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

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 (NO₃⁻) 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 (¹⁵NO₃⁻) 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 ¹⁵NO₃⁻ from the different horizons. We found the highest initial ¹⁵N 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 ¹⁵N 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 ¹⁵N 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 NO₃⁻ from the soil, while vertical flow through the legacy-sediment-rich soil profile that originates in the surface has the potential to retain more NO₃⁻. 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 NO₃⁻ to nearby stream waters.