Deep nutrients and soil fungal communities support tomato fruit yield and quality in dry farm management systems.

Abstract Changing climates are causing agricultural water shortages at unprecedented scales and magnitudes, especially in regions historically reliant on irrigation. Identifying and understanding systems of farming that allow continuity in agricultural operations in times of water scarcity are increasingly urgent needs. Vegetable dry farming relies on winter rains stored in soils to reduce irrigation to 0-2 events per season and has become prevalent on California’s Central Coast in recent decades. Until now, this system has been unexplored in scientific literature beyond extension publications, despite its promise as a model for low-water agriculture in arid regions. Dry farm management presents a unique challenge given that low water content restricts nutrient access in surface soils, which farmers typically target for fertility management. Managing soil nutrients at depth, as well as microorganisms that help plants access nutrients and alleviate water stress (e.g. arbuscular mycorrhizal fungi, or AMF) could be crucial to dry farm success. We engaged in a collaborative research design process with farmers managing seven commercial dry farm tomato fields to identify and answer three key management questions: 1. What are the depths at which nutrients influence harvest outcomes given low water content in surface soils?, 2. Are commercially available AMF inoculants effective at improving harvest outcomes?, and 3. How does the broader fungal community change in dry farm soils, and are those changes associated with harvest outcomes? Only soil nutrients below 60cm depth were correlated with tomato yield and fruit quality. We identified a fungal class, Sordariomycetes, as a “signature” fungal group in dry farm soils that distinguished them from irrigated management and correlated with positive fruit quality, while commercial AMF inoculation showed little benefit. These findings can inform management practices that optimize fruit yield and quality, and can guide farmers and policymakers alike in efforts to minimize agricultural water use.