Assessing Nitrogen Sources and Transformations in Deep Soil Profiles of a Sandy, Limestone-Based Watershed.

Nitrate pollution continues to be a significant problem in anthropogenically-affected watersheds, with known negative impacts on both receiving waters and aquifers. An important part of creating a nitrogen budget, includes assessment of the processes between the inputs and the outputs, and with nitrate that would include the processes affecting its attenuation, nitrification and denitrification. In this study, we examine patterns of stable nitrogen isotopes, extractable nitrate, and denitrification rates in deep profiles of soil within watershed (northern Florida, USA) in an effort to better quantify N sources in different land uses. Soil/subsoil was sampled to depths up to 150 feet at 15 sites. Denitrification was highest in the top meter of soil, regardless of the land use except at a site with a buried peat horizon approximately 11m below the surface. This indicates that there is potential for denitrification to occur within the vadose zone of the soil. The stable isotopes of nitrate (¹⁵N and ¹⁸O) correlated with the occurrence of denitrification with depth causing the nitrate isotope end member in the groundwater to be poorly indicative of the source.  The findings of this study will assist in the development of more accurate N budgets and also demonstrate the pitfalls of using isotopic signatures to interpret N sources in a complex watershed.