UC Davis

The negative effects of population growth and agricultural intensification on soil resources are further exacerbated by drought in arid regions like the California Central Valley. Deficit irrigation management techniques are used to navigate water shortages and work to maintain adequate production while limiting water inputs. This thesis examines the effect of four deficit irrigation management schedules via subsurface drip on the physicochemical properties of soil cropped with processing tomatoes, a late-season crop commonly grown under deficit irrigation. First, I investigated soil physical and chemical properties over the course of the season as the soils were exposed to increasing water stress. Second, I investigated the spatial arrangement of microbial populations and variability of soil physicochemical properties as the wetting zone of the soil decreases throughout the season. The experiment occurred in two consecutive years, 2021 and 2022, in Five Points, California. Processing tomatoes were irrigated with a control treatment, or either low, medium or high water stress treatments, each replicated three times. Soil physical and chemical properties such as pH, electrical conductivity, microbial biomass carbon, nitrate nitrogen, ammonium nitrogen, and potentially mineralizable nitrogen were measured in both experiments four times throughout the season. Spatially, I analyzed phospholipid fatty acid groups in the soil at 10 cm, 25 cm, and 45 cm away from the irrigation emitter. In both years, tomato yields did not significantly differ between the deficit irrigation treatments. Nitrate-N decreased as the season progressed, while potentially mineralizable nitrogen and ammonium-N increased with distance from the irrigation emitter. The total concentration of bacteria decreased with distance from the irrigation emitter in the medium stress deficit irrigation treatment, but not substantially more so than in the other irritation treatments, and our measured concentrations did not fall below ranges of total bacteria found in previous literature. I conclude that deficit irrigation does not significantly impact yield in processing tomatoes or reduce other soil properties such as pH, potentially mineralizable nitrogen, and microbial biomass carbon, sufficiently enough to affect soil health.