SURFACE WATER AND SEDIMENT GEOCHEMISTRY IN UNDERSTANDING MOBILITY OF NITRATES IN MESIC KANSAS GRASSLAND

Nitrate pollution is a significant problem for farmers that use nitrogen containing fertilizers and manure for their crops. To understand ambient nitrate dynamics in unmanaged grasslands in the Central United States, we measured δ¹⁵N, δ¹⁸O, and δD at the Konza Prairie Biological Station, a mesic grassland located in eastern KS, USA. The Konza Prairie contains surficial waters, as creeks with varying amount of nitrates derived from adjacent sediment cover above the bedrock which also act as medium for transportation. These sediments range from alluvium to bedrock materials which are mostly limestones and shales. Consistent flow in these creeks is largely attributable to fairly high precipitation rates of northeast Kansas (30-year mean – 840mm/ year) and a shallow water table. To understand the biogeochemistry of nitrates in unmanaged grassland, 40 water samples have been collected along a full stretch of the major creeks on site (Kings and Shane Creek). These samples are being analyzed for δD, δ¹⁸O, and δ¹⁵N to determine the manner by which recharge water (precipitation) allows nitrates to be leached into sediment, and to determine if additional nitrate is contributed from weathering of the bedrocks. Anion content of these water samples has shown the primary compositions are sulfates, chlorides, nitrates, fluorides, and carbonates. Although an obvious correlation among the anions is not present, data shows high carbonate content due to the presence of shallow bedrock. It also shows that nitrates are higher in amount within the deeper wells compared to surface creeks possibly due to nitrogen/nitrate contributions from subsurface sediments. There is a considerable difference in nitrate levels at this grassland site that are grazed by bison, compared to ungrazed regions. Future analyses will compare nitrate dynamics in nearby agricultural lands. Additionally, we use subsurface sediments via geoprobe to collect 2m cores adjacent to the creeks to determine δ¹⁵N within core sediments as indicators of nitrate leaching from this grassland into the surface and ground waters. The results of this study will provide broader context for ambient nitrate cycling in unmanaged grassland, and provide a baseline comparison for nitrate levels in surrounding agricultural lands.