Carbon allocation to root exudates in a mature mixed F. sylvatica – P. abies forest under drought and one year after drought release.

In recent years, important processes controlling ecosystem carbon dynamics have been connected to fine-root exudation of soluble carbon compounds. Root exudation patterns may change depending on plant interactions and plant susceptibility to and recovery from drought. Recent investigations suggest that root exudation tends to increase with stress events, but quantification of the amount of carbon released from roots across soil depths with differing water availability and species interactions are missing.

We tested if root exudation rates were negatively correlated with soil water content across soil depths during and after drought. We further tested if species in mixture, often considered to be less stressed under drought, exuded less carbon than species in monospecific environments. Exudates were sampled in a mature Fagus sylvatica L. and Picea abies (L.) Karst. forest at the end of a five-year throughfall exclusion period and again one year after the drought ended. We quantified root exudates and their variation with soil depth for both tree species in monospecific and mixed species zones.

Carbon exudation significantly increased in fine roots exposed to dry soils (< c. 10vol-% SWC), with fine roots growing in driest surface soils exuding the most. Under drought, the proportion of net assimilated carbon allocated to exudates doubled for F. sylvatica and tripled for P. abies, respectively. One year after drought release, carbon allocation to exudates was not different for F. sylvatica between previously drought stressed and control trees, while the proportion was still significantly increased in previously drought stressed P. abies compared to control trees. This indicates long-term disruptions in carbon allocation patterns even after the end of a drought period in evergreen vs. broadleaved temperate tree species. Both species exuded significantly more carbon when in monoculture than when in a mixed zone, especially in the surface soil layer.

Our results demonstrate that carbon is released preferentially in the surface soil layers exposed to more variable soil water contents and exudation amounts are maintained by allocating bigger proportions of net-assimilated carbon into exudates even among variable carbon assimilation rates throughout periods of drought and re-wetting. In addition, plant interactions significantly influenced root exudation patterns. Further studies are planned to understand if differences in exudate quantity correlate to differences in functioning of released exudates in monospecific and mixed environments by analyzing the chemical profile of the exudate metabolome.