Effects of multispecies restoration on soil extracellular enzyme activity stoichiometry in Pinus massoniana plantations of subtropical China

Multispecies restoration is considered a feasible practice to tackle the decline in ecosystem services provided by monocultures. However, our understanding of how multispecies restoration influences microbial catabolic and anabolic pathways in coniferous monocultures based on enzymatic stoichiometry remains limited. Here, to assess the microbial resource limitation and carbon (C) use efficiencies in topsoil and subsoil after multispecies restoration, we compared the soil enzyme activities and stoichiometry of two multispecies plantations (broadleaf-oriented transformation and replenishment with local broadleaved trees), two stand ages of Pinus massoniana monocultures, and secondary forests in subtropical China. According to the specific enzyme activities, the labile C decomposition was suppressed but the recalcitrant C decomposition was stimulated in the subsoil in multispecies restorations. Additionally, the vector analysis and microbial acquisition ratios consistently indicated that multispecies plantations aggravated the co-limitation of C and phosphorus (P) in the subsoil and that the limitation was equal to the level of secondary forests. We also found significant decreases in C use efficiencies in multispecies plantations compared with pine monocultures. These findings suggested a trade-off between obtaining multiple nutrients and increasing C use efficiency in the subsoil of forests with different restoration approaches. Furthermore, the microbial acquisition ratios and C use efficiencies were mainly influenced by soil pH and microbial biomass stoichiometry. Overall, our study integrated the microbial resource limitation and C use efficiency to provide an enzymatic perspective for underground C-cycling and nutrient mineralization after forest restoration, highlighting the critical role of multispecies plantations in inducing strong shifts in microbial metabolism, especially in the subsoil.