Nitrate removal in stream ecosystems measured by 15N addition experiments: Denitrification

We measured denitrification rates using a field N-15-NO3- tracer-addition approach in a large, cross-site study of nitrate uptake in reference, agricultural, and suburban-urban streams. We measured denitrification rates in 49 of 72 streams studied. Uptake length due to denitrification (S-Wden) ranged from 89 m to 184 km (median of 9050 m) and there were no significant differences among regions or land-use categories, likely because of the wide range of conditions within each region and land use. N-2 production rates far exceeded N2O production rates in all streams. The fraction of total NO3- removal from water due to denitrification ranged from 0.5% to 100% among streams (median of 16%), and was related to NH4+ concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling SWden were specific discharge (discharge / width) and NO3- concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (Uden) and NO3- concentration indicated a partial saturation effect. A power function with an exponent of 0.5 described this relationship better than a Michaelis Menten equation. Although Uden increased with increasing NO3- concentration, the efficiency of NO3- removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO3- load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO3- concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO3- concentration.