Alfalfa reduces winter nitrate leaching relative to organic and conventional annual vegetable systems: Resin bag field measurements and modeling with HYDRUS-1D

Leaching of nitrate (NO₃^-) to groundwater is a major concern in California, where groundwater NO₃^- levels often exceed public safe drinking water thresholds. The state has enacted legislation to implement monitoring programs and management plans that will minimize future NO₃^- loadings to groundwater based on modeled NO₃^- leaching; however, a need remains for empirical NO₃^- leaching data and assessment of model suitability to specific systems. Moreover, debate remains around the ability of different management practices like cover cropping, replacement of chemicals with organic inputs, and cultivating perennials to reduce NO₃^- leaching in California's annual vegetable systems. We measured winter NO₃^- leaching over the wintery rainy season (October to March) within systems of the Century Experiment, a long-term cropping systems experiment in northern California evaluating effects of cover cropping, certified organic management, and alfalfa (Medicago sativa L.) incorporation in tomato (Lycopersicon esculentum Mill.)-maize (Zea mays L.) rotations. Anion exchange resin bags were installed at the bottom of the crop rooting zone (~65 cm) following tomato and prior to the onset of fall rains to adsorb leaching NO₃^- over the winter. Empirical resin bag NO₃^- leaching values were compared to modeled leaching results using HYDRUS-1D, which estimates water movement and reactive solute transport in soils. The rotation with alfalfa was the only system that reduced winter NO₃^- leaching (21.8 kg ha^-1), compared to conventional management (bare winter fallow after tomato) (47.1 kg ha^-1). Compared to conventional, certified organic management (44.7 kg ha^-1) and inclusion of a winter cover crop (58.2 kg ha^-1) had no significant impact on NO₃^- leaching. HYDRUS-1D model estimates for NO₃^- leaching were in good agreement with empirical field measurements in conventional and cover cropped systems, but less for certified organic and greater for the alfalfa systems. Results from this study show that perennial crops have potential to mitigate NO₃^- leaching losses across an agricultural landscape, and models like HYDRUS can provide useful estimates of NO₃^- leaching in some agricultural systems.