The Shallow and Deep Hypothesis: Subsurface Vertical Chemical Contrasts Shape Nitrate Export Patterns from Different Land Uses

Abstract

Eutrophication has threatened water resources worldwide, yet mechanistic understanding on controls of nutrient export remains elusive. This work tests the shallow and deep hypothesis: subsurface vertical chemical contrasts regulate nitrate export patterns under different land use conditions. We synthesized data from 228 watersheds and used reactive transport modeling (500 simulations) under broad land use, climate, and geology conditions. Data synthesis indicated that human perturbation has amplified chemical contrasts in shallow water (e.g., soil water) versus deep waters (e.g., groundwater), inducing primarily flushing patterns (concentrations increase with streamflow) in agriculture lands and dilution patterns (concentrations decrease with streamflow) in urban watersheds. Results revealed a quantitative relationship between export patterns and shallow-versus-deep concentration contrasts, underscoring the often-overlooked role of nutrient distribution over depth. Results challenge the commonly held perception that legacy stores in agricultural lands induce chemostasis where concentrations vary negligibly with streamflow. They suggest that nitrate concentrations from agricultural lands will escalate during large hydrological events, which can exacerbate nutrient export problems as flooding events intensify in the future climate.

Effects of Heterogeneous Stream‐Groundwater Exchange on the Source Composition of Stream Discharge and Solute Load

“…If solute concentration in groundwater is homogeneous there is no difference between water and solute source composition. Homogeneous groundwater concentrations are typically assumed or are implicitly derived if instream concentrations under base‐flow conditions are used as a surrogate for groundwater concentrations (e.g., Pinder & Jones., 1969; Zhi & Li., 2020).…”

Significant stream chemistry response to temperature variations in a high-elevation mountain watershed

“…3, including P, PO 4 , NO 3 ) are among those that have seen large variations. These solutes generally exhibited flushing C-Q patterns, indicating a more abundant presence in shallow soils [35][36][37][38] . Fe-containing minerals, including various forms of iron oxides, commonly exist in soils and play an essential role in biogeochemical cycles 39 .…”

Toward catchment hydro‐biogeochemical theories

“…11 in Zhi et al, 2019). The same relationship (with different parameters) can also explain nitrate export patterns in more than 200 watersheds under different land use conditions across USA (Zhi & Li, 2020). In a global water chemistry data analysis comparing the relative controls of C–Q patterns from above (climate) and below (depth of solute generation), the depth of solute generation has been found to be predominant (Botter et al, 2020).…”

“…This unique depth‐dependent characteristic often determines the compositions of end‐member source waters and can translate into C–Q relationships that reflect these characteristics. The power‐law exponent b in C–Q relationships has been used to infer the chemical contrasts in shallow and deeper zones (Ameli et al, 2017; Wen et al, 2020; Zhi et al, 2019; Zhi & Li, 2020). These examples echo early studies of C–Q relationships using end‐member mixing analysis (Johnson, Likens, Bormann, Fisher, & Pierce, 1969; Langbein & Dawdy, 1964).…”