Figure 1 . A conceptual figure and hypothesis about nitrate
export patterns, exemplified in Concentration-Discharge (C-Q)
relationship under different land use conditions (i.e., Ag =
Agriculture, Undev = Undeveloped, and Urban). Triangle wedges at the
left of each hillslope describe the depth profile of N abundance with
dark and light red represents high and low N abundance, respectively.
Here we define two “loose” end-members: the “shallow” water (from,
for example, surface runoff, shallow soils, or shallow pipes) and the
“deep” water (e.g., from deeper zones such as groundwater). Stream
water is often dominated by shallow water under high flow conditions and
by deep water under low flow conditions. We propose the shallow
versus deep hypothesis: nitrate concentration contrasts in shallow water
(Csw) and deep water (Cdw) drive the
export patterns. If this hypothesis is true, we expect higher
concentrations in shallow water in agricultural lands, leaning 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, possibly
leaning toward a dilution pattern. Nitrogen in forests and pristine
sites can come from the decomposition of organic matters in shallow
soils and leaching from N-containing rocks in deeper zones. They are
often tightly cycled with very low concentrations. These characteristics
can lead to diverse patterns