Effects of lithology on asymbiotic N2 fixation in subtropical secondary forests, southwest China

Asymbiotic N2 fixation (ANF) is a major nitrogen (N) input pathway to terrestrial ecosystems. However, there is considerable uncertainty about how lithology affects ANF. Here we measured ANF rates in soil, litter, and moss in forests underlain by limestone (ForestLim), dolomite (ForestDol), and clasolite (ForestCla), respectively, in southwest China. Effects of lithology on soil ANF rate varied seasonally. The rate was highest in ForestDol during the wet season, but was highest in ForestLim during the dry season. The overall soil ANF rate was significantly higher in ForestLim than in ForestCla. Litter ANF rate was significantly higher in ForestLim and ForestDol than in ForestCla regardless of season. There was no significant difference in moss ANF rate among the three types of forests in both seasons. The annual N2 fixation rate was highest in ForestLim (1.72 ± 0.27 kg N ha-1 yr-1) but lowest in ForestCla (0.70 ± 0.08 kg N ha-1 yr-1). Stepwise multiple linear regression analyses revealed that the strongest explanatory factors were soil available phosphorus, ammonium and moisture for soil ANF; and were nitrogen and moisture for litter ANF. Our results suggest that lithology differentially affected ANF in soil, litter, and moss, and hence should be considered in Earth system models to facilitate better prediction of N inputs via biological N2 fixation under global change.