Plants secrete carbon compounds into soils through their roots, affecting microbes in such a way that nutrient availability in the soil can increase. This root exudation can cost over 10% of the carbon fixed through photosynthesis, but this cost is thought to be largely offset by the benefits of altering soil microbial activity. Given the amount of carbon that plants can release into soils, root exudation is an important link between the above- and belowground carbon cycling. Despite the importance and magnitude of this process, our understanding of how stresses such as drought affect the amount of exuded carbon is relatively poor. So, does water stress increase or decrease exuded carbon?
Recently in Tree Physiology, Catherine Preece and colleagues set out to determine how drought stress affects the amount of carbon secreted by roots of Mediterranean oak (Quercus ilex). Mediterranean oak is a widespread species throughout the Mediterranean region, which means that changes in root exudation by this species would have consequences for the soil biology and carbon cycle of the entire region. Using oak seedlings, they exposed groups of trees to drought ranging from 0 to 21 days, measuring the carbon exuded into the soil after drought and after rewatering. Drought stress causes stomata on leaves to close, reducing the supply of carbon for photosynthesis in trees. We would then expect root exudation to cost proportionally more carbon under drought stress, such that trees would reduce root exudation to conserve carbon.
But Catherine Preece and colleagues found the opposite response: root exudation increased over 20% under the most severe drought conditions! For trees with a limited supply of carbon to their leaves, this is proportionally costlier in terms of carbon than it seems at first glance.
Why would trees invest in such an expensive process under stress? It’s possible that the exuded carbon could make it easier for roots to grow by providing lubrication through the dry soil, or it could provide a burst of nutrients; both possibilities could aid expansion of the root system to find water. Another possibility is rather grim for the plant – the drought stress could have caused roots to die, causing more carbon to leak into the soil. In any case, the authors found that root exudation recovered completely after six weeks, no matter the intensity of the drought stress.
What do these findings mean? Since drought frequency will increase with climate change, carbon cycling between vegetation and soil could become increasingly driven by drought stress. This could decrease the predictability of carbon flux to soils as its magnitude becomes driven by unpredictable drought events.