Linking water age, nitrate export regime, and nitrate isotope
biogeochemistry in a tile-drained agricultural field
Abstract
Recent theoretical advances related to time-variant water age in
hydrologic systems have opened the door to a new method that probes
water mixing and selection behaviors using StorAge Selection (SAS)
functions. In this study, SAS functions were applied to investigate
storage, water mixing behaviors, and nitrate (NO3-) export regimes in a
tile-drained corn-soybean rotation field in the Midwestern United
States. The natural abundance stable nitrogen and oxygen isotopes of
tile drainage NO3- were also measured to provide constraints on
biogeochemical NO3- transformations. The SAS functions calibrated using
chloride measurements at tile drain outlets revealed a strong young
water preference during tile discharge generation. The use of a
time-variant SAS function for tile discharge generated unique water age
dynamics that reveals an inverse storage effect driven by activation of
preferential flow paths and mechanically explains the observed
variations in NO3- isotopes. Combining the water age estimates with NO3-
isotope fingerprinting delineated NO3- export dynamics at the tile-drain
scale, where a lack of strong contrast in NO3- concentration across the
soil profile results in chemostatic NO3- export regimes. For the first
time, NO3- isotopes were embedded into a water age-based transport model
to model reactive NO3- transport under transient conditions. Results
from this modeling study provided a proof-of-concept for the potential
of coupled water age modeling and NO3- isotope analysis in elucidating
complex mechanisms that control the coupled water and NO3- transport.
Further integration of water age theory and NO3- isotope biogeochemistry
is expected to significantly improve reactive NO3- transport modeling.