Hydraulic lift may buffer rhizosphere hyphae against the negative effects of severe soil drying in a California Oak savanna

Many studies have shown that the total abundance of hyphae in the soil covaries seasonally with soil moisture. We investigated the extent to which soil hyphal abundance varies as a function of depth and moisture availability within the soil profile during the dry season, and determined whether soil moisture compensation via hydraulic lift (HL) buffers rhizosphere fungi from the effects of severe soil drying. We measured soil water potential, isotopic composition of soil water and total hyphal length in a California coast live oak stand and adjacent grassland at the beginning and end of the 5-month summer drought period. Throughout the summer, oaks maintained predawn water potential values (−0.4±0.1 MPa) that were significantly above those recorded in the 0–200 cm soil depth interval, strongly suggesting root access to groundwater. Direct evaporation of soil water was much more intense and affected deeper layers of the profile in the grassland compared to the oak stand, as indicated by extremely negative water potential values and very enriched isotopic composition of soil water near the surface. Significantly higher soil water potential and less isotopically enriched soil water at 15–40 cm depth in the oak stand were consistent with oak root exudation of isotopically depleted groundwater or deep soil water not exposed to evaporation. Hyphal length in the soil profile declined markedly during the summer drought period in the grassland, particularly in upper layers (41–75% decrease at 0–40 cm depth), indicating rapid turnover of the arbuscular mycorrhizae (AMF) dominated hyphal carbon pool after grass senescence. By contrast, soil hyphal length in the ectomycorrhizal (EM)/AM oak stand remained remarkably constant throughout the summer drought period, with the only exception of the topsoil layer exposed to direct evaporation (49% decrease at 5 cm depth). The sustained exudation of water from roots to soil through HL may have buffered rhizosphere hyphae against the negative effects of extreme soil desiccation in the oak stand. These data suggest that HL by deep-rooted trees may influence the biogeochemical cycling of carbon and nutrients in seasonally dry ecosystems through effects on rhizosphere fungi.