Impacts of conversion from secondary forests to larch plantations on the structure and function of microbial communities

In response to increasing timber demands, extensive areas of secondary forests have been converted to larch (Larix gmelinii) plantations in northeast China. It has been reported that the conversions have led to reduction in soil organic matter and nutrient availability. Soil microbes play crucial roles in nutrient cycling. However, the effects of forest conversion on soil microbes are not yet well understood in temperate forest ecosystems. In this study, the structure and function of bacterial and fungal communities were compared between larch plantations and adjacent secondary forests relative to surface litter and two soil layers (0-10 cm and 10-20 cm) using Illumina MiSeq sequencing. We found that the impacts of the forest conversion on soil microbes mainly occurred in the top (0-10 cm) soil layer. Soils in secondary forests were associated with a higher proportion of copiotrophic bacteria (Proteobacteria and Actinobacteria). In contrast, larch plantation soil was enriched with Acidobacteria, which are oligotrophs that appear to be well suited to acidic conditions. These differences in soil bacterial communities confirm that the forest conversion resulted in soil acidification and a large decline in available soil nutrient in larch plantations. Furthermore, the results of bacterial functional genes, inferred by PICRUSt, and the results of extracellular enzyme assays indicate that fungi likely dominated soil organic carbon decomposition in forest soils. Sordariomycetes was the most abundant fungal class in soils of secondary forests. The abundance of Sordariomycetes fungi was related to higher levels of almost all enzymes measured, except for cellobiohydrolase and β-1,4-N-acetyl-glucosamidase. These two enzymes were positively associated with Agaricomycetes fungi in soils of secondary forests. The fungal community in the soils of larch plantations was dominated by the class Agaricomycetes, which was positively associated with activities of both hydrolytic and oxidative enzymes. These results indicate that fungal classes, Sordariomycetes and Agaricomycetes, are most likely responsible for the decomposition of soil organic matter in secondary forests, while Agaricomycetes fungi might be more important in controlling decomposition in larch plantations.