Nitrate-nitrogen in effluent from agricultural tile drains in California

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Abstract
Soil profile characteristics significantly affect nitrate N movement through soil. The coarse-textured profiles have high redox potentials throughout, with apparently very low [bacterial] denitrification potential. Nitrate in these profiles moves with the water as a direct consequence of irrigation practices. Irrigation, which creates downward hydraulic gradients and considerable leaching, removes considerable amounts of nitrate from the soil. The concentration in the tile effluent is a consequence of the amount of fertilizer applied, fertilizer taken up by the plant and the leaching fraction. A low-leaching fraction may result in relatively high nitrate-N concentration in the tile effluent, with a relatively low total loss as compared to higher-leaching fractions. The profiles with layers of high clay content had zones where the redox potential was low, and apparently denitrification occurred. Layers of clay restrict water movement, thus reducing the leaching fraction and promoting anaerobic conditions favorable for denitrification. Significantly, the changes in chloride to nitrate-N ratio observed in the soil profile were related to the position of the clay layer.