Root exudates from drought-affected plants increase soil respiration across a range of grassland species

Root exudates play an important role in ecosystem carbon (C) cycling. Drought can alter the quality and quantity of root exudation, but it is not clear how root traits affect these changes, and what the implications are for soil C cycling. Seventeen common grassland species of three functional groups were subjected to a two-week drought followed by one week of recovery, after which root exudates were collected and analysed for their total C content, as well as for the respiration they triggered. Across all species but especially in legumes, drought increased specific root exudate-induced respiration rates. Both specific root exudation rate and specific respiration rate were positively correlated to root diameter and root nitrogen content, implying a link with “outsourcing” and “fast” strategies, and this correlation was strengthened after drought. These findings suggest that increased specific respiration rates as a result of drought-induced changes in root exudation is a plant strategy for coping with drought that may result in a loss of soil C after a drought has ended. These findings may help understand the impacts of drought on the capacity of soils to store C, and offer insight into the role of plants in this process.