Evaluating Cover Crop and Irrigation Management in Terms of Water Use, Energy Costs and Greenhouse Gas Emissions in Tomato Rotations.

In view of the predicted reduced availability of water to irrigate crops due to climate change and efforts to curb greenhouse gas (GHG) emissions, agricultural management practices need to be evaluated in terms of water use and global warming potential. We assessed the use of cover crops and subsurface drip irrigation (SDI) in tomato (Lycopersicon esculentum) rotations at the UC Davis Sustainable Agriculture Facility Russell Ranch and on-farm sites in Yolo County, CA, with regard to soil rainwater storage, groundwater recharge, irrigation water consumption, and GHG emissions by measuring runoff in cover cropped and winter-fallow fields and calculating groundwater recharge during the rainy and tomato growing seasons, appraising crop performance, and measuring nitrous oxide (N₂O) emissions, as well as energy consumption, for two years in furrow- and drip-irrigated tomato rotations that were winter-fallow or cover cropped with either Triticale (T. hexaploide) or a bell beans (Vicia faba)/vetch (Vicia dasycarpa)/oats (Avena sativa) mixture. Cover cropped fields stored up to 40 mm more rainwater than winter fallow soils. Applied water ranged from 384 to 537 mm under SDI, and from 301 to 1018 mm under furrow irrigation (FI). The highest yields per unit water applied were recorded in the cover cropped SDI treatments with 211, followed by winter-fallow SDI with 154, and the three FI treatments with 148 to 163 kg ha^-1 mm^-1 water applied. In both years, N₂O emissions were significantly lower under SDI than FI. Direct emissions of N₂O and GHG emissions associated with fertilizer manufacture accounted for 54 to 69, irrigation for 8 to 18, and diesel fuel to power farm equipment for 22 to 32 % of total GHG among the different management practices. Overall, SDI produced lower total GHG emissions than FI, while cover cropping combined with FI increased them compared to FI winter-fallow in one of the two years.