UC Davis

Restoring soil health by using cover crops can improve agricultural productivity and sustainability. This practice can improve soil structure, organic matter, and fertility. Cover crops, typically grown outside the main growing season, have the potential to reduce soil erosion, promote water infiltration, limit pest and disease outbreaks, and improve nutrient cycling. However, cover crops may deplete soil moisture, which can increase the water demand of the following growing season in semi-arid climates such as California’s Central Valley. For decades, agriculture in California’s Central Valley has pumped groundwater to meet irrigation demand. This has led to widespread depletion of groundwater aquifers, land subsidence, water quality degradation, and domestic wells running dry. In recent years, efforts have attempted to intentionally replenish aquifers with excess surface water when available. One such method is agricultural managed aquifer recharge (Ag-MAR), a practice that involves intentionally flooding an agricultural field during the winter or early spring to recharge groundwater. However, agricultural fields often contain high nitrate levels from fertilizer and manure applications. The challenge, therefore, is to increase recharge without contaminating groundwater. Non-leguminous cover crops can offer a potential solution to the nitrate leaching risk associated with Ag-MAR events by taking up excess nitrogen from soil and sequestering it in biomass. In this study, we intentionally flooded a vineyard planted with a cover crop for two weeks in March 2022 to investigate its effect on cover crop survival, and nitrate leaching and nitrogen dynamics in root zone. A split-plot experiment with main plots arranged as a randomized complete block design was set up in a 0.8-hectare Thompson seedless grape vineyard at the Kearney Research and Extension Center in California. Three cover crop treatments were tested on flooded and control (winter precipitation only) subplots. Triticale, a grass cover crop, was planted in vineyard alley ways in November 2021. In early March 2022, half of the cover crop plots were mowed, and residue was disked into the soil before flooding for two weeks. The other half of cover crop plots were left standing during flooding and terminated after flooding using an herbicide. Before, during, and after flooding, we tracked triticale and grapevine biomass. We measured carbon and nitrogen from porewater (depths: 20, 60, and 100 cm) using lysimeters. We also measured carbon and nitrogen content from soil samples (depths: 0-10, 10-20, 50-60, and 90-100 cm). Results demonstrate the complexity of crop tolerance to flooding and cover crop nitrogen dynamics. Grape yields decreased due to flooding, but triticale biomass did not. Triticale did not reduce the initial amount of nitrate in soil despite taking up 85-92 kg N/ha before flooding began. This indicates that residual soil nitrate was too high and N uptake by the cover crop too low to cause a significant reduction. Porewater NO3--N concentrations dropped to 0 mg/L within the first five days of continuous flooding, indicating that most residual nitrate leached. Cover crops did not reduce the total amount of nitrate leached during flooding. More study is needed to determine best practices for minimizing nitrate leaching and crop injury during Ag-MAR