Back from the past? Assessment of nitrogen removal ability of buried historic wetland soils before and after a 1‐year incubation on a restored floodplain

Abstract

Stream, floodplain, and wetland restorations enhance water quality and ecological function; however, soil health is prioritized infrequently in restoration planning and 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 for nitrogen (N) removal, we characterized pre‐Euro‐American settlement wetland soils buried below legacy sediments and now exposed along incised streambanks across the mid‐Atlantic. We compared carbon (C) and N contents; C:N ratios; nitrate‐N and ammonium‐N concentrations; denitrification rates; functional genes for denitrification (nosZ) and nitrification (amoA for ammonia oxidizing archaea [AoA] + ammonia oxidizing bacteria [AoB]); and phospholipid fatty acid biomasses of historic wetland soils with contemporary wetland soils before and after an 1‐year incubation in a recently restored floodplain. Compared to modern wetland soils, historic hydric soils 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 1‐year incubation, many of these concentrations, rates, and gene counts increased in historic soils, though not substantially. Ultimately, our results suggest that while inclusion of historic, hydric soils and their legacy microbiomes is valuable for N‐removal in floodplain restoration, the recovery of historic, hydric soils is predictably slow, and attainment of restoration goals, such as increased denitrification, may require multiple years.

Dissimilatory Nitrate Reduction to Ammonium (DNRA) Can Undermine Nitrogen Removal Effectiveness of Persistently Reducing Riparian Sediments

“…10,39 Both sites were also similar in terms of sediment texture -with both sites having a very high percent (>90%) of silt and clay in the riparian soil profiles. 48 Previous studies have shown that sites that are persistently anoxic and saturated tend to support elevated DNRA rates and provide a favorable habitat for obligate anaerobes and fermentative bacteria that contribute to this process. 19−21 For example, Jantti et al 19 found that soil DNF capacity was almost completely lost, and DNRA dominated in estuarine soils when hypoxia lasted for several months.…”