Thinning-enhanced understory solar radiation promotes litter decomposition in subtropical pine forests

Litter decomposition is a key control on carbon and nutrient cycling in subtropical forests, yet how thinning and understory solar radiation are related to this process remains insufficiently understood. We combined a stand structural adjustment (thinning) experiment in Pinus yunnanensis forest with a radiation-exclusion experiment within thinned stands to examine whether thinning was associated with faster litter decomposition and whether transmitted solar radiation affected decomposition under post-thinning conditions. In thinned stands, P. yunnanensis needle litter decomposed faster than in unthinned stands, with higher decomposition constants and greater mass loss. In the radiation-exclusion experiment, litter exposed to transmitted solar radiation decomposed faster than litter under radiation-blocked conditions, with oak litter decomposing fastest, pine litter slowest, and mixed litter showing synergistic effects that were strengthened under transmitted radiation. Solar radiation was also associated with higher bacterial and fungal diversity, shifts in community composition, distinct microbial biomarkers, and changes in predicted microbial phenotypes and inferred functional profiles related to stress tolerance and degradation of complex plant polymers and secondary compounds. These results suggest that increased understory solar radiation may contribute to faster litter decomposition and nutrient release after thinning and that these responses are accompanied by shifts in litter-associated microbial communities and predicted functional potential. Our findings highlight understory solar radiation as an important, yet often underappreciated, factor associated with litter decomposition and suggest that incorporating understory light regimes and litter composition, particularly mixed litter layers, may improve predictions of post-thinning carbon and nutrient dynamics in subtropical pine forests.