Changes in soil microbial community structure and function after afforestation depend on species and age: Case study in a subtropical alluvial island.

It is well established that land use change can have a profound impact on soil physicochemical properties but the associated changes in soil microbial communities are poorly understood. We used long-term research sites in a subtropical alluvial island of eastern China to measure changes in soil physicochemical properties and microbial community abundance and composition (via phospholipid fatty acid (PLFA) analysis) and function (via extracellular enzyme activity) across different land use types developed on the same soil matrix, including a camphor (Cinnamomum camphora) plantation, a chronosequence of differently aged dawn redwood (Metasequoia glyptostroboides) plantings, a deforested land and a rice paddy. We hypothesized that afforestation could improve soil quality by enhancing carbon (C) and nitrogen (N) contents, microbial biomass and enzyme activities, but that this effect would vary depending on forest age and tree species. Soil C and N concentrations, PLFA abundances and activities of decomposition enzymes (β-glucosidase, urease, alkaline phosphatase and catalase) in older plantations all increased significantly compared to cropland. These variables changed little or decreased in deforested land compared to cropland. These variables also increased with planting age in the dawn redwood plantings. Soils under camphor plantations had higher soil nutrient contents, microbial biomass and lower enzyme activities than dawn redwood soils with similar age. We also found some significant relationships between soil chemical and biological properties: PLFA abundances were positively related to soil organic matter (SOM) contents; the fungal-to-bacterial ratio and fungal relative abundance were correlated positively with SOM contents and negatively with C/N ratio; both soil PLFA abundances and enzyme activities were positively linked with soil inorganic N content and potential net N mineralization rate; ratio of specific C, N and P (phosphorus) acquisition activities was limited to 10: 1: 10 across land use types. Our study underscores the fact that land use type can have a profound impact on soil microbial communities; in addition, tree species and planting age also play significant roles in afforestation.