, DOM+N addition significantly increased MBC, abundance of fungi and AMF, fungi to bacteria ratio (F:B), and activities of four enzymes [β-glucosidase (βG), N-acetyl glucosaminidase (NAG), PhOx, and Perox] in the Chinese-fir soil but not in the Castanopsis soil. The DOM+N addition also had a significant effect on composition of main microbial groups in the Chinese-fir soil but not in the Castanopsis soil. These results suggest the enhanced PE following DOM+N in the Chinese-fir soil was likely mediated by enhanced enzyme production associated with increased fungal abundance. Our study highlights that the effects of increases of DOM on soil C cycling is largely affected by N availability and mediated by the effects on the abundance of soil microbial groups and enzyme activities. Our result also demonstrated a case in which effects of DOM and N addition on soil C cycling differ between a Castanopsis forest and a Chinese-fir plantation forest, with Chinese-fir soil being more sensitive to N addition. This is an important finding that needs to be taken into consideration in estimating the soil C pools.