QUANTIFYING THE NITROGEN RETENTION CAPACITY OF LEGACY SEDIMENTS AND RELICT HYDRIC SOILS

Eutrophication is the leading cause of impairment in surface waters in the U.S. 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. The goal of our research was to determine how NO₃^- is cycled through the soil of a legacy sediment strewn stream before and after soil drying. The integration of legacy sediment research and soil drying was motivated by recent analyses showing that even in the relatively wet environment of the mid-Atlantic, there is a pulse of nitrate (NO₃^-) in streams following drought. Assessing which soil layers experience the largest NO₃^- flush following sequential leaching of dry soils could provide critical information for determining sources of N to streams, and directly inform best management practices. We extracted separate intact soil columns with 10.16 cm diameter that extended ~40 cm into each of the three significant soil layers at Big Spring Run in Lancaster, Pennsylvania (surface soil, legacy sediment, and buried relict hydric layer). Isotopically labeled nitrate (¹⁵NO₃^-) was added to these columns to quantify NO₃^- retention in each soil layer. The columns were then left to air-dry until steady dry volumetric soil moisture contents had been reached. Lastly, the columns were rewet with N-free water to quantify the loss of ¹⁵NO₃^- from the different soil layers during sequential leaching events. We found the highest initial ¹⁵N retention in the legacy sediment (29±6%) and buried relict hydric soil (25±5%) layers, with retention in the surface soil (10±1%) layer being significantly lower than the deeper layers. As expected, rewetting dry soil resulted in ¹⁵N losses in all soil layers. The greatest losses of ¹⁵N following rewetting occurred in the buried relict hydric soil, followed by the legacy sediment and surface soil layers, respectively. The ¹⁵N remaining in the soil following the sequential leaching events post-drought was highest in the legacy sediment, intermediate in the surface soil, and lowest in the buried relict hydric soil. Higher ¹⁵N retention in the stable soil N pool following sequential leaching suggests that legacy sediments may immobilize some NO₃^-, reducing its losses to nearby stream waters.