Modeling Nitrogen Cycling in Hyporheic Zones: A Comparison of First-Order and Monod-Type Kinetics

Dynamic interactions between surface water and groundwater induce pronounced temporal and spatial variability in redox conditions and substance concentrations within hyporheic zones, giving rise to highly complex nitrogen transformation dynamics. However, under environmentally heterogeneous and data-limited conditions, the level of kinetic complexity required to adequately represent nitrogen processes remains poorly constrained. In this study, we use soil microcosm experiments representative of hyporheic environments to systematically evaluate the applicability and modeling performance of first-order and Monod-type kinetics for simulating nitrogen transformations. Time-series measurements of ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), nitrite nitrogen (NO₂^--N) and dissolved organic carbon (DOC) were used to constrain nitrogen transformation rates, while functional gene abundances quantified by quantitative PCR served as indicators of microbial functional potential. Two kinetic frameworks, consisting of parsimonious first-order kinetics and Monod-type kinetics that explicitly incorporate substrate limitation, were independently calibrated to the experimental observations.
Our results indicate that both kinetic frameworks reproduced the overall temporal evolution of nitrogen species, including the general trends of ammonium oxidation and nitrate reduction. However, only the Monod-type kinetics captured substrate-dependent process controls and reactions associated with anoxic microenvironments, even when overall concentration variability was limited. While the first-order kinetics provide an efficient representation of net nitrogen turnover, the Monod-type kinetics offer a more mechanistic description of pathway sensitivity and environmental regulation that is essential for interpreting nitrogen transformation processes in hyporheic zones. The derived kinetic parameters therefore provide scenario-dependent priors for reactive biogeochemical modeling and highlight the importance of explicitly representing substrate limitation and redox regulation using Monod-type kinetics when coupling biogeochemical dynamics with hydrologic variability.