Drought Effects on Soil Carbon Stability Mediated by Rhizodeposition and Microbes

Drought will increase in frequency and intensity in many areas of the world and has the potential to turn entire ecosystems from a sink to a source of C. Soil represents one of the largest C pools on earth, and small changes in the balance between inputs and outputs may have extreme consequences for total atmospheric CO2 concentrations. Outputs are determined by microbial decomposition of soil organic matter (SOM) which can be divided in pools of different inherent stability and turn-over. A major stable pool of C is represented by organo-mineral complexes (C bound to silt and clay), which is primarily controlled by plant-derived inputs to soil and soil microbes. Drought effects on plants, microbes and their interactions could cause changes to the stable pool of C, however information on this topic is lacking. In this thesis I: (i) reviewed and quantified drought-induced effects on soil respiration and microbial communities by meta-analysis; (ii) quantified the effects of drying and rewetting on wheat-derived C stabilization and N cycling; (iii) quantified and qualified drought-induced effects on root exudation of soybean and sunflower; (iv) examined drought effects to C stabilization in the field. Results show that drought can induce intense losses of C by increasing soil respiration following rewetting. Highest losses were produced in combination of intense drought and C-rich soils. At the same time drying and rewetting can cause intense stress on plants, reducing biomass and C inputs to soil. However plants can adopt different strategies to drought-induced changes which are reflected in different rates and quality of root exudates. In field drought did not change the size of the mineral-associated or more stable soil C, highlighting resistance of grassland soils. Specific microbial groups were linked to stable soil C at different depths and legumes were shown to be a key functional group in mediating drought effects and increasing stable C in soil.