Stream nitrate and DOC dynamics during three spring storms across land uses in glaciated landscapes of the Midwest

Many studies have investigated nitrate and dissolved organic carbon (DOC) export mechanisms during storms in forested mountainous to hilly catchments. However, the number of studies across land uses in artificially drained landscapes of the Midwest remains relatively small despite the importance of nitrate and carbon losses to streams on water quality in this region. This study investigates water, nitrate, and DOC delivery (timing) to streams during storms, and the mechanisms (flowpaths) affecting nitrate and DOC flushing trajectories during three spring storms in an agricultural watershed and a mixed land use watershed in glaciated landscape of the Midwest. Hydrograph separation techniques using oxygen-18 isotopes of water and the analysis of major cation flushing trajectories were used to determine the flowpath associated with the export of nitrate and DOC in each watershed. Higher anthropogenic N inputs in the agricultural watershed are associated with higher stream nitrate concentration during storms, while DOC concentration in streams across land uses is mainly influenced by storm characteristics/discharge. In both watersheds, DOC concentration quickly increases and decreases with discharge. In the mixed land use watershed, the peak in nitrate is consistently delayed relative to the peak in DOC; however, nitrate peaks vary in relation to discharge in the agricultural watershed. The comparison of the nitrate/DOC concentration patterns to the concentration patterns of major cations during the storms studied suggests that DOC is likely exported via a combination of overland flow and preferential flow through soil macropores in both watersheds. Data suggest nitrate is exported with groundwater, either as tile-drain flow or subsurface flow to the streams. Results also indicate that the connectivity between nitrate/DOC reservoirs and the stream is an important control mechanism on the timing of delivery of nitrate and DOC to the stream in the watersheds studied. Finally, results indicate that even in agricultural watersheds where tile-drain flow is an important contributor to stream flow during storms, there is not necessarily a consistent export mechanism for all solutes. These results underscore the need for further studies investigating the relative importance of matrix flow, overland flow and macropore flow at the watershed scale in order to fully understand event scale processes controlling the transport of solutes to streams in glaciated landscapes of