Forest conversion alters microbial decomposition of soil organic matter

Soil microorganisms are critical to maintain carbon (C) balance by decomposition of plant residues and microbial necromass. The effects of deforestation on mechanisms and factors of microbially mediated decomposition of organic compounds in soil remain unclear. Topsoil (0–20 cm) was sampled from native forests, plantations, and croplands at six sites in subtropical China using space-for-time substitution. Metagenomic sequencing combined with the Carbohydrate-Active Enzymes (CAZyme) database was used to trace microbial C-degradation potentials under deforestation. Forest conversion altered the community composition of C-degrading microorganisms, leading to a shift from oligotrophic Acidobacteria to dominance of other low-abundance taxa. The effects of forest conversion to cropland on microbial C-degradation potentials was stronger than that of conversion to plantations. After forest conversion to cropland, the abundance of genes involved in the degradation of plant-derived hemicellulose and cellulose and bacteria-derived peptidoglycan increased by 11 %–19 %, whereas the abundance of genes encoding the degradation of plant-derived lignin and fungi-derived glucans decreased by more than 20 %. This indicated a shift in microbial C-degradation preference from recalcitrant compounds to labile substrates. The increase in total phosphorus content in soil and pH, and the decrease in the carbon-to‑nitrogen ratio after deforestation, were the most important factors driving the change of CAZyme C-degradation genes. The C-degradation genes partially explained microbial enzyme activities, such as decreased of GH39 (β-xylosidase) with the corresponding enzyme activities. Overall, changes in microbial C-degradation genes and soil properties after forest conversion shifts microbial C-degradation preferences and impact soil C cycling by land-use change.