Effects of secondary succession on soil fungal and bacterial compositions and diversities in a karst area

We explored the trends in soil fungal and bacterial patterns and their responses to plant and soil characteristics with increasing site age in abandoned farmland in karst areas. Illumina sequencing of 16S rRNA and ITS genes was used to characterize the soil bacterial and fungal diversities in farmland, farmland abandoned for 3, 6, 20, and 40 years, and old-growth forests in Southwest China. Plant diversity, community-weighted mean (CWM) leaf and branch traits, soil physical and chemical properties and metal element concentrations were also investigated. Bacterial diversity decreased slightly with increasing site age, while fungal diversity first increased and then decreased. Ascomycota was the dominant fungal phylum, and its abundance decreased significantly, from 83.21% in farmland to 49.66% in old-growth forests, while that of Basidiomycota increased significantly from 4.52 to 35.43%. The soil bacterial community was dominated by Proteobacteria, Actinobacteria, Acidobacteria, and Chloroflexi. The fungal and bacterial diversities were mainly affected by soil properties. At the older sites, high levels of soil nitrate nitrogen (N), ammonium-N, total N, soil organic carbon, calcium, and magnesium and relatively low levels of soil potassium and available phosphorus (P) resulted in decreased bacterial diversity. Fungal diversity was positively affected by soil total P and pH and negatively affected by soil iron and copper. The bacterial and fungal compositions were jointly affected by soil properties and CWM leaf and branch traits. Our findings indicate that soil bacterial and fungal diversities and compositions changed significantly during secondary succession in karst areas. Microbial diversity was determined by soil properties, and compositions were jointly driven by plant and soil properties.