The identity of a single plant species can shift a salt marsh's alkalinity output by more than a factor of two, and that is a finding that should reshape how we estimate blue carbon budgets. A new mesocosm experiment demonstrates that porewater total alkalinity (TA) concentrations in vegetated sediments averaged nearly 4200 µmol kg⁻¹, almost double the 2400 µmol kg⁻¹ measured in unvegetated controls, but the variation among species is what demands our attention. Monocultures of *Triglochin maritima* pushed TA above 5000 µmol kg⁻¹, while *Puccinellia maritima* produced values indistinguishable from bare sediment. This is not a simple story of "plants help"; it is a story of which plants, at what stage of growth, and under what soil conditions.
These results connect directly to ongoing work across our publication. Previous research on Salt Marsh Vegetation Boosts Organic Carbon Storage in Yellow Sea Wetlands established that vegetation enhances organic carbon sequestration, but the new alkalinity data reveal a separate, plant-driven mechanism for carbon dioxide removal through coastal carbonate dynamics. Meanwhile, findings on Crab Activity Impacts Sediment, Shaping Salt Marsh Resilience remind us that biological interactions extend beyond plants alone, bioturbation by crabs alters sediment structure and biogeochemistry, potentially modulating the very porewater conditions that drive alkalinity export. The integrated picture is that marsh function is determined by a web of species-specific traits, not by vegetation cover alone.
Our take is pragmatic: restoration and carbon-accounting projects cannot treat salt marshes as uniform alkalinity factories. The experiment shows that plant growth stage and belowground biomass are tightly coupled with TA production, and that more reducing soil conditions, often a consequence of root activity, amplify alkalinity generation. Yet species richness did not consistently boost TA; mixtures converged toward intermediate values, and some mixtures with lower absolute alkalinity actually showed higher TA:DIC ratios, meaning they offered greater relative buffering capacity per unit of dissolved inorganic carbon. For practitioners, this implies that selecting for high-alkalinity species like *Triglochin maritima* could maximize export, but at the cost of reduced buffering resilience if diversity is lost.
The open question that lingers is how these mesocosm-scale patterns translate to the field, where tidal flushing, salinity gradients, and competition play out over seasons. A specific detail to watch: the study's finding that TA:DIC ratios were higher in lower-alkalinity treatments suggests that even marshes with modest absolute alkalinity production may play an outsized role in buffering coastal acidification. That nuance, not just how much alkalinity is exported, but how effectively it stabilizes local pH, should be a priority for the next wave of empirical, longitudinal observations.