Changes in litter input exert divergent effects on the soil microbial community and function in stands of different densities.

Soil microbial communities influence soil biogeochemical cycling by affecting the production of extracellular enzymes and the release of carbon dioxide. Changes in litter input or stand density due to thinning can affect soil microbial communities and their function by altering soil biochemical properties. However, it is unclear how or to what extent different amounts of litter input affect soil microbial communities and their function in forest stands with different densities. Therefore, we simulated litter removal, 50 % litter reduction, normal litter input, and double litter increase under field conditions by applying different amounts of litter to soils with different stand densities in the laboratory. We then measured soil biochemical properties, microbial communities, enzyme activity, and respiration rate. Our results revealed that the responses of soil dissolved organic carbon and total nitrogen to litter input were more pronounced in the high-density forest stand with poor soil than in the low-density forest stand with nutrient-rich soil, which was mainly reflected in that the addition of litter significantly decreased the concentration of dissolved organic carbon while increasing the content of total nitrogen in the soil of the high-density forest stand. In comparison to the soil carbon component, the nitrogen component of the soil was more affected by stand density. The responses of soil fungal and bacterial communities to leaf litter treatment varied with stand density, as reflected primarily in changes in the relative abundances of Ascomycota, unclassified_K_fungi, and Proteobacteria, and changes in the relative abundances of their functional groups (ectomycorrhizal fungi, saprophytic fungi, pathogens, parasites, and bacteria involved in the nitrogen cycle). Soil fungal community responses to changes in litter input are more sensitive in the high-density forest with nutrient-poor soil than in the low-density forest stand. Furthermore, litter input inhibited the activities of soil β-glucuronidase, N-acetyl-β-d-glucosaminidase, and acid phosphatase more strongly in the low-density forest stand. Litter manipulation primarily affected enzymatic activity in the high-density forest stand by changing the diversity and composition of the soil fungal community. However, in the low-density forest stand, litter treatment affected soil enzyme activity, primarily through changes in soil bacterial and fungal community composition, as well as soil respiration through changes in bacterial richness (Chao 1) and community composition. We conclude that how the change in litter input impacts the soil microbial community and its function, or the magnitude of the effects, is largely dependent on soil quality. Relationships among soil variables, microbial communities, and function differ between stand densities. Our study contributes to an enhanced understanding of the impact of changes in litter input due to climate change or anthropogenic activities on soil biogeochemical cycles and can also guide rationally formulating forest management approaches to improve microbial function under climate change.