INTRODUCTION
Eutrophication in rivers, lakes, and coastal areas worldwide has persisted for decades due to intensive agriculture and urbanization (Diaz & Rosenberg, 2008; Galloway et al., 2004; Le Moal et al., 2019; Royer et al., 2006; Van Cappellen & Maavara, 2016; Van Meter et al., 2017). In the U.S. alone, over 60% of U.S. estuaries and coastal water bodies have been degraded by excessive nutrient inputs (Howarth et al., 2002). The concentration levels and export patterns of nitrate, a major dissolved nitrogen (N) species, are essential to understand and quantify N export from terrestrial to aquatic ecosystems. Export patterns are often quantified using concentration - discharge (C-Q) relationships with a power law of C = a Qb . The C-Q relationships determine loads of solute export (load = C x Q) and reflect the response of earth systems to hydrological changes. The value of b indicates different export patterns: a positive, high value means a flushing pattern where concentrations increase with streamflow, whereas a negative b indicates decreasing concentrations with streamflow (Godsey et al., 2009; Moatar et al., 2017; Musolff et al., 2015). Chemostatic patterns (with absolute b values close to zero) occur when concentrations vary little compared to hydrological variations. These export patterns have important implications on how and how much nitrate exports across different hydrological regimes. High bvalues with pronounced flushing patterns indicate highly sensitive, escalated export during extreme hydrological events such as flooding.
Agriculture and urbanization have significantly modified subsurface physical and chemical structures, including the spatial distribution of N (Figure 1). Agricultural lands are often characterized by shallow flow via tile drainage and heavy fertilization that enriches N in shallow soils (Van Meter et al., 2016; Woo & Kumar, 2019). Urban watersheds represent a different type of human perturbation, often characterized by impervious surfaces that facilitate surface runoff during storms and sewer pipes that enhance rapid shallow subsurface flow (Grimm et al., 2008). Nitrogen in urban watersheds can come from a variety of surface and underground sources including atmospheric deposition, lawn fertilizers, and domestic and industrial wastewater (Baker et al., 2001; Divers et al., 2014). Buried leaky sewer systems often elevate the N input to urban groundwater (Groffman et al., 2003; Pickett et al., 2011). These manipulations do not occur in undeveloped, pristine watersheds, where N content is often low and tightly cycled (Weitzman & Kaye, 2018). These differences, in particular their vertical distribution of N content, can lead to distinct C-Q patterns across different land use conditions.
Given the tremendous human influence, an open question that is key to understand nutrient export is:How do subsurface physical and chemical structures from different land uses in governing nitrate export patterns? Contrasting observations in literature do not converge to a unifying framework that answers these questions. In agricultural lands, both dilution and flushing have been observed (Jiang et al., 2010; Miller et al., 2017). The common perception is that chemostasis or biogeochemical stationarity prevails in agriculture lands, primarily due to the large legacy store of nitrogen (N) that induces transport limitation and buffers concentration variability (Basu et al., 2010; Basu et al., 2011; Thompson et al., 2011). In undeveloped, forest watersheds, nitrate concentrations have been shown to often peak during spring floods (Creed & Band, 1998; Creed et al., 1996; Pellerin et al., 2012; Sickman et al., 2003), whereas chemostasis has not been as commonly observed (Huang et al., 2012; Jacobs et al., 2018). Although much less examined, existing urban studies have observed both dilution (Duncan et al., 2017) and flushing patterns (Poor & McDonnell, 2007; Wollheim et al., 2005).
Growing literature has recognized the importance of subsurface structure, particularly the hydrological and biogeochemical contrasts in shallow and deeper zones in governing nitrate levels, denitrification potentials (Bishop et al., 2004; Creed et al., 1996; Kolbe et al., 2019), and solute export in general (Musolff et al., 2017; Zhi et al., 2019). Seibert et al. (2009) developed a flow-concentration integration model that quantitatively links variations of stream chemistry to flow rates and vertical profile of shallow soil water chemistry. Zhi et al. (2019) showed that vertical chemical contrasts between shallow soil water and deeper groundwater can explain diverse export patterns in multiple watersheds under a gradient of climate, geology, and land cover conditions. Here we propose the shallow versus deep hypothesis: the N concentration contrasts in shallow and deep zones determine nitrate export patterns under different land use conditions . In particular, we hypothesize that nitrate concentration contrasts in shallow water (Csw) and deep water (Cdw) differ in different land uses, and that the magnitude of concentration contrasts governs export patterns. Here “shallow water” and “deep water” are broadly defined as waters from different depths contributing to streams (Figure 1). The shallow water can come from the shallow surface / subsurface zone that dominates streamflow at high flow; the deep water may derive from deeper subsurface zones and predominate at low flow. These shallow and deep waters are loosely defined. In pristine sites, the shallow water may be the shallow soil water; in agriculture sites, it can be the combination of surface runoff, tile drainage, and soil water; in urban lands, it may be the runoff from impervious surfaces and shallow subsurface pipes. The deep water is the groundwater that interacts with streams and carries chemical signatures that are distinctive from shallow water.
If this hypothesis is true, we expect higher concentrations in shallow water in agricultural lands, leaning more toward a flushing pattern. In contrast, in urban watersheds, concentrated nutrients accumulated in leaky pipes in deeper subsurface are often higher than shallow, rapid runoff on impervious surfaces, leading more toward a dilution pattern (Duncan et al., 2017). Nitrogen in forests and pristine sites can come from decomposition from organic matter in shallow soils and leaching from N-containing rocks (Mayer et al., 2002; Montross et al., 2013). In addition, they are often tightly cycled. These characteristics can lead to varied patterns. We test this hypothesis combining a national-scale synthesis for 228 sites in the United States data synthesis and mechanism-based reactive transport modeling. Our goal is to propose a general, simple conceptual model that can explain and quantify export patterns under diverse conditions.