The saturation of N cycling in Central Plains streams:: 15N experiments across a broad gradient of nitrate concentrations

We conducted (NO3-)-N-15 stable isotope tracer releases in nine streams with varied intensities and types of human impacts in the upstream watershed to measure nitrate (NO3-) cycling dynamics. Mean ambient NO3- concentrations of the streams ranged from 0.9 to 21,000 mu g I-1 NO3--N. Major N-transforming processes, including uptake, nitrification, and denitrification, all increased approximately two to three orders of magnitude along the same gradient. Despite increases in transformation rates, the efficiency with which stream biota utilized available NO3-decreased along the gradient of increasing NO3-. Observed functional relationships of biological N transformations (uptake and nitrification) with NO3- concentration did not support a 1st order model and did not show signs of Michaelis-Menten type saturation. The empirical relationship was best described by a Efficiency Loss model, in which log-transformed rates (uptake and nitrification) increase with log-transformed nitrate concentration with a slope less than one. Denitrification increased linearly across the gradient of NO3- concentrations, but only accounted for similar to 1% of total NO3- uptake. On average, 20% of stream water NO3- was lost to denitrification per kin, but the percentage removed in most streams was < 5% km(-1). Although the rate of cycling was greater in streams with larger NO3- concentrations, the relative proportion of NO3- retained per unit length of stream decreased as NO3- concentration increased. Due to the rapid rate of NO3- turnover, these streams have a great potential for short-term retention of N from the landscape, but the ability to remove N through denitrification is highly variable.