How agricultural field management modulates downstream transport of antibiotics, bacteria and nutrients in runoff water using edge-of-field monitoring.

Agricultural land application of dairy manure supplies essential nutrients but may simultaneously introduce bacteria, residual antibiotics, and excess nutrients into water bodies through runoff, posing significant risks to water quality and public health. Biochar, a carbon-rich material produced by pyrolysis, has emerged as a promising soil amendment for mitigating contamination due to its large surface-area sorption capacity. Still, significant uncertainty persists due to the high variability of soil properties and the complex dynamics of soil under various management practices. A three-year (2022-2024) edge-of-field monitoring study was conducted across two contrasting soil types (clay loam and sandy loam) in Texas, USA, evaluating four management treatments: inorganic fertilizer with tillage (F-T), manure with no-till (M-NT), manure with tillage (M-T), and manure plus biochar amendment under no-till management (MB-NT). Runoff transport of Escherichia colibacteria, sulfamethoxazole (SMX), tetracycline (TC), and nutrients (ammonium, nitrate, and soluble reactive phosphorus) was quantified using generalized linear mixed models and regression analysis. Robust machine learning approach using a random forest model, coupled with SHAP (Shapley Additive exPlanations) interpretability analysis, was used to make predictions and identify dominant transport drivers. Rainfall intensity was the primary driver of contaminant export at both sites, while soil type substantially modulated responses to management practices. Clay soils exhibited strong intrinsic buffering with minimal treatment differentiation, whereas in sandy loam, MB-NT suppressed E. colitransport relative to M-T management treatment. RF models demonstrated strong predictive performance for soluble reactive phosphorus (R² = 0.82), TC (R² = 0.77), and nitrate (R² = 0.69), while SHAP analysis identified rainfall and soil moisture as dominant predictors for nutrients. Ancillary climatic variables (temperature, day length) were more critical for bacterial and antibiotic dynamics. Findings demonstrate that biochar amendment under no-till management offers an effective, soil-specific remediation strategy for mitigating agricultural nonpoint source pollution, particularly on coarse-textured soils. Further research across different management and environmental conditions is needed to optimize biochar management strategies for sustained mitigation of agricultural contaminants.