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

We conducted ¹⁵NO ₃ ^- stable isotope tracer releases in nine streams with varied intensities and types of human impacts in the upstream watershed to measure nitrate (NO ₃ ^- ) cycling dynamics. Mean ambient NO ₃ ^- concentrations of the streams ranged from 0.9 to 21,000 μg l^-1 NO ₃ ^- -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 NO ₃ ^- -decreased along the gradient of increasing NO ₃ ^- . Observed functional relationships of biological N transformations (uptake and nitrification) with NO ₃ ^- 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 NO ₃ ^- concentrations, but only accounted for ∼1% of total NO ₃ ^- uptake. On average, 20% of stream water NO ₃ ^- was lost to denitrification per km, but the percentage removed in most streams was <5% km^-1. Although the rate of cycling was greater in streams with larger NO ₃ ^- concentrations, the relative proportion of NO ₃ ^- retained per unit length of stream decreased as NO ₃ ^- concentration increased. Due to the rapid rate of NO ₃ ^- 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.