Low-level nitrate export from confined dairy farming detected in North Carolina streams using (delta) 15N

Animal waste-produced nitrate in streams can be detected with natural abundance nitrogen isotopes even when stream nitrate concentrations are low and derive predominantly from natural soil nitrate sources. The objective of this limited study was to demonstrate the utility of such an approach with a minimal number of samples and minimal supporting data. These constraints are important in terms of the usefulness of the small-scale isotopic approach as a component of long-term, large-scale watershed and river basin monitoring, given the expense of nitrate isotopic measurements. The study was able to identify a small animal waste nitrate export signal over a significant background signal of soil nitrate and possible fertilizer nitrate in the stream draining the site. During the course of a year, a second-order stream adjacent to a carefully managed research dairy farm near Raleigh, NC had mean nitrate-N of 0.4 mg l^-1 with mean (delta)15N-NO3 values of +8.7‰ downstream of the farm. Secondary dairy lagoon liquids used for spray irrigation of nearby fields had a mean ammonium concentration of 87.5 mg l^-1, and a mean (delta)15N of +20.7‰. A small, intermittent first-order stream draining the dairy sprayfields and emptying into the second-order stream had a mean nitrate-N level of 3.7 mg l^-1 and mean (delta)15N-NO3 of +16.0‰. Even at the relatively low nitrate levels and slightly elevated (delta)15N-NO3 values of the second-order receiving stream, an observed increase in (delta)15N values with increasing nitrate concentration in this stream indicates that a small amount of animal waste-produced nitrate is being exported. This type of monitoring could be applied to many small watersheds as part of a larger watershed or river basin study where sampling density is constrained by logistical and economic factors. This is the case in the Neuse River Basin, where this study was an early part of a developing basin-wide Nitrogen Isotope Tracer Program. This type of monitoring can be particularly useful where identification of targets for reduction or stabilization of N export from livestock farms is desired, and the animal waste nitrate signal is superimposed on a larger natural background nitrate or fertilizer nitrate + natural background nitrate signal.