Water, Nutrients, Roots, and Mutualists: Vertical Patterns in Semiarid Woodlands across the Substrate Age Gradient of Arizona (SAGA).

Understanding and predicting ecosystem responses to changing environmental conditions requires extensive knowledge of belowground patterns and processes. Fundamental to these goals in arid and semiarid ecosystems is knowledge of the availability of soil resources within the soil profile and the acquisition strategies of plants (i.e., vertical fine root and mycorrhizal distribution) to obtain these potentially limiting resources. We examined the vertical distribution of soil water and nutrients (carbon, nitrogen, and phosphorus), along with fine roots and associated ectomycorrhizae of piñon pine (Pinus edulis), across a three million year substrate age gradient in northern Arizona. Five soil profiles were excavated beneath trees to two meters depth at each of four sites (substrate ages ~1, 55, 750, and 3,000 ky) and soil horizons described. Each horizon was sampled for total and available nutrient pools, fine root biomass, ectomycorrhizal (EM) colonization and restriction fragment length polymorphism patterns, and soil physical and chemical properties. Water and nutrient pools varied considerably across the substrate age gradient, with significant storage of these resources at depth at all sites. Solum depth increased with substrate age, and surface (A) horizon organic C pools contributed proportionally less to the total solum pool with increases in soil development. Concomitantly, there was a significant decline in fine root density in the A horizons across the age gradient, while total fine root biomass within the 2-m profiles was not different. Ectomycorrhizal colonization of fine roots at depth was similar to values in the A horizons, but the EM community structures at depth were quite different than in the surface horizons. Our results suggest that deep roots are important for soil resource acquisition in these semi-arid ecosystems, and that surface soil patterns of water, nutrients, roots, and ectomycorrhizae are often not reflective of patterns in the subsoil.