Do root exudates stabilize in coastal wetland soils?

Salt marshes are highly effective long-term carbon sinks. The capacity of these ecosystems to sequester carbon is controlled by the balance of plant primary production and microbial decomposition. Besides the input of litter, plants are able to excrete organic carbon into the soil by transporting recently fixed carbon compounds from the living roots into the surrounding soil. Despite playing an important role in the global carbon cycle, studies on root exudates in tidal wetlands are rare. This study reports findings on (1) the detection and (2) the stabilization of root exudates in a wetland plant-soil system. Conducting a ¹³CO₂ pulse-labeling study, we (1) tested if Spartina anglica Hubb. supplies a relevant (i.e. detectable) flux of recently fixed carbon via root exudates to the soil environment. The biogeochemical conditions of a typical wetland were simulated by planting S. anglica, a dominant grass in large parts of the European salt marsh area, in waterlogged mesocosms. The aboveground biomass was labeled by acidifying 0.1 g of ¹³C-pure bicarbonate inside a cylindric transparent acrylic glass chamber. Labeling was conducted once daily over a period of ten days. Isotope-ratio mass-spectrometry was used to track the ¹³C label through different compartments of the plant-soil system, including leaves, roots and bulk soil. We found a rapid translocation of recently fixed carbon to belowground plants tissues. (2) Subsamples of the labeled bulk soil were used to study the decay and stabilization of root exudates in the soil. A full factorial pot experiment (labeled vs. unlabeled x vegetated vs. non-vegetated) was conducted, where a total of 12 mesocosms were sampled over approx. 15 months and the bulk soil was analyzed for its δ¹³C-signature. This long-term approach showed that root exudates stabilize in coastal wetland soils under anoxic soil conditions and thereby could play an important role in their carbon sequestration capacity.