Testing assumptions for nitrogen transformation in a low-gradient agricultural stream

Abstract

Summary Common assumptions of the nitrogen cycle in agricultural streams point to biologic transformation of nitrate being tightly linked to benthic carbon turnover within fine sediments of the streambed. While the nitrogen and carbon linkage has been supported in agricultural streams using multi-week stream injection studies, few studies have tested these assumptions using multi-year time-series of carbon and nutrient species. We made elemental and isotopic measurements from 8 years of weekly suspended sediment samples on the main-stem, 14 months of dissolved nitrate samples on the main-stem and tributaries, and point observations of sediment samples from benthic algae and stream banks in an agricultural stream to test the assumptions. Results from Empirical Mode Decomposition of carbon and nitrogen time series suggest agreement with the prevailing assumption and coupling of benthic carbon dynamics with nitrate from late spring through fall during the 8 year sampling period for the temperate stream. During late spring, summer and fall, autotrophic growth and organic matter decomposition assist with controlling temporary sequestration of nitrate and denitrification in stream sediments, respectively. Contrary to conventional wisdom, our data results suggest decoupling of carbon and nitrogen dynamics from winter through mid-spring for much of the 8 year sampling period. During the winter and spring, nitrate loadings from upland fertilizer application and delivery of upland sediments by storm events are shown to instantaneously increase sediment nitrogen and instantaneously decrease transported nitrate. The result is attributed to abiotic transfer of transient nitrate storage during the winter and early spring due to variably charged sesquioxides within streambed sediments. The results provide the first study, to our knowledge, of the potential importance of nitrate sorption in the stream nitrogen cycle and potential implications are that sorption could retard nitrate loadings from downstream transport and increase the potential for denitrification beyond which would be expected with purely biologic based assumptions.