Assessing the Role of Land Use in Watershed Nitrate Export Using Triple Oxygen Isotopes

Quantifying the influence of land-use patterns on the amount and source(s) of nitrate (NO3) exported from watersheds is critical for understanding and mitigating the effects of nutrient pollution on downstream waterbodies. The isotopic composition of NO3 is valuable for fingerprinting of NO3 sources, including manure and atmospheric nitrate. To assess loads, sources, and potential transformations of NO3 in the Chesapeake Bay (CB) watershed, stream samples from fourteen sub-watersheds of the CB were collected semi-monthly and during multiple storm events from October 2015-September 2016 (Water Year 2016). The watersheds range in size (500 - 127,900 ha) and in dominant land-use (forest, urban, agriculture). The samples were analyzed for nitrate concentrations and isotopes (δ15N, as well as the triple oxygen isotope composition, defined as Δ17O ≅ δ17O - 0.52 x δ18O). Stream loads of nitrate were estimated using WRTDS (Weighted Regressions on Time, Discharge, and Season). NO3 deposition (dry and wet) was fairly uniform across all watersheds (2.1 - 3.0 kg NO3-N ha-1), whereas stream NO3 varied greatly (0.6 - 11.8 kg NO3-N ha-1). Stream loads of NO3 were positively related to the percent of agricultural land (r2 = 0.67, p < 0.005) and negatively related to the percent of forested land (r2 = 0.61, p < 0.005). Preliminary isotope data indicate a positive relationship between δ15NNO3 and the proportion of agricultural land (r2 = 0.48, p < 0.0001), which suggests that nitrate inputs in predominantly agricultural watersheds are manure or pools of partially denitrified fertilizer. A positive relationship between Δ17ONO3 and the percent of forested land (r2 = 0.19, p < 0.005) suggests that forests export a greater proportion of atmospheric nitrate than other systems. Full NO3 isotope data for Water Year 2016 ( 400 samples) will be presented from all watersheds to elucidate the amount and sources of NO3 exported from a variety of land-uses.