Biogeochemical recovery and role of ancient hydric soils in contemporary floodplain restorations

Stream restoration has become a popular management practice to mitigate sediment and nutrient pollution and to meet regulatory water quality targets. Many of these restorations, however, fail to account for the pre-disturbance conditions underlying historical legacy impacts (farming, damming, deforestation, etc.) and how they can be leveraged for more environmentally effective restorations. Here, we assessed the biogeochemical recovery of ancient, previously buried, hydric soil on a restored floodplain surface and its potential for providing denitrification ecosystem services. Relict hydric soil along with contemporary wetland soils were evaluated for three years at the Gramies Run restoration site in Maryland. Porewater sampling was conducted monthly, and soil samples were collected every six months, both of which were evaluated for total carbon (C) and nitrogen (N), nitrate and ammonium-N, N process rates (denitrification and net nitrification and mineralization) and living microbial biomass. Oxidation of hydric soils released inorganic N, but the dissolved N concentrations were low (≤ 1 mg/L). Denitrification rates in the hydric soil were low in year 1 (1–4 μg kg⁻¹ h⁻¹) but slowly increased to higher values by year 3 (10–47 μg kg⁻¹ h⁻¹). Our observations suggest that relict hydric soils will require three or more years to provide restoration ecosystem services like improved denitrification and slow gains must be considered in water quality expectations from restoration. We also recommend that hydric soils be retained in-situ, undisturbed, where they are at their original elevations and soil moisture conditions for quicker recovery.