Effects of drought and nutrient deficiency on the allocation of recently fixed carbon in plant-soil-microbe system.

Carbon (C) allocation plays an important role in plant adaptation to water and nutrient stresses. However, the effects of drought and nutrient deficiency 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 13CO2 continuous labeling was used to trace C allocation in the plant-soil-microbe system. We found that drought significantly reduced plant 13C, but increased 13C accumulation in soil. The decreased plant 13C under drought was attributed to the decrease of 13C in stem and root rather than that in leaf. The excess 13C fraction in microbial biomass (MB13C) was reduced by N deficiency, but was not affected by the combination of drought and N deficiency, indicating that drought weakened the effects of N deficiency on MB13C. In contrast, MB13C increased under the combination of drought and P deficiency, suggesting that drought enhanced the effects of P deficiency on MB13C. Drought and nutrient deficiency regulated belowground 13C allocation. Specifically, drought and P deficiency increased the allocation of 13C to root, and N deficiency regulated the allocation of 13C to microbial biomass C and dissolved organic C in soil. Notably, soil 13C decreased with increasing plant 13C, while MB13C first decreased and then increased with increasing plant 13C. Overall, our study demonstrated that drought and nutrient deficiency 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.