Linkages Between Dissolved Organic Matter Quality and Denitrification Potential in Surface Water and Groundwater Mixing Zones

Abstract

Nitrate contamination in surface water (SW) and groundwater (GW) represents a pervasive environmental challenge worldwide. The river-riparian-aquifer transition zone exerts a pivotal role in mediating nitrate export, primarily through promoting denitrification fueled by organic matter. In lowland riverine reaches, how compositional characteristics of organic matter regulate denitrification rates remains poorly elucidated, particularly in lowland reaches where processes are complicated by the combined effects of SW-GW mixing, and heterogeneous environmental conditions. This study conducted field sampling of water and sediment across the riparian continuum (SW, SW-GW mixing zone, and GW) along a representative lowland reach of the Han River. Denitrification potentials were quantified using the acetylene block technique, while the compositions and optical properties of organic matter were characterized via spectroscopic analyses. Partial least squares regression (PLSR; R² = 0.95) revealed that DOM quality dominates denitrification potential, with denitrifiers preferentially utilizing labile, microbially derived DOC components (C2), followed by terrestrial humic-like components (C4) derived from riverbank sediments. During low-flow periods, when labile protein-like DOM was less available, SW-GW mixing (river fraction of 10%–90%) was associated with enhanced utilization of sediment-derived humic-like C4 (p < 0.05). This pattern is consistent with a priming-like substrate-switching mechanism, whereby small inputs of labile DOM and dynamic hydrological mixing stimulate the microbial use of semi-labile sediment-derived DOM. Such a mechanism helps sustain high denitrification potential in the mixing zone, highlighting the need to consider not only DOM quantity and quality, but also hydrologically mediated sediment OM processing in shallow riparian aquifers.