Regulated deficit irrigation exhibits limited effects on soil health and microbial communities in processing tomatoes

Abstract

Regulated deficit irrigation (RDI) has been increasingly adopted in California processing tomato ( Solanum lycopersicum L.) systems as a strategy to reduce agricultural water use in water‐limited regions. While late‐season RDI can achieve substantial water savings with minimal yield penalties, its implications for soil health and microbial activity remain poorly understood, particularly in subsurface drip‐irrigated systems where soil wetting is spatially constrained. We evaluated the effects of RDI on soil physicochemical and biological properties across the growing season and within tomato beds at two commercial processing tomato sites in California's Central Valley. Treatments included a grower standard control and three RDI regimes of increasingly reduced fractions of crop evapotranspiration (approximately 50%, 43.5%, and 37%). Soil health indicators included electrical conductivity (EC), pH, nitrate, ammonium, potentially mineralizable nitrogen (PMN), and microbial biomass carbon (MBC). Microbial communities were characterized using phospholipid fatty acid (PLFA) analysis. No effects were detected for most soil health indicators, including EC, pH, inorganic nitrogen pools, and MBC, although statistical power was limited for some comparisons. PMN exhibited limited change under Medium RDI but did not show sustained declines under all RDI treatments. PLFA profiles indicated overall microbial community stability, with no consistent shifts toward drought‐tolerant taxa, even under higher RDI levels. Instead, site‐specific characteristics, spatial heterogeneity within beds, and seasonal dynamics exerted stronger control over soil health than the RDI regime. These results suggest that late‐season RDI in subsurface drip‐irrigated processing tomato systems can reduce water inputs without substantially degrading soil health or microbial community composition in the short term.