Nitrate sources and transformation pathways in salt-affected agricultural ditches: biogeochemical responses along contrasting salinity gradients

Agricultural nitrate pollution, largely associated with high-intensity fertilizer application and untreated or insufficiently treated waste inputs, has become a major driver of water quality degradation worldwide. Agricultural ditches, as key recipients of surface runoff and lateral seepage, function as both transport conduits and biogeochemical hotspots for nitrogen transformation. In salt-affected ditches, high nitrogen inputs and salinity can interact to enhance nitrate accumulation and mobility, thereby increasing the potential for downstream transport. To bridge this knowledge gap, we investigated two representative saline agricultural ditches located in northwestern China, a region characterized by an arid climate and saline–alkaline soils. We employed stable isotope analysis (δ¹⁵N-NO₃^– and δ¹⁸O-NO₃^–) combined with a Bayesian stable isotope mixing model (MixSIAR) to identify nitrate sources and transformation pathways. Results showed that manure and sewage (M&S) and chemical fertilizer (CF) were the dominant contributors, accounting for 42.6% and 42.1%, respectively, in the first ditch and 43.8% and 36.0% in the fifth ditch. Soil nitrogen (SN) contributed relatively less (15.3% and 20.2%, respectively). The results of both nitrate isotope analysis and chloride ion tracing revealed that the nitrate in the ditches originated primarily from agricultural activities and anthropogenic discharge. Interestingly, the nitrate transformation pathways differed notably between the two ditches, which is primarily driven by variations in nitrogen substrate availability and salinity levels. Hydrochemical and isotopic patterns suggest reduced nitrate removal potential under higher salinity, consistent with constraints on microbial nitrate reduction. These findings highlight the pivotal role of salinity in influencing nitrogen cycling and underscore the need for integrated management strategies that simultaneously address nutrient inputs and control salinity. Effective mitigation measures should prioritize seasonal fertilizer management and manure handling improvement, especially during winter irrigation periods when residual nutrients are mobilized. In addition, adaptive drainage strategies are needed to maintain nitrate removal efficiency under increasing salinity stress. This study provides isotope-based constraints to support science-based policymaking and adaptive water governance under salinization pressure.