THE SHALLOW AND DEEP HYPOTHESIS: SUBSURFACE CHEMICAL CONTRASTS SHAPE NITRATE EXPORT PATTERNS FROM DIFFERENT LAND USES

Eutrophication has threatened water resources worldwide yet mechanistic understanding on controls of nutrient export remain elusive. This work tests the shallow and deep hypothesis: subsurface chemical contrasts regulate nitrate export patterns from different land uses. We synthesized data from 228 watersheds and used reactive transport modeling (500 simulations) under broad land use, climate, and geology conditions. Data synthesis showed 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 subsurface concentration contrasts, underscoring the often-overlooked role of N distribution over depth. Results challenge the commonly-held perception that legacy stores in croplands induce chemostasis with negligible variations. They suggest nitrate export will exacerbate as extreme events such as flooding intensify in the future.