Tree species effects on asymbiotic N2 fixation in subtropical karst and non-karst forests

Asymbiotic dinitrogen (N2) fixation (ANF) plays an important role in determining forest nitrogen (N) availability. Yet, the controls on ANF variation within or between forests remain poorly understood. In the present study, ANF in bulk leaf litter (or litter ANF hereafter) and soil (or soil ANF hereafter) within the crown radius of the dominant tree species was investigated in a karst forest over limestone and a nearby non-karst forest over clasolite, southwest China. Lithology exerted significant (P < 0.05) influences on ANF in litter but not in soil. ANF in litter was 4.9 times higher in the karst forest than in the non-karst forest. Tree species had significant effects on litter ANF in the karst and non-karst forests with the rates varying 23-fold and 71-fold, respectively. Significant effect of tree species on soil ANF was only encountered in the non-karst forest. The strongest explanatory variables were substrate and forest specific, but generally included litter moisture, total nitrogen, total phosphorus, calcium and soil organic carbon. The findings suggest that tree species and lithology may provide new mechanisms explaining the great variation of ANF within and between forests, respectively.