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

Abstract

We measured uptake length of ¹⁵NO₃⁻ in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO₃⁻ uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban-urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO₃⁻ concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO₃⁻ uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79% of the variation in log uptake length (S_Wtot). Uptake length increased with specific discharge (Q/w) and increasing NO₃⁻ concentrations, showing a loss in removal efficiency in streams with high NO₃⁻ concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO₃⁻ removal. The fraction of catchment area as agriculture and suburban-urban land use weakly predicted NO₃⁻ uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO₃⁻ uptake lengths via directly increasing both gross primary production and NO₃⁻ concentration. Gross primary production shortened S_Wtot, while increasing NO₃⁻ lengthened S_Wtot resulting in no net effect of land use on NO₃⁻ removal.