Mixing of Pinus massoniana and broadleaved tree species alters stoichiometric imbalances between plants and soil microbes and their resources in subtropical plantations

Abstract

Substituting broadleaved trees for some existing coniferous industrial timber trees is an increasingly common cultivation practice to increase productivity, and non-isometric variations in soil carbon (C), nitrogen (N), and phosphorus (P) availability may make it challenging for plants and soil microbes to meet their elemental requirements. However, how plants and soil microbes cope with imbalanced stoichiometry induced by changes in the dominant species to maintain homeostasis remains unclear. We investigated the C:N:P stoichiometry in Pinus massoniana monocultures and in mixed plantations of Pinus massoniana and five individual broadleaved trees (Bretschneidera sinensis, Manglietia conifera, Cercidiphyllum japonicum, Michelia maudiae, and Camellia oleifera). We found that species substitution alleviated soil N-P imbalances for plants; it also alleviated the imbalances in soil accessible resources and litter C:P & N:P for microbes, thus reducing plant and microbial P limitations but aggravated potential microbial C limitations. Lowering the annual N and P uptake ratio and reabsorbing more P relative to N were two important ways that plants decreased the soil N-P imbalance. Increasing microbial biomass, enzymatic activities, and the microbial element utilization ratios C:P and N:P were important mechanisms by which microbes decreased C:P and N:P imbalances with substrates. The synergistic response of plant P uptake vs. microbial P assimilation and plant nutrient resorption efficiency (N:P) vs. microbial element utilization ratio (N:P) implied a mutually reinforcing relationship between plants and microbes in response to stoichiometric imbalances. Overall, these results advance the understanding of the plant- and microbe-driven alleviation of P limitation synergistically in P-scarce fragile ecosystems.