Effects of different land-use types on the activity and community of autotrophic microbes in karst soil

CO2 fixation by soil autotrophic microbes could make considerable contribution to soil organic matter (SOM). However, the effects of different land-use types on the activity and community of soil autotrophic microbes are still unclear. In this study, the autotrophic carbon fixation (ACF) rates of karst soils with four land-use types were assessed by 13CO2 labelling. After 21 days incubation under alternate 12 h light with darkness, the ACF rate was 0.18 mg C·kg-1·day-1 in bare land soil, approximately 0.39 mg C·kg-1·day-1 in crop land and shrub land soils, while sharply increased to 1.54 mg C·kg−1·day−1 in grass land soil. The 13C nuclear magnetic resonance analysis of SOM showed that the fixed 13CO2 was mainly incorporated into carbonyl carbon in grass land soil, but largely incorporated into recalcitrant aromatic carbon in other land-use types, especially in bare land. Longer incubation time (21 days versus 7 days) greatly elevated ACF rate only in grass land soil (by about 4 times) under continuous light conditions, suggesting that longer incubation in light conditions largely stimulated microbial phototrophic CO2 fixation in grass land soil. Metagenomic analysis showed that the soil autotrophic microbial communities were prominently different among the four land-use types, and the autotrophic populations using bicarbonate as inorganic carbon species were most important for explaining the variations of autotrophic communities among the land-use types. Additionally, total nitrogen and soil organic carbon contents could be the key controlling factors of both ACF rates and structure of autotrophic microbial communities. Eventually, the change of soil properties caused by land-use type substantially affected the activity and community of soil autotrophic microbes, as well as the fate and stability of the assimilated CO2.