Biogeochemical Recovery of Oligohaline Wetland Soils Experiencing a Salinity Pulse

Abstract

Oligohaline wetlands exist at a dynamic interface along a river-to-estuary gradient where salinity changes frequently due to fluctuating tidal cycles and weather events. Large storms can cause short-term saltwater intrusion into these low-lying coastal areas, exposing oligohaline wetland plants and soils to above-normal salinities and severe stress on wetland organisms. The objective of this study was to determine how oligohaline wetland soils respond to a single pulse saltwater intrusion, similar to what may occur during hurricane-associated storm surge. Soil response variables were measured in intrusion-Impacted and Reference soils before, during, and after a 6-wk saltwater intrusion event to determine potential impacts on microbial biomass and activity and associated nutrient dynamics. We observed no significant change in basal CO₂ or CH₄ production in oligohaline soils (interstitial salinity = 1.3 ± 0.1 practical salinity units [psu]; mean ± 1 SD) exposed to 20 psu saltwater, while substrate-induced methanogenesis was negatively correlated with salinity. Microbial biomass C (MBC) responded positively to saltwater intrusion by doubling in concentration at the 0- to 5-cm depth interval. Saltwater intrusion had no impact on porewater nutrient concentrations; however, extractable NH₄⁺ decreased as salinity increased. Although some significant changes in microbial activity, abundance, and nutrient availability occurred due to saltwater intrusion, these impacts were generally transient, with post-intrusion conditions resembling pre-intrusion conditions. These results suggest that short-term and transient saltwater intrusion may have little longer-term effect on wetland soil biogeochemistry. However, compounding effects of frequent saltwater intrusion pulses due to strong, regularly occurring storm events could cause longer-lasting shifts in biogeochemical functioning of these wetlands.