Interacting effects of vegetation and hydrogeomorphic complexity on nitrate in agricultural waterways

Abstract:

Intensive cultivation in the California Central Valley has resulted in the loss of ~95% of riparian habitat. In addition, small streams have been replaced by denuded agricultural waterways to convey irrigation drainage high in nitrate (NO₃). A variety of strategies are being used to increase riparian vegetation along agricultural waterways, but many of the ecological functions of these novel habitat features are unknown. An extensive study was conducted in summer 2014 to investigate which riparian ecosystem features are associated with variable NO₃ levels in waterways draining irrigated cropland. Eighty sites were selected to represent a wide range of riparian vegetation, hydrogeomorphic features, and agricultural contexts. Sites were visited three times to correspond to the early, mid, and late irrigation season. At each visit, water samples were taken 300m apart and analyzed for turbidity, pH, temperature, and NO₃. To characterized hydrogeomorphic complexity, the frequency of features associated with transient storage and increased retention time were quantified. Vegetation cover and structure on banks, foreshores, and in channels was also measured. Using a generalized linear model, we tested for the interaction of upstream [NO₃-N] with vegetation and hydrogeomorphic variables to predict downstream [NO₃-N], the dependent variable. As expected, upstream [NO₃-N] strongly predicted downstream [NO₃-N] and no features directly predicted downstream [NO₃-N]. The frequency of hydrogeomorphic features, however, interacted with upstream [NO₃-N] to predict downstream [NO₃-N], indicating an effect of hydrogeomorphic complexity on NO₃ from up to downstream. Vegetation factors did not interact with upstream [NO₃-N], but woody vegetation cover was positively correlated with the frequency of hydrogeomorphic features. These findings support the use of woody riparian vegetation to increase NO₃ retention in agricultural waterways, via an indirect association with hydrogeomorphic complexity.