Effects of thinning intensity on understory vegetation and soil microbial communities of a mature Chinese pine plantation in the Loess Plateau

Thinning can effectively improve forest production and maintain ecological stability. However, the changes in soil microbial community compositions after thinning are still not well understood. In this study, we investigated the changes in the soil microbial community of mature Chinese pine (Pinus tabuliformis) plantations in the Loess Plateau after 11years of four different thinning intensity treatments. Furthermore, the responses of the soil microbial community to changes in understory plants and soil properties were analyzed. The ratios of wood removal investigated were 0 (CK), 15% (LIT), 30% (MIT) and 45% (HIT). Compared with the CK, thinning significantly increased the Shannon index, species richness, coverage and biomass of the understory plants, and these values were highest for the HIT. The soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), nitrate nitrogen (NO₃^--N) and available phosphorus (AP) concentrations increased with increasing thinning intensity. Thinning intensity did not significantly affect soil microbial community diversity indices. With respect to the dominant bacterial groups, the relative abundance of Proteobacteria was much higher in the HIT, while that of Acidobacteria was much higher in the LIT and CK. For the dominant fungal groups, the relative abundance of Basidiomycota was lowest in the HIT, while that of Ascomycota was highest in the same treatment. Redundancy analysis (RDA) showed that SOC, TN, and AP significantly correlated with soil bacterial communities and that SOC, TN, TP, AP and NO₃^--N significantly correlated with soil fungal communities. The understory vegetation influenced soil fungal communities rather than soil bacterial communities. These findings suggest that the aboveground vegetation diversity and soil nutrients were improved with the increased thinning intensity after 11years. The copiotrophic groups (e.g. Proteobacteria) and oligotrophic groups (e.g. Acidobacteria) differed significantly among the four thinning treatments, indicating a dependence of the soil microbial community composition on soil nutrients.