NITRATE DYNAMICS IN A KANSAS GRASSLAND - EFFECT ON ECOSYSTEM HEALTH

Understanding the effect of local weather on the nitrate cycling and mobility of nitrates from sediments to groundwaters was the major thrust of our work. Precipitation affects the nitrogen cycle in terms of transporting excess nitrates on land with surface waters from one location to the other. To understand ambient nitrate dynamics in an unmanaged grassland environment, we studied the stable isotope chemistry of various substrates at the Konza Prairie Biological Station. Sediments collected ranged from alluvium or loess to bedrock materials which are mostly limestone and shale. 40 water samples have been collected along a full stretch of the major creeks onsite (Kings and Shane Creeks). These samples were analyzed for δD, δ¹⁸O, and δ¹⁵N to determine the manner by which recharge water allows nitrates to be leached into sediment, and to determine if additional nitrate is contributed from weathering of the bedrocks. Anion chemistry of the water indicates the presence of sulfates, chlorides, nitrates, fluorides, and carbonates. Samples collected from groundwater wells contained nitrates in amounts that ranged from 0.004-0.025 mg/L and the surface waters ranged from 0.003 – 1.286 mg/L. δ¹⁸O-δD relation reflects isotopic fractionation as a function of local evaporation and transpiration. However the streams by the groundwater wells, show a relative meteoric origin and being derived from precipitation without subsequent modification such as that due to isotopic fractionation. δ¹⁵N of the creeks ranged 0.07-6.13 per mil, while the groundwaters ranged 3.98-4.75 per mil. Subsurface sediments were collected with a hydraulic auger upto 2.4 m depth to study spatial variation of δ¹⁵N and δ¹³C. Major findings indicated the presence of C3 and C4 derived soil organic carbon. There is also an approximate 1:10 correlation between the total N and C content of these core sediments including NO₃ and NH₄ mostly concentrating in the upper 0.6 – 1.2 m of the sediment profile. δ¹⁵N of sediments from 0.6-1.2m depth ranged from 3.36 to 7.53 per mil. This data indicates that little fractionation happens in nitrates while in transition from sediments to waters. The results of this study provide a broader context for ambient nitrate cycling in unmanaged grassland and a baseline comparison for nitrate levels in surrounding agricultural lands.