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

Background and Aims Carbon (C) allocation plays important role in plant adaptation to water and nutrient stress. However, the effects of drought and nutrient deficit on the allocation of recently fixed C in plant-soil-microbe system remain largely unknown. Methods We studied the response of C allocation of Sophora moorcroftiana (an indigenous pioneer plant in Tibet) in plant-soil-microbe system to drought, nitrogen (N) and phosphorus (P) deficit using a microcosm experiment. The 13CO2 continuous labeling was used to trace C transport through the plant-soil-microbe system. Results Drought significantly reduced C allocation to stem and root but increased allocation to soil. Deficit of N and P significantly increased C allocation to root under well-watered conditions, while P deficit significantly increased allocation to stem but decreased allocation to leaf under drought conditions. Carbon allocation to microbes was mainly affected by nutrient deficit, and 13C amounts in microbial biomass was decreased by N deficit and increased by P deficit. Stem 13C amount was positively related to net photosynthetic rate and leaf 13C amount, suggesting that plants preferentially allocate C to stem. Soil 13C amount decreased and 13C amount in microbial biomass first decreased and then increased with increasing plant 13C amount, indicating that high plant C supply did not drive high amounts of C transferred to soil and microbes. Conclusion We proved that drought and nutrient deficit interactively affected C allocation in plant-soil-microbe system, and revealed a U-shaped relationship between plant C supply and the amounts of C transferred to microbes.