Carbon allocation to fine root exudation – mechanisms and quantification in temperate trees and forests under drought

Background/Question/Methods

In recent years, important processes controlling ecosystem carbon (C) dynamics and plant susceptibility to drought have been connected to fine root exudation of soluble C compounds. Recent investigations suggest that root exudate release increases under drought, but mechanisms and quantification of the amount of released C, in light of changes in root growth and distribution under drought, are missing. We tested if fine roots in dry surface soils exuded more C than roots in deeper moist soils and if root exudation rates were negatively correlated with soil water content across soil depths. We further hypothesized that plants would allocate newly assimilated C preferably towards roots in dry soils. Exudates were sampled in A) a mature Fagus sylvatica L. and Picea abies (L.) Karst. forest that was exposed to throughfall exclusion for five consecutive growing seasons and B) in a greenhouse in 13CO2 labeled saplings of split-rooted Abies balsamea (L.) Mill. and Tsuga canadensis (L.) Carrière trees. We quantified root exudates and their variation with soil depth under experimental drought and control conditions (A) and tracked the 13CO2 tracer towards a moist and a dry pot along the split-rooted saplings (B).

Results/Conclusions

In the mature forest, we found that C exudation increased in fine roots exposed to drier soils, with the highest exudation rates in the fine roots in surface soils. When scaled to the whole tree level, the proportion of root exudation from total assimilation increased two- (F. sylvatica) to three-fold (P. abies) for drought stressed mature trees, amounting to 1.0% (0.11 ± 0.04 g m-2 d-1, F. sylvatica) to 2.5% (0.12 ± 0.04 g m-2 d-1, P. abies) of net carbon assimilation, respectively. Therefore, trees maintained constant root exudation, although C assimilation was reduced by more than 50% under drought. In the smaller sapling trees, labeled C appeared in stem respiration 1 day after canopy 13CO2 labeling of A. balsamea and T. canadensis saplings irrespective of treatment, but release into the soil was faster by 1 day in droughted compared to well-watered T. canadensis saplings. Our results demonstrate that fine root exudation rates are enhanced, and newly assimilated C is preferentially used for exudation into dry soils in temperate species. In natural forest ecosystems, C is released especially in the surface soil layers exposed to more extreme drought conditions and whole-tree exudation is maintained even under strongly decreased C assimilation.