Nitrate reduction across soils transitioning from coastal forest to wetland are hotspots for denitrification

Abstract

Sea level rise drives spatial migration of coastal ecosystems and can lead to the accelerated replacement of coastal forests with tidal wetlands. During this transition, changes in inundation frequency and saltwater intrusion dramatically alter soil biogeochemistry and redox conditions. Soil biogeochemical cycles in steady-state upland and wetland ecosystems are well studied, but pathways and rates in rapidly changing ecosystems are largely unconstrained. We characterized reduction of reactive nitrogen (N) via denitrification and dissimilatory nitrate reduction to ammonia (DNRA) at four sites where coastal forest is undergoing ecosystem state change and becoming wetland throughout the Chesapeake Bay. Sites were chosen to vary in soil characteristics, vegetation, and salinity regime. Soils were incubated with ¹⁵N-labeled nitrate to estimate potential rates of denitrification and DNRA. Denitrification rates ranged from 0.011 to 26 nmoles N g^-1 hr^-1 and DNRA rates ranged from -0.30 to 5.6 nmoles N g^-1 hr^-1. DNRA rates were low with no clear pattern across the sites. Denitrification rates were higher in transition zone soils than in upland soils at three of the four sites (p < 0.05). Rates of denitrification in wetlands were lower than that observed in transition soils when hydrogen sulfide (H₂S) concentrations were high, favoring DNRA, but not where H₂S was low. Rates of denitrification across sites and zones were driven by soil moisture and related to organic matter content. Our data suggest that transition zone soils act as hotspots for N removal due to intermittent inundation and oscillating redox conditions. The N cycling characteristics of soils changing from forest to wetland are unique enough from steady-state upland and wetland ecosystems to address as a distinct ecotone in forecasts of future ecosystem function and N exchange at the coastal terrestrial-aquatic interface.