Linking water and nutrients through the vadose zone: a fungal interface between the soil and plant systems

Abstract

Plant water availability, use, and management have largely focused on physical processes of infiltration and the role of roots in uptake and transpiration. However, roots and mycorrhizal fungi redistribute water in complex patterns. Here I describe some of our observations and experiments showing that mycorrhizal fungi play key roles in moving water for both transpiration and to facilitate nutrient acquisition under dry conditions. Mycorrhizal fungal hyphae grow from both surface and deep roots even into bedrock to help extract water under dry conditions. In both deep and surface roots, mycorrhizal fungi acquire water from pores too small for roots and root hairs to access, and at distances from roots and root hairs. Mycorrhizal fungi are also able to utilize hydraulic-lifted water from plants to obtain nutrients in extremely dry surface soils. The importance of these root symbionts in water and nutrient dy- namics, and as integrators of surface and deeper water dynamics need further investigation. Predicting water use by irrigated crops in mesic lands has become relatively accurate, facilitating manipula- tion of water application and for predicting crop growth and production. However, these models are largely based on the relatively even distribution of soil structural elements through the root zone, coupled with an understanding in the vertical spatial distribu- tion of water inputs. Even then, however, water movement within the soil profile is chaotic, resulting in complex patterns of water movement under unsatu- rated conditions. Much of the research on water movement and use has been undertaken as a physical process, but here I discuss the role of mycorrhizal fungi in redistributing soil moisture for plant use and for plant re-allocation to mycorrhizal fungi to acquire surface nutrients. Water and nutrients are tightly coupled in regulating ecosystem processes including productivity and de- composition. However, in arid land soils, water and nutrients are spatially segregated. Most of the nutrients are applied (as fertilizers) or continually cycled in the upper soil layers and the highest nutrient availability is generally in a thin organic layer between the litter and the mineral soils, and in the upper mineral soil. In