Uneven-aged mixed planting alleviated soil microbial carbon limitation while exacerbating microbial phosphorus limitation in Pinus massoniana forests

Uneven-aged mixed planting affects the structure and function of soil microbial communities. However, the impact of uneven-aged mixed planting involving various tree species on soil microbial nutrient limitation remains insufficiently understood. We selected four uneven-aged mixed forests, established through the underplanting with Schima superba , Elaeocarpus sylvestris, Phoebe bournei and Taxus wallichiana within Pinus massoniana forests. A pure forest of P. massoniana served as the control. The study involved determining soil physicochemical properties, assessing extracellular enzyme activities, measuring microbial biomass and performing metagenomic sequencing to analyze soil microbial nutrient limitation. The results indicate that the soil extracellular enzyme stoichiometric ratios were significantly lower than those in pure forest, and the ratios in stands underplanted with slow-growing shade-tolerant tree species ( P. bournei , T. wallichiana ) were lower than those in stands underplanted with fast-growing light-demanding tree species ( S. superba , E. sylvestris ). The vector length (VL) in uneven-aged mixed P. massoniana forests were significantly lower by 20.83 %-34.17 % than those in pure forest, while the vector angle (VA) was significantly higher by 2.21 %-6.60 % compared to pure forest. The microbial carbon use efficiency (CUE) in mixed forests was significantly higher by 1.82 %-7.27 % than those in pure forest. However, except for PT ( P. massoniana-T. wallichiana ), the microbial nitrogen use efficiency (NUE) and phosphorus use efficiency (PUE) in the other mixed forests were significantly lower than those in pure forest by 17.65 %-27.45 % and 6.67 %-20.00 %, respectively. Soil bacteria were more susceptible to carbon limitation, whereas fungi were more affected by phosphorus limitation. Soil nutrients and microbial biomass are crucial factors that regulate soil microbial nutrient limitation. Uneven-aged mixed planting alleviated the soil microbial carbon limitation in P. massoniana forests but exacerbated soil microbial phosphorus limitation. Underplanting slow-growing shade-tolerant tree species more effectively alleviated microbial carbon limitation, while underplanting fast-growing light-demanding tree species further exacerbated microbial phosphorus limitation. • Uneven-aged mixed planting alleviated microbial C limitation while exacerbating P limitation. • Underplanting slow-growing shade-tolerant tree species alleviated microbial C limitation. • Underplanting fast-growing light-demanding tree species exacerbated microbial P limitation. • Uneven-aged mixed planting improved microbial CUE but reduced NUE and PUE. • Soil bacteria were more susceptible to C limitation while fungi were more affected by P limitation.