Carbon (C) allocation plays an important role in plant adaptation to water and nutrient stresses. However, the effects of drought and nutrient deficiencies on the allocation of recently fixed C in the plant-soil-microbe system remain largely unknown. Herein, we studied the response of C allocation of Sophora moorcroftiana (an indigenous pioneer shrub in Tibet) to drought, nitrogen (N) deficiency and phosphorus (P) deficiency using a microcosm experiment. The (CO2)-C-13 continuous labeling was used to trace C allocation in the plant-soil-microbe system. We found that drought significantly reduced plant C-13, but it increased C-13 accumulation in soil. The decreased plant C-13 under drought was attributed to the decrease of C-13 in stem and root rather than that in leaf. The excess C-13 fraction in the microbial biomass ((MBC)-C-13) was reduced by N deficiency, but it was not affected by the combination of drought and N deficiency, indicating that drought weakened the effects of N deficiency on (MBC)-C-13. By contrast, (MBC)-C-13 increased under the combination of drought and P deficiency, suggesting that drought enhanced the effects of P deficiency on (MBC)-C-13. Drought and nutrient deficiencies regulated the belowground C-13 allocation. Specifically, drought and P deficiency increased the allocation of C-13 to root and N deficiency regulated the allocation of 13C to microbial biomass C and dissolved organic C in soil. Notably, soil C-13 decreased with increasing plant C-13, while (MBC)-C-13 first decreased and then increased with increasing plant C-13. Overall, our study demonstrated that drought and nutrient deficiencies interactively affected C allocation in a plant-soil-microbe system and provided insights into C allocation strategies in response to multiple resource (water and nutrient) stresses under environmental changes.
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