In-stream Processing of N: Implications for Regional N Losses

In many regions with high N inputs from human activity, large unmeasured N sinks have been inferred by comparing watershed inputs and outputs. One of the most likely N sinks is N uptake or denitrification within or adjacent to the stream channel. Recent research conducted as part of the LINX project helps to quantify some of these sinks and pathways. In 10 headwater streams, 15-NH4 was added at tracer levels and tracked through the various pools and fluxes of the N cycle. Results show that whole-stream enrichments (where N concentration is increased over background concentrations to determine N uptake) underestimate ambient rates of N uptake; that nitrification can be a very large fraction of NH4 uptake; that NH4 removal is related to Q and not biotic characteristics of the stream; and that NO3 uptake is often 10-fold slower than NH4 uptake. Given the very rapid N cycling in headwater streams, N losses in large rivers may be much more closely tied to these headwater processes than previously recognized. This is especially likely in watersheds where non-point source inputs are dominant.