Free-living nitrogen fixation is an important pathway for nitrogen inputs into soil and litter ecosystems. Here we compared free-living nitrogen fixation in forest soils and litter layers between limestone and clastic rock. We found that free-living nitrogen fixation activity is largest in litter followed by limestone soils and lowest in clastic rock soils. Compared to clastic rock soils, limestone soils had higher pH, exchangeable calcium, and greater abundances of diazotrophs and phoD-harboring bacteria, along with increased microbial network complexity and stability. Network analysis identified phoD-harboring bacteria as having strong associations with diazotrophs and arbuscular mycorrhizal fungi, indicating their key role in phosphorus supply for soil free-living nitrogen fixation. In contrast, litter free-living nitrogen fixation activity remained consistent across lithologies due to similar microbial populations. These results suggest that higher soil pH and calcium in limestone promote free-living nitrogen fixation activity by fostering beneficial microbial associations, an effect absent in litter. Presence of higher soil pH and calcium in limestone promote free-living nitrogen fixation activity in soils by fostering beneficial microbial associations, whereas lithology has no significant effect on the activity in litter, according to sampling analysis conducted in the southwest China.
Xiao et al. (Thu,) studied this question.