Mechanisms driving the seasonality of catchment scale nitrate export: Evidence for riparian ecohydrologic controls

Abstract

Considerable variability in the seasonal patterns of stream water nitrate (NO 2 3 ) has been observed in forested watersheds throughout the world. While many forested headwater catchments exhibit winter and early spring peaks in NO 2 3 concentrations, several watersheds have peak concentrations during the summer months. Pond Branch, a headwater catchment in Maryland monitored for over 10 years, exhibits recurrent and broad summer peaks in both NO export from June to September is particularly surprising, given that these summer months typically have the year's lowest discharge. A key challenge is identifying the source(s) of NO 2 3 and the mechanism(s) by which it is transported to the watershed outlet during the summer. In this study, we assessed multiple hypotheses (not mutually exclusive) that could account for the seasonal trend including proximal controls of groundwater-surface water interactions, instream processes, and riparian groundwater-N cycling interactions, as well as two distal controls: geochemical weathering and senescence of riparian vegetation. A combination of long-term weekly and limited duration high-frequency sensor data reveals the importance of riparian ecohydrologic processes during base flow. In this watershed, patterns of seasonal stream water NO 2 3 concentrations and fluxes depend fundamentally on interactions between groundwater dynamics and nitrogen (N) cycling in the riparian zone. Groundwater tables control nitrification-denitrification dynamics as well as hydrologic transport. Our results suggest that in many watersheds, a more sophisticated exploration of NO 2 3 production and NO 2 3 transport mechanisms is required to identify critical points in the landscape and over time that disproportionately drive patterns of watershed NO 2 3 export.

Evaluating hillslope and riparian contributions to dissolved nitrogen (N) export from a boreal forest catchment

“…Other recent studies have similarly reported riparian soils to be sources of N to streams in temperate [e.g. Duncan et al ., ] and Mediterranean [ Lupon et al ., ] catchments. While the underlying mechanisms in these studies differ from those highlighted here, together such observations suggest that this landscape pattern may be more common than is currently appreciated.…”

Boosted Regression Tree Models to Explain Watershed Nutrient Concentrations and Biological Condition

“…Duncan et al . (), in turn, identified proximal controls, i.e., interactions between groundwater dynamics and nitrogen processes in the riparian zone, on NO 3 − concentrations during the summer months in their study watershed. However, other distal controls (e.g., senescence of vegetation) were considered as potential influences on NO 3 − concentrations and fluxes during different portions of the year.…”

Nitrate retention capacity of milldam-impacted legacy sediments and relict A horizon soils

“…In contrast, legacy-sediment-rich fill terraces have been shown to dampen N removal pathways in the long-buried relict soils which they overlie, while also acting as potential sources of nitrate (NO − 3 ) to waterways (Weitzman et al, 2014). Climate-driven export of N from watersheds is known to occur (Howarth et al, 2006;Lewis and Grimm, 2007;Kaushal et al, 2008aKaushal et al, , 2010Duncan et al, 2015), with N stored during "dry" years or seasons and flushed from watersheds during "wet" years or seasons. Such drying-rewetting cycles have been linked to regionwide pulses of high NO − 3 concentrations in tributaries of the Chesapeake Bay .…”