Contrasting responses of the priming effect to nitrogen deposition in temperate and subtropical forests

The priming effect (PE) of soil organic carbon (SOC) decomposition has been widely demonstrated to be affected by elevated nitrogen (N) deposition, yet the response of the PE to N deposition in diverse climatic regions remains elusive. This study aimed at exploring the effects of N deposition on the PE and their controlling factors in temperate and subtropical forest ecosystems. We conducted an incubation experiment with temperate and subtropical forest soils that had been experimentally exposed to varying levels of N deposition. A global meta-analysis of the responses of the PE to N deposition in forest ecosystems was also conducted. We found that the response of the PE to increasing N deposition depended on the climatic region. In temperate forests, the PE was strengthened by low N deposition whereas inhibited by high N deposition, with an approximate threshold of 50 kg N ha−1 yr−1. Contrastingly, no pronounced impact was observed in subtropical forests regardless of N levels. These findings generally align with our meta-analysis of global forest studies, except for low N treatment in temperate forests. In N-limited temperate forests, the alterations in PE under N deposition were primarily controlled by soil recalcitrant organic carbon content. Conversely, in P-limited subtropical forests, the lack of N impacts on PE was attributed to the persistent ratio of fungal to bacterial PLFA. Correspondingly, the microbial mechanisms underlying the process of priming also varied in diverse climatic regions, particularly emphasizing the stronger role of fungi in subtropical forests. These results highlight the noteworthy alteration of SOC dynamics in temperate forests under increasing N deposition, and suggest that previous findings might have overestimated the effect of N deposition on the PE in subtropical forests.