The role of hydraulic lift on mycorrhizal-mediated dynamics

Background/Question/Methods Drought plays a major role in structuring communities. Microbes are also impacted. Yet fungal hyphae, especially those forming mycorrhizal associations remain active and respiring in the near surface environment despite extremely dry soils. Previously, we demonstrated that vertical spatial stratification in soil moisture could create hydraulic redistribution of water, especially for those individual plants whose deep roots reach groundwater. Our current goal is to understand the breadth of redistribution and the impact on cycling of C and N. To characterize activity, we continuously measure soil moisture, temperature, and respiration in a highly variable California mixed-conifer forest using a sensor network. We observe roots, mycorrhizal fungi, and water drops daily using an automated minirhizotron. We intermittently measure water (dD, d¹⁸O), carbon (d¹³C) and nitrogen (d¹⁵N) to assess sources and activity.

Results/Conclusions During wet winters, soil pores fill throughout the profile. As soils dry down, individual hyphae cross air gaps in the soil and access individual water droplets. Hyphae continue to explore the soil seeking ever-smaller pores as soil water declines. As the soil surface continues to dry down, isotope ratios show that plants tap winter water deeper in the granite matrix. Both field observations and greenhouse isotope chamber studies show that water redistribution occurs from plants able to tap the deep water through mycorrhizal hyphae, and into seedlings through the common mycorrhizal network. N from dry zones may also be acquired and allocated among plants and fungi via the hydrated hyphae. Ectomycorrhizal mycorrhizal fungi remain present in extremely dry (<-5MPa) soils as long as the plants are able to reach deep water. Hyphae shrink in diameter but remain present, often for several months. Arbuscular mycorrhizal fungi persist in drier conditions, in pastures and slopes with little groundwater. As fall/winter precipitation re-fills soil pores, hyphae rehydrate and soil respiration increases. Both ecto- and arbuscular mycorrhizal fungi are well adapted to persist in these environments with annual drought cycles and highly variable precipitation years. Implications of these strategies to a warmer environment will be explored.