Long-Term Cover Crop Effects on Soil Hydraulic Properties, Nitrate Leaching, and Crop Water Use.

Winter cover crops are known to increase infiltration and take up residual nitrate left over from cash crops. Less studied are the effects of cover crops on water movement within the soil profile with varied irrigation systems. Soil hydraulic properties, water fluxes in the soil profile, and crop water use were compared in soils at the UC Davis Sustainable Agriculture facility Russell Ranch, where cover crop (CC) and winter-fallow (WF) treatments have been in place for 19 years. Neutron probes, as well as tensiometers and soil moisture sensors were used to monitor water movement in the soil profile in furrow- (FI) and subsurface drip-irrigated (SDI) tomato (Lycopersicon esculentum Mill.) rotations during the growing and winter rainy seasons. Immobilization of nitrate by either Triticale (Triticosecale) or bell bean (Vicia faba L.) cover crops and was assessed by a ¹⁵N tracer method. Under FI, the average drainage below the root zone according to water balance calculations during three growing seasons was 329 mm or 30% of the applied water in the CC and 129 mm or 14% of the irrigation water in the WF treatments. Under SDI, drainage averaged 32 mm or 5% of the applied water. The beginning of the irrigation and the winter rainy seasons were the periods with the highest likelihood of drainage below the root zone in SDI soils. Under SDI, soil water potential measurements indicated water uptake by plant roots from a greater volume and depth in CC than in WF treatments. Soil water potential measurements indicated better infiltration in CC than WF soil under FI, but under SDI, there was less lateral movement of irrigation water from the drip tape towards the sides of the beds in CC than in WF soil. Of the two cover crops, Triticale had a deeper root system and took up 15 times more ¹⁵N-nitrate than bell beans. The results imply that in these soils long-term cover cropping increased hydraulic conductivity, which calls for optimizing nitrogen (N) management to synchronize N uptake and supply via fertigation and use of a nitrate-scavenging CC, such as Triticale, to minimize nitrate leaching losses.