Empirical tests for effects of riparian buffers on whole watershed nitrate discharges

Widespread efforts to protect and restore riparian areas for water quality benefit continue despite mixed results from studies examining the influence of riparian buffers in whole watersheds.  We used a dataset designed to isolate influence of land cover patterns on water quality, recently developed measures of buffer configuration, and new analytical approaches to test for whole-watershed effects on stream nitrogen concentrations from 321 watersheds. We employed significance tests and corrected Akaike Information Criterion (AICc) in linear models to assess significance of buffer effects across watersheds, whether including descriptions of buffer configuration explained more variability in nitrate concentrations than models using only land cover proportions, and whether buffer effects remain consistent across physiographic provinces. We found statistically significant effects (or that buffer models were more likely according to AICc), quantified variation in nitrate discharges from buffered and unbuffered cropland, and estimated whole-watershed buffer removal percentages. Our effort represents the first empirical test of riparian filtering across watersheds allowing explicit quantification of the influence of buffers on stream nitrate concentrations.  We discuss management implications from marked physiographic and spatial variability in buffer effects, indicating that aggregate filtering by buffers across watersheds is not as consistently important as suggested by published studies.