Impact of Low-Residue and Full-Term Cover Crops On Nitrate Leaching From Intensive Vegetable Ground.

The Salinas Valley supplies over 90% of US lettuce production from April to October. High land rents and the need for keeping ground open during the winter to accommodate spring planting schedules have limited the use of winter cover crops. In addition, high residual soil nitrate levels in the soil at the end of the production season and the lack of deep-rooted winter rotation crops result in substantial nitrate losses from leaching and runoff during the winter storm events. This project evaluated the use of low-residue winter cover crops which have potential to allow the use of a cover crop while not disrupting spring planting operations; this is accomplished by killing the cover crop before it produces too much residue that would interfere with subsequent tillage operations. A trial was conducted during the winter 2010-11 at the Hartnell East Campus Research Station to determine the potential of low-residue cover crops to trap and sequester residual soil nitrate. Low-residue cover crop was compared with a full-term cover crop and an uncover-cropped control. Cereal rye ‘AGS 104’ was grown following the production of two lettuce crops the previous summer which left >30 ppm residual soil nitrate-nitrogen levels. Cover crop seed was drilled on November 19, 2010 and a germinating rain occurred the following day. Three treatments were established: 1) full-term cover crop (grown for 107 days); 2) low residue cover crop (grown for 62 days and then killed with glyphosate); and 3) uncover-cropped control. Suction lysimeters were installed at 60 cm deep in the soil to collect samples of the nitrate- nitrogen concentration of soil water at that depth; lysimeters were connected to a suction pump and the vacuum was maintained at ~20kPa. Changes in soil moisture storage were measured with Decagon Echo 10HS soil moisture probes at 30 and 60 cm deep in the soil. The low-residue and full-term cover crops took up 60 kg N ha-1 on January 28, 2011; however, levels of N in the low- residue cover crop declined following the herbicide application and contained 30 kg N ha-1 on March 9, 2011. In contrast, the full-term cover crop continued to uptake N and contained 140 kg N ha-1 on March 9. Soil samples to 90 cm in the soil indicated that the full-term and low-residue cover crops had similar low levels of nitrate at all depths than the uncover-cropped control treatment on February 9, but by March 9, the full term cover crop had lower levels of nitrate than both the low-residue cover crop and the bare treatment. This trial indicated that low-residue cover crops can take up moderate levels of residual soil nitrate-N, but after killing the cover crop with an herbicide, the N in their biomass is rapidly mineralized and can be lost by leaching during subsequent rain events.