| Summary: | <i>Research Highlights</i>: Moderate thinning can effectively improve forestry production and change the microenvironment of understory vegetation. <i>Background and Objectives</i>: Microbial communities control the decomposition and transformation of forest organic matter; however, the influence of thinning on microbes in the organic horizon remains unclear. <i>Materials and Methods</i>: In this study, we subjected four plots of Chinese pine plantations in Badaling, Beijing to different thinning intensities, including no thinning (T0), low-intensity thinning (T10), medium-intensity thinning (T20), and high-intensity thinning (T50). The changes in chemical properties and microbial community compositions observed in the organic horizon, which comprised undecomposed litter (L layer) and half-decomposed litter (F layer), were analyzed after thinning. Microbial community compositions were evaluated using phospholipid fatty acid (PLFA) methods. <i>Results</i>: The results showed that the abundances of gram-negative bacteria (GN) and total bacteria (B) under the T10 thinning condition were the highest among the four thinning intensities, and the abundance of arbuscular mycorrhizal fungi (AMF) in T20 was higher than under other thinning intensities. The abundance of gram-positive bacteria (GP) and actinobacteria (ACT) in T10 was lower than in both T0 and T50. The abundance of total PLFAs and fungi (FU) was higher in the L layer, whereas the abundance of GP, GN, B, ACT, and AMF was higher in the F layer. <i>Conclusions</i>: Our results demonstrated that the L layer better reflects the influence of thinning on litter. Redundancy analysis (RDA) results indicated that the organic carbon (LOC), dissolved organic carbon (DOC), and ammonium nitrogen (NH<sub>4</sub><sup>+</sup>-N)contents of litter were primarily responsible for the observed changes in microbial community structure, with LOC alone explaining 62.6% of the total variance among the litter substrate factors selected. Overall, moderate-intensity thinning of <i>Pinus tabulaeformis</i> Carr. plantations created more favorable conditions for microbial communities in the organic horizon.
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