Evolution of groundwater nitrate during managed aquifer recharge: Insights from DOM characteristics and stable...

River-induced managed aquifer recharge (MAR) is widely applied to alleviate groundwater depletion, yet how MAR reshapes the nitrate source-sink balance through alterations in dissolved organic matter (DOM) properties remains poorly understood. Here, we integrate hydrochemistry, ultrahigh-resolution DOM molecular characterization (FT-ICR-MS), stable carbon and nitrogen isotopes, microbial community analysis, and interpretable machine learning to systematically unravel DOM-driven nitrate transformation mechanisms under MAR conditions. MAR substantially enhances groundwater-surface water connectivity, leading to elevated groundwater levels, dissolved oxygen, dissolved organic carbon, and nitrate concentrations. Recharge-derived DOM is characterized by lower molecular weight, higher O/C ratios, and increased nitrogen enrichment, resulting in enhanced thermodynamic favorability and bioavailability. Isotopic signatures combined with microbial functional predictions indicate that MAR simultaneously stimulates nitrate production via organic nitrogen mineralization and nitrification, while promoting nitrate removal through denitrification, yielding a dynamically rebalanced nitrate system. Notably, MAR drives a mechanistic shift in nitrate evolution from a refractory DOM-constrained slow metabolic regime to an efficient regime fueled by highly bioactive substrates. Machine learning analysis further identifies the nominal oxidation state of carbon (NOSC), O/C ratio, and molecular mass as key molecular determinants governing DOM reactivity and nitrate responses. This study establishes an integrated DOM molecular-microbial-isotopic framework for elucidating nitrate transformation under MAR, thereby offering new insights into groundwater quality regulation and nitrogen pollution mitigation in managed recharge systems.