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

Abstract

We measured denitrification rates using a field 15 NO 3− 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 Wdenn) 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 N 2 O production rates in all streams. The fraction of total NO 3− removal from water due to denitrification ranged from 0.5% to 100% among streams (median of 16%), and was related to NH 4+ concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling S Wden were specific discharge (discharge / width) and NO 3− concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (U den) and NO 3− 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 U den increased with increasing NO 3− concentration, the efficiency of NO 3− removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO 3− load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO 3− concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO 3− concentration.