Low temperature-mediated changes in soil free-living nitrogen fixation functional groups in tree species at different successional stages in subtropical forests

Late-successional trees have a relatively high growth rate at low temperature, which is important for their competitive advantage over mid-successional trees. However, we have a limited understanding of the underlying mechanism for such an advantage.

We transplanted two mid-successional tree seedlings (Castanopsis chinensis and Schima superba) and two late-successional species (Machilus chinensis and Cryptocarya chinensis) from Dinghushan (subtropical forest) to South China Normal University. The seedlings were incubated in pots with the same soil and routinely managed with regular watering and weeding. After 7 years of seedling culture, five pots of each tree species were selected for greenhouse cultivation to simulate the effect of low temperature (16 °C) on the studied tree species. A control treatment (ambient temperature) was also established.

After 3 years, low temperatures changed the bacterial and diazotrophic communities, decreased the relative abundance of Actinobacteria in the mid-successional species Schima superba and Castanopsis chinensis, and increased the relative abundance of Rubrivivax in the late-successional species Machilus chinensis and Cryptocarya chinensis. In the low-temperature treatment, late-successional M. chinensis and Cryptocarya chinensis increased N fixation and net N mineralization rates compared with those in mid-successional S. superba and Castanopsis chinensis. Meanwhile, the relative abundance of the fungal community changed insignificantly, indicating that bacteria play a major regulatory role in nutrient cycling at low temperatures.

Our results suggest that shifts in the bacterial community and changes in functional group structures may play a key role in soil nitrogen cycling during the succession of evergreen broadleaf forest ecosystems at low temperatures during the dry season.