Nitrate retention capacity of milldam-impacted legacy sediments and relict A horizon soils

Abstract

Abstract. While eutrophication is often attributed to contemporary nutrient pollution, there is growing evidence that past practices, like the accumulation of legacy sediment behind historic milldams, are also important. Given their prevalence, there is a critical need to understand how N flows through, and is retained in, legacy sediments to improve predictions and management of N transport from uplands to streams in the context of climatic variability and land-use change. Our goal was to determine how nitrate (NO − 3 ) is cycled through the soil of a legacy-sediment-strewn stream before and after soil drying. We extracted 10.16 cm radius intact soil columns that extended 30 cm into each of the three significant soil horizons at Big Spring Run (BSR) in Lancaster, Pennsylvania: surface legacy sediment characterized by a newly developing mineral A horizon soil, mid-layer legacy sediment consisting of mineral B horizon soil and a dark, organic-rich, buried relict A horizon soil. Columns were first preincubated at field capacity and then isotopically labeled nitrate ( 15 NO − 3 ) was added and allowed to drain to estimate retention. The columns were then air-dried and subsequently rewet with N-free water and allowed to drain to quantify the drought-induced loss of 15 NO − 3 from the different horizons. We found the highest initial 15 N retention in the mid-layer legacy sediment (17 ± 4 %) and buried relict A soil (14 ± 3 %) horizons, with significantly lower retention in the surface legacy sediment (6 ± 1 %) horizon. As expected, rewetting dry soil resulted in 15 N losses in all horizons, with the greatest losses in the buried relict A horizon soil, followed by the mid-layer legacy sediment and surface legacy sediment horizons. The 15 N remaining in the soil following the post-drought leaching was highest in the mid-layer legacy sediment, intermediate in the surface legacy sediment, and lowest in the buried relict A horizon soil. Fluctuations in the water table at BSR which affect saturation of the buried relict A horizon soil could lead to great loses of NO − 3 from the soil, while vertical flow through the legacy-sediment-rich soil profile that originates in the surface has the potential to retain more NO − 3 . Restoration that seeks to reconnect the groundwater and surface water, which will decrease the number of drying-rewetting events imposed on the relict A horizon soils, could initially lead to increased losses of NO − 3 to nearby stream waters.Published by Copernicus Publications on behalf of the European Geosciences Union.

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

“…Our results are more consistent with authors who have observed reduced biological activity in buried organic‐rich horizons, especially in terms of denitrification (e.g. Koval 2011; Weitzman et al 2014; Weitzman & Kaye 2017). Clague et al (2013) cited less labile organic matter, low microbial biomass and diversity, and/or microbial communities without denitrifying genes as potential explanations for low denitrification capacity in buried historic organic soils compared to surface sediments.…”

Draining the Landscape: How Do Nitrogen Concentrations in Riparian Groundwater and Stream Water Change Following Milldam Removal?

“…This was based on well‐established studies indicating that denitrification is enhanced under saturated and anoxic soil conditions associated with near‐surface groundwater levels while nitrification is favored under well‐drained and oxic soil conditions (Burt & Pinay, 2005; Cirmo & McDonnell, 1997; Gold et al., 1998). This assessment was also supported by work of Weitzman and Kaye (2017) who reported elevated rates of nitrification in surficial and well‐drained riparian sediments and suggested these soils could serve as potential nitrate‐N sources for streams. The nitrification and N leaching rates could especially be elevated if riparian sediments contain buried organic‐rich soil layers which can be mineralized following drainage and oxidation (e.g., Gurwick, Groffman, et al., 2008, Gurwick, McCorkle, et al., 2008; Hill, 2011).…”

Concentration–discharge relationships describe solute and sediment mobilization, reaction, and transport at event and longer timescales

“…Fertilizer inputs associated with agricultural land uses in the White Clay Creek catchment may also contribute to legacy stores of exogenous solutes in soils. The presence of widespread legacy stores of NO 3 − and SO 4 2− , particularly if they are deep in the soil profile (Weitzman & Kaye, ), might explain the similar C‐Q behaviour of exogenous and geogenic solutes at both event and longer timescales. Similarities in the threshold behaviours of NO 3 − , SO 4 2− , and Cl − revealed by the piecewise regression analysis (Figure , Table ) further support the activation of deep source pools of these solutes in the White Clay Creek watershed.…”