Possibility of exogenous organic carbon input to increase global soil nitrogen supply potential: A meta-analysis

Soil nitrogen (N) mineralization is regulated by the input of exogenous organic carbon (C, ExoC) substrate via changing C availability. Although ExoC input occurs widely among global terrestrial ecosystems, it is still not clear how ExoC input affects soil gross N mineralization (GNM). Here, 265 observations from 63 peer-reviewed publications were collected for a global meta-analysis to investigate the effect and the drivers of ExoC input upon soil GNM. The results showed that ExoC input significantly increased GNM by 40.8%, and the influence depended on the types of ExoC substrate. The input of complex ExoC substrate (straw, animal manure, cellulose, etc.) increased soil GNM by 44.8%, while simple ExoC substrate (glucose, amino acids, oxalic, etc.) had no significant effect on GNM. Only in cropland the ExoC input promote the rate of soil GNM, with no significant effect found in either forest or grassland. Furthermore, the results revealed that the application rate of ExoC was the paramount factor affecting soil GNM, given their significant positive correlations. This study also emphasized that initial soil pH (i.e. soil pH before ExoC input) and pH of ExoC, rather than the change of bulk soil pH, played an important role in regulating soil GNM after the ExoC input, and both were significantly negatively correlated with GNM. In addition, soil GNM was also negatively correlated with mean annual temperature but positively related with soil total N and mean annual precipitation. The findings of this meta-analysis contribute to a comprehensive understanding of how soil N availability and cycling respond to ExoC input on a global scale. That means the factors related to rate of ExoC, soil total N, initial soil pH, the pH of ExoC, and climate factors and the interaction among above factors may affect global soil N supplying potential.