Effects of thinning intensity and understory removal on soil microbial community in Pinus massoniana plantations of subtropical China

Monitoring the soil microbial communities following forest management practices is vital since soil microorganism play a critical role in sustaining forest productivity, nutrient cycling and soil CO2 emission. Here, biomass and the structures of soil microbial communities in aerially seeded Pinus massoniana plantations across three growing and three non-growing seasons following canopy tree thinning and understory removal were investigated using PLFA method, and soil environmental factors were analyzed. Both overall microbial community structures and mean microbial biomass did not significantly differ among control (CK), heavily thinning (HT), light thinning (LT) and understory removal (UR), but were significantly affected by time following treatment. In spite of non-significant effects of treatment, thinning and UR tended to decrease soil microbial biomass in nongrowing seasons, and to increase the microbial biomass in the 2nd and 3rd growing seasons following treatment. Soil moisture, carbon to nitrogen ratio and nitrogen (i.e. total N and mineral N) were the main drivers of soil microbial community changes. Lower soil moisture and higher soil nitrogen led to significant differences in soil microbial biomass in the 2nd year after treatment. Thinning significantly increased fungi to bacteria ratio (FB ratio), which was significantly positively correlated with soil heterotrophic respiration. Our study confirms that forest management practices such as thinning and understory removal exert strong effects on environmental conditions (i.e. microclimate and soil nutrient availability), which further influences FB ratio, and consequently might impact soil C cycling and forest productivity.