Impact of root exudates on microbial carbon cycling in salt marshes

Rhizosphere processes such as root exudation play a major role in carbon cycling by influencing microbial activity. Although the effect of root exudation, both positive and negative, in terrestrial systems has been widely studied, the corresponding effect in coastal systems is unknown. This gap in knowledge is particularly critical because coastal systems sequester 111.4 Tg C /year, a large fraction of which can be attributed to the vegetation. In this study, we characterized the root exudates of the dominant plant species in the pioneer zone, Salicornia spp. and Spartina anglica, and examined how vegetation affects the sediment biogeochemistry in this zone. Field site analysis revealed a high influence of vegetation on microbial sediment respiration as in situ CO₂ emissions from vegetated sediments were 3.6-fold and 4.2-fold higher for Salicornia spp. and Spartina anglica plots, respectively, than CO₂ release in unvegetated sediment. The sediment content of organic carbon and nitrogen and porewater ammonium concentrations (an indicator of organic matter degradation) were however not elevated in the vegetated sediment. Analysis of root exudate composition revealed that fumarate, acetate, and formate accounted for 30–38% of total root-released carbon in both salt marsh species, indicating a stronger environmental than species-specific influence on root exudation. Further, in a microcosm experiment, we evaluated the impact of root exudation on organic carbon cycling in rhizosphere sediments of the two dominant plant species. We focused on the coupling of organic carbon oxidation to sulfate (SO₄^2-) and ferric iron (Fe(III)) reduction. The addition of model root exudates to the rhizosphere sediment of Salicornia spp. and Spartina anglica resulted in a 2.4-fold and 1.3-fold increase of CO₂ emissions compared to the controls, respectively. The CO₂ release even exceeded added organic carbon oxidation, indicating potential microbial “priming” and enhanced organic carbon mineralization in the sediment. Organic carbon addition caused increased sulfate reduction but had no significant effect on iron reduction, emphasizing the dominance of sulfate reduction for organic carbon oxidation in salt marsh sediments. The rapid microbial response to organic carbon addition highlights the stimulation of microbial activity by root exudation. Thus, root exudates are an important component in predicting the stability of salt marsh carbon sinks and global carbon cycling.