Differential response of soil bacteria and fungi to carbon and respiration components in abandoned grasslands on the Loess Plateau, China

Aims Abandoned grasslands may affect soil carbon and respiration components; however, it is unclear how microorganisms are involved in this process. Methods In this study, ten-year intervals of a 50-year sequence of restored abandoned grassland was selected on the Loess Plateau, with farmland used as a control. A total of 18 soil samples were collected. Soil samples were performed to indoor experiments and high-throughput sequencing to determine soil physicochemical properties, carbon components, enzyme activity, respiration components, and microbial community composition and diversity. Results The results showed that the vegetation structure and species diversity of abandoned grassland changed over time, which led to changes in soil physicochemical properties. Soil inorganic carbon (from 9.71 to 18.60 g/kg), organic carbon (from 2.05 to 9.36 g/kg), resistant organic carbon (from 0.92 to 7.22 g/kg), labile organic carbon (from 1.03 to 2.14 g/kg) and autotrophic respiration (from 0.57 to 1.93 μmol·m −2 ·s −1 ) increased with the prolonged abandonment period, but heterotrophic respiration declined in 40–50 years (reduce from 4.92 to 4.63 μmol·m −2 ·s −1 ). Meanwhile, we found a significant correlation between carbon and respiration components. As well, the composition and diversity of microorganisms are also greatly impacted. The main bacterial phyla were Actinobacteriota, Proteobacteria, Acidobacteria, and Chloroflexota. The main fungal phyla were Ascomycota, Mortierellomycota, and Basidiomycota, and the fungal diversity index increased with the prolonged abandonment period. Redundancy analyses showed that soil carbon components, enzyme activity, and respiration components were positively correlated with Acidobacteria, Mortierellomycota, fungal diversity (Ace, and Chao 1) and negatively correlated with Cyanobacteria and Glomeromycota. Conclusions The process of vegetation restoration on the Loess Plateau has increased soil carbon content and enzyme activities, thereby increasing soil microbial activity and diversity. Enhanced soil microbial activity promoted soil respiration, but fungi had a greater impact on soil respiration compared to bacteria. This suggests that we should pay more attention to the role of soil fungi in the future management of soil carbon pools in grassland ecosystems.