Root and mycorrhizal distribution through the soil profile across a 3 million year semi-arid substrate age gradient

Understanding and predicting ecosystem responses to 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 with respect to their dynamic distribution 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 fine roots and associated ectomycorrhizas (EM) of pinyon pine (Pinus edulis) across a three million year substrate age gradient in northern Arizona. Soil profiles were excavated beneath trees to two meters depth, soil horizons were described, and each horizon was sampled for fine root biomass, mycorrhizal colonization and RFLP patterns, and soil physical and chemical properties. The depth of the soil solum increased with substrate age. Concomitantly, the depth of fine root distribution increased with age and there were considerable fine root densities below 1 m depth at the two older sites that were not statistically different than surface horizons. There was also a significant decline in fine root density in the A horizon across the age gradient. Ectomycorrhizal colonization of fine roots at depth was also similar to values in the surface soil horizons. Further, there is preliminary evidence that the EM community structure and dominant RFLP patterns at depth are quite different than in the surface horizons. These fine root and EM data will then be used with stable isotope analyses to address the potentially different functional roles of these resource acquiring tissues within the soil profile.