Back from the past? Assessment of nitrogen removal ability of buried historic wetland soils before and after a one-year incubation on a restored floodplain

Stream, floodplain, and wetland restorations have enhanced water quality and to some extent ecological function; however, soil health is prioritized infrequently in restoration planning or monitoring. Buried, historic, hydric soils – common across U.S. mid-Atlantic valley bottoms beneath legacy sediments – are not included in most floodplain restoration designs, though they may retain favorable biogeochemical characteristics and host legacy microbial communities that could support ecosystem recovery if exhumed and preserved. To assess the efficacy of including historic hydric soils in floodplain restoration, we characterized pre-Colonial wetland soils buried below legacy sediments and now exposed along incised streambanks across the mid-Atlantic. We compared carbon (C) and nitrogen (N) contents; C:N ratios; nitrate-N and ammonium-N concentrations; denitrification rates; functional genes for denitrification (nosZ) and nitrification (amoA for AoA+AoB); and phospholipid fatty acid (PLFA) biomasses of historic wetland soils with contemporary wetland soils before and after a one-year incubation in a recently restored floodplain. Compared to modern wetland soils, historic hydric soils that are now buried by legacy sediment are less nutrient-rich, have fewer functional genes for and lower rates of denitrification, and possess significantly less microbial biomass. Following the one-year incubation, many of these concentrations, rates, and gene counts increased in historic soils, though incubated modern soils showed greater improvements. Ultimately, our results suggest that while inclusion of historic, hydric soils and their legacy microbiomes is valuable in floodplain restoration, the recovery of historic, hydric soils is slow and attainment of restoration goals, such as increased denitrification, may require multiple years.