Pine root exudation increased under drought and even more under eCO2 and drought

Tree processes belowground are highly complex and strongly affect the soil carbon. Trees assimilate carbon and allocate up to 20% of the carbon to the rhizosphere as root exudates. Rhizosphere processes in the forest soil might have a significant global effect. Abiotic factors such as intensified drought and elevated atmospheric CO2 (eCO2) will influence tree carbon fluxes as predicted under climate change. As trees play a vital role in maintaining Earth’s carbon balance, ecological studies on trees are crucial, especially in light of climate change.

The effect of the predicted elevation of atmospheric CO₂ and drought on the rhizosphere was studied on 2-years-old pine saplings in climate-controlled growth rooms. The results showed up to a twofold increase in assimilation, increasing shoot biomass, while root exudation rate remained unchanged in eCO₂ compared to ambient CO₂. Root exudation increased under drought, despite reduced assimilation. Under combined drought and eCO₂ treatment, exudation rate increased even more by 56%, suggesting assimilated surplus carbon might have been stored in the roots under eCO₂. In addition, we found an increase of soluble sugars in the stem under combined drought and eCO₂ treatment, specifically glucose and fructose, indicating that indeed surplus carbon is stored during well-watered times under eCO₂.

Our results are unique because they show for the first time that pines were able to increase their root exudation under drought due to eCO₂, potentially enhancing their resilience during drought recovery.