Changes in root exudate induced respiration reveal a novel mechanism through which drought affects ecosystem C cycling

1) Root exudates play an important role in ecosystem response to climate change, but the functional consequences of drought-induced changes in the quality of root exudates are unknown. Here, we addressed this knowledge gap in a unique experimental approach.
2) We subjected two common grassland species that differ widely in their growth strategies and root systems, the grass Holcus lanatus and the forb Rumex acetosa, to two weeks of drought. We collected root exudates and soils at the end of the drought and after two weeks of recovery, and re-added all root exudates to all soils in a fully reciprocal set up to measure root exudate induced respiration.
3) We found that soil treatment was unimportant for determining root exudate induced respiration. In contrast, root exudates collected from plants that had experienced drought clearly triggered more soil respiration than exudates from undroughted plants. Importantly, this increased respiration compensated for the lower rates of root exudation in droughted plants.
4) Our findings reveal a novel mechanism through which drought can continue to affect ecosystem carbon cycling, and a potential plant strategy to facilitate regrowth through stimulating microbial activity. These findings have important implications for understanding plant and ecosystem response to drought.