Impact of hydrologic disturbances and water chemistry on soil biogeochemistry in an upland coastal forest

Abstract

Hydrologic disturbances driven by sea level rise and storm surges are altering biogeochemical processes in coastal forests, transforming upland ecosystems into wetlands. However, the effects of repeated flooding on belowground dynamics during the early stages of this transition are not well understood. This study investigates how repeated hydrologic disturbances with different water chemistries impact soil biogeochemistry dynamics in upland coastal forest soils. We conducted a mesocosm flow-through incubation experiment, applying freshwater (FW) and brackish water (BW) pulses to intact soil cores from a temperate coastal forest previously exposed to these treatments in separate field plots. Continuous measurements of CO2, CH4, and N2O fluxes were coupled with δ13C-CH4 isotopic analysis, porewater chemistry (DOC, SO42-, S2-, Fe2+, Mn2+, NH4+, NO3-+NO2-, ORP, pH), and Fourier-transform infrared spectroscopy (FTIR) of soil organic functional groups. Results showed that BW treatments resulted in stronger legacy effects, enhanced reducing conditions, increased CH₄ and N₂O fluxes, and alterations in soil organic matter. Elevated S2- and δ13C-CH4 signatures indicated the co-occurrence of sulfate reduction and methanogenesis via the methylotrophic pathway. Accumulation of NH4+ and N2O suggested potential dissimilatory nitrate reduction to ammonium (DNRA) and incomplete denitrification. Increases in Fe²⁺, Mn²⁺, and DOC indicated the destabilization of organo-mineral associations, while FTIR analysis revealed changes in soil functional groups, including degradation of polysaccharide structures and incorporation of sulfur- and nitrogen-containing groups. Our findings underscore the vulnerability of upland coastal forest soils to repeated inundation, revealing water chemistry–dependent responses and highlighting the complex biogeochemical feedbacks triggered by brackish water intrusion.