Impacts of nitrogen addition on net primary productivity in the typical mixed broadleaved-Korean pine forest

The impact of nitrogen (N) on net primary productivity (NPP) of forest ecosystem has become focus of the researchers since N deposition is increasingly serious. A six-year field experiment was conducted in the typical mixed broadleaf-Korean pine forest in Xiaoxing'an Mountains, northeastern China, to explore the effects of N addition on carbon (C) density and NPP of forest ecosystem and its components. Urea was selected as N source and four levels of N addition were control (N0, 0kg/(ha·a), low N (N1, 30kg/(ha·a), medium N (N2, 60kg/(ha·a) and high N (N3, 120kg/(ha·a). Results showed that N addition had no significant effects on C density of vegetation C pool, detritus C pool and soil organic C pool (P>0.05). N addition had no significant effects on the NPP of whole forest ecosystem, however, it inhibited NPP of coniferous tree leaf but promoted that of broadleaved trees (P < 0.05). Soil total N concentration had no significant difference among different N addition treatments, but it had a highly significantly positive correlation with soil organic C concentration (P < 0.01). We suggest that soil total N concentration has a significant influence on soil organic C in forest ecosystem, rather than N addition amount.