Water Movement, Nitrate Leaching, and Crop Performance in Cover-Cropped Subsurface Drip-Irrigated Processing Tomato-Corn Rotations.

Cover crops are known to take up residual nitrate left over from cash crops and can thus reduce nitrate leaching. Less studied are the effects of cover crops on soil solution movement, which affects water and, in addition to soil mineralization, nutrient availability to crops during the irrigation season.  We measured water and nitrate fluxes by three independent methods in subsurface drip-irrigated (SDI) tomato (Lycopersicon esculentum Mill.) – corn (Zea mays) rotations for 2.5 years during growing and winter rainy seasons in winter-fallow, and Triticale (Triticosecale) and bell bean (Vicia faba L.) cover crop treatments at the UC Davis Sustainable Agriculture Facility Russell Ranch. The beginning of the irrigation and the winter rainy seasons were the periods with the highest drainage and nitrate leaching below the root zone. Of the two cover crops, Triticale was more effective in taking up nitrate early on during the rainy season, but 105-day soil incubation results showed no net nitrogen (N) mineralization from the addition of Triticale residue in this treatment.  Soil moisture measurements during the irrigation season indicated that throughout the root zone there was less lateral, and faster vertical movement of irrigation water in the cover cropped than winter-fallow soils, and this may have affected crop development and yields, which were lower in cover cropped than winter-fallow soils in two out of three years. To reduce nitrate leaching early in the growing season, irrigation and N fertigation rates must be optimized and adjusted based on nitrate levels in the soil profile. The use of a nitrate-scavenging cover crop, such as Triticale, is recommended but the optimal termination date and method of residue incorporation needs further refinement.