Nitrogen addition impacts on soil phosphorus transformations depending upon its influences on soil organic carbon and microbial biomass in temperate larch forests across northern China

Globally increasing atmospheric nitrogen (N) deposition can affect soil phosphorus (P) transformations, yet the pathways and underlying mechanisms are largely unclear. Here, we sampled soils from five larch stands along a soil fertility gradient in northern China that have experienced long-term N addition (>5 years), and measured soil P fractions and biochemical properties. We found that the magnitude and direction of N addition effects on soil inorganic P and organic P fractions significantly varied with initial soil C:N ratio. Specifically, N addition significantly reduced almost all soil inorganic P fractions in the larch stand with the highest soil C:N ratio (18.35). In contrast, N addition significantly elevated NaOH-extractable organic P and total organic P in the larch stand with the highest soil C:N ratio while reduced labile organic P and total organic P in the stand with the lower soil C:N ratio (11.74). N addition-induced changes in inorganic P fractions were significantly correlated with that of soil organic C. Moreover, N addition-induced changes in organic P fractions were significantly correlated with those of microbial biomass and acid phosphatase activity. Taken together, our results indicate that N addition influenced soil P availability and fractions mainly by affecting soil organic C and microbial properties, which can improve our understanding of mechanisms of changes in soil P availability under the context of increasing N deposition.