Changes in the composition of soil microbial communities and their carbon-cycle genes following the conversion of primary broadleaf forests to plantations and secondary forests

The soil organic carbon (C) cycle is primarily mediated by soil microorganisms and their genes that function in the C cycle (C-cycle genes), both of which are strongly affected by land cover disturbance. However, the mechanism underlying microbially mediated soil C loss after conversion of primary natural broadleaf forests (BF) to plantation forests (PF) and secondary forests (SF) remains unknown. Here, we measured soil physicochemical properties and soil microbial community properties, and examined their linkages with microbial C-cycle genes. Forest conversion dramatically decreased the richness of the soil fungal community but not of the bacterial community, and altered the composition of both communities. Analysis of C-cycle genes revealed that the abundance of genes associated with C fixation, methane metabolism, and C degradation decreased by 51.3%, 57.9%, and 67.0%, respectively with the conversion of BF to PF; and by 6.3%, 4.1%, and 15.6%, respectively, with the conversion of BF to SF. The reductions in the abundance of C-cycle genes, especially the reduction of hemicellulose- and lignin-degradation genes, were primarily associated with the declines in the abundance of forest conversion-sensitive microbes indexed by operational taxonomic units (fsOTUs, beta = 0.41). fsOTUs were taxonomically diverse and included members frequently co-occurring with numerous other microbes in the microbial communities, indicating that the manipulation of fsOTUs by forest management could improve soil fertility and soil C sequestration. Forest conversion-induced shifts in fsOTUs abundance were associated with changes in soil potassium permanganate oxidizable organic carbon (PXC) concentration, dissolved organic carbon (DOC) concentration, and soil pH. Our results indicate that alterations in soil substrate supply (e.g., DOC and PXC) and soil pH induced by forest conversion may strongly shape fsOTUs structure and decrease the abundance of hemicellulose and lignin degradation genes, and consequently increase C loss.