Effects of instream processes, discharge, and land cover on nitrogen export from southern Appalachian Mountain catchments

Abstract

Catchments with minimal disturbance usually have low dissolved inorganic nitrogen (DIN) export, but disturbances and anthropogenic inputs result in elevated DIN concentration and export and eutrophication of downstream ecosystems. We studied streams in the southern Appalachian Mountains, USA, an area dominated by hardwood deciduous forest but with areas of valley agriculture and increasing residential development. We collected weekly grab samples and storm samples from nine small catchments and three river sites. Most discharge occurred at baseflow, with baseflow indices ranging from 69% to 95%. We identified three seasonal patterns of baseflow DIN concentration. Streams in mostly forested catchments had low DIN with bimodal peaks, and summer peaks were greater than winter peaks. Streams with more agriculture and development also had bimodal peaks; however, winter peaks were the highest. In streams draining catchments with more residential development, DIN concentration had a single peak, greatest in winter and lowest in summer. Three methods for estimating DIN export produced consistent results. Annual DIN export ranged from less than 200 g ha−1 year−1 for the less disturbed catchments to over 2,000 g ha−1 year−1 in the catchments with the least forest area. Land cover was a strong predictor of DIN concentration but less significant for predicting DIN export. The two forested reference catchments appeared supply limited, the most residential catchment appeared transport limited, and export for the other catchments was significantly related to discharge. In all streams, baseflow DIN export exceeded stormflow export. Morphological and climatological variation among watersheds created complexities unexplainable by land cover. Nevertheless, regression models developed using land cover data from the small catchments reasonably predicted concentration and export for receiving rivers. Our results illustrate the complexity of mechanisms involved in DIN export in a region with a mosaic of climate, geology, topography, soils, vegetation, and past and present land use.

Watershed‐scale vegetation, water quantity, and water quality responses to wildfire in the southern Appalachian mountain region, United States

“…Nitrate concentration and export under baseflow conditions were generally greater in burned than unburned watersheds and increased with greater high burn severity extent. Nitrate concentration in unburned and less severely burned watersheds was consistent with reference watersheds previously studied in the southern Appalachians (Clinton & Vose, 2006; Elliott & Vose, 2005; Swank & Vose, 1997; Webster, Stewart, Knoepp, & Jackson, 2018). However, the magnitude of NO 3 ‐N responses in the more severely burned CA‐TO clearly exceeded that of previous prescribed burn studies in the region (Clinton et al, 2003; Elliott & Vose, 2005; Knoepp & Swank, 1993; Vose, Laseter, & McNulty, 2005), and even exceeded whole‐watershed clearcutting experiments (Swank et al, 2001; Webster, Knoepp, Swank, & Miniat, 2016).…”

“…In addition, NO 3 -N concentration in burned watersheds increased with increasing streamflow under baseflow and stormflow conditions whereas concentration generally remained the same or decreased with increasing streamflow in unburned watersheds, a finding supported by soil inorganic nitrogen concentration measurements over a range of burn severity ( Figure S2). Similar increasing flow dependence of NO 3 -N concentration with increasing disturbance severity has been observed in forested southern Appalachian watersheds (Webster et al, 2016;Webster et al, 2018). While NO 3 -N concentrations in severely burned watersheds are well below the EPA drinking water standard of 10.0 mg L −1 (U.S. Environmental Protection Agency, 2018), increased loading of NO 3 -N in addition to NH 4 -N and PO 4 as a result of wildfire to downstream receiving waters could contribute to eutrophication and associated water quality impairment of water supply reservoirs when a large proportion of the watershed is burned at high severity.…”

Nitrate in Streams During Winter Low‐Flow Conditions as an Indicator of Legacy Nitrate

“…The stream baseflow index (BFI) has been used to evaluate the relative importance of groundwater to streamflow and more recently has been used to understand the contribution of groundwater‐sourced nitrate to stream systems (Tesoriero et al, 2013; Webster et al, 2019). Streams with high BFI values receive more of their streamflow from groundwater discharge compared to streams with low BFI values, which are dominated by rainfall runoff.…”

“…Watershed models can be useful but are resource intensive at small scales, and larger, regional models often do not capture site‐specific details needed for management. Analyses of stream low flow (LF) or baseflow conditions currently are among the best methods to understand nitrate discharge from groundwater at the catchment scale (Miller et al, 2016; Tesoriero et al, 2013; Webster et al, 2019).…”

Can small stream solute–land cover relationships predict river solute concentrations?

“…Solute behaviours also depend on watershed‐specific factors such as topography, geology, soils, climate, and presence of lakes (e.g., Hynes, 1975; Shogren et al, 2019). Many studies have shown that anthropogenic changes to land use alter solute concentrations in streams (e.g., Bolstad & Swank, 1997; Hunsaker & Levine, 1995; Moerke & Lamberti, 2006; Osborne & Wiley, 1988; Stets et al, 2020; Webster et al, 2019). Some of the earliest studies looked at how clear‐cutting increased solute concentrations, particularly nitrate (Likens et al, 1970) and how various forest management practices modify stream solute response to tree removal (e.g., Swank, 1988).…”