Salt marsh porewater alkalinity varies with plant species identity and growth stage

Abstract

Salt marshes are increasingly recognized as important sources of total alkalinity (TA), contributing to coastal carbon sequestration. While hydrological drivers are well studied, plant-mediated effects on TA export remain poorly constrained. Here, we assess how plant species identity and diversity influence TA production in a marsh mesocosm experiment including eight species and two- and five-species mixtures. On average, porewater TA concentrations were nearly twice as high in vegetated sediments (~4200 µmol kg - ¹) compared to unvegetated controls (~2400 µmol kg - ¹). However, responses varied strongly among species, with monocultures ranging from values comparable to unvegetated sediments (e.g., Puccinellia maritima ) to >5000 µmol kg - ¹ ( Triglochin maritima ). TA increased with plant growth stage, belowground biomass, sediment water content, and more reducing soil conditions, indicating tight coupling between plant traits and sediment biogeochemistry. However, TA did not increase consistently with plant species richness, as mixtures generally converged toward intermediate values. Lower TA production was often associated with higher TA:DIC ratios, suggesting a greater relative buffering capacity despite lower absolute alkalinity generation. These findings suggest that species identity and development may be important factors influencing salt marsh alkalinity production, with implications for coastal carbonate dynamics and blue carbon budgets.