Saltwater intrusion into tidal freshwater marshes alters the biogeochemical processing of organic carbon

Abstract. Environmental perturbations in wetlands affect the integrated plant-microbial-soil system, causing biogeochemical responses that can manifest at local to global scales. The objective of this study was to determine how saltwater intrusion affects carbon mineralization and greenhouse gas production in coastal wetlands. Working with tidal freshwater marsh soils that had experienced ~ 3.5 yr of in situ saltwater additions, we quantified changes in soil properties, measured extracellular enzyme activity associated with organic matter breakdown, and determined potential rates of anaerobic carbon dioxide (CO₂) and methane (CH₄) production. Soils from the field plots treated with brackish water had lower carbon content and higher C : N ratios than soils from freshwater plots, indicating that saltwater intrusion reduced carbon availability and increased organic matter recalcitrance. This was reflected in reduced activities of enzymes associated with the hydrolysis of cellulose and the oxidation of lignin, leading to reduced rates of soil CO₂ and CH₄ production. The effects of long-term saltwater additions contrasted with the effects of short-term exposure to brackish water during three-day laboratory incubations, which increased rates of CO₂ production but lowered rates of CH₄ production. Collectively, our data suggest that the long-term effect of saltwater intrusion on soil CO₂ production is indirect, mediated through the effects of elevated salinity on the quantity and quality of autochthonous organic matter inputs to the soil. In contrast, salinity, organic matter content, and enzyme activities directly influence CH₄ production. Our analyses demonstrate that saltwater intrusion into tidal freshwater marshes affects the entire process of carbon mineralization, from the availability of organic carbon through its terminal metabolism to CO₂ and/or CH₄, and illustrate that long-term shifts in biogeochemical functioning are not necessarily consistent with short-term disturbance-type responses.