『Abstract
　We conducted ¹⁵NO3^- 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 21,000μg l^-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 〜1％ of total NO3^- uptake. On average, 20％ of stream water NO3^- 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 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.

Keywords: Denitrification; Nitrate; Nitrification; Saturation; Stream; Uptake』

Introduction
Methods
　Study sites
　Field methods
　Laboratory methods
　Calculations
　Statistical analyses
Results
　Stream chemical and physical parameters
　Stream metabolism and biomass
　Nutrient dynamics
　Nutrient dynamics in relation to NO3^- concentration
Discussion
　The relationship between spiraling and concentration
　Denitrification and N2O production
　Nitrification
　Biological N demand
　Wal-Mart Ditch
Conclusions
Acknowledgements
References