The expanding role of deep roots during long‐term terrestrial ecosystem development

Models of ecosystem development and response to environmental variation must incorporate change in vertical soil space as well as over time. Insufficient measurement of subsurface soil properties represents a major observational bias in ecosystem studies. We address these changes in horizontal (time) and vertical (soil profile) space along a three‐million‐year, semi‐arid, piñon‐juniper woodland substrate age gradient with characteristic progressive and retrogressive ecosystem development phases and a shift from nitrogen (N) and water to phosphorus (P) limitation. We present a novel pedological approach using isotopic tracers and biogeochemical analyses to address fine root distribution, depth of plant uptake and relative nutrient availabilities. We show that (a) the quantity of fine roots remains constant with ecosystem development but their distribution in the soil profile becomes increasingly deeper and less concentrated in the surface soil; (b) mean depth of tree uptake becomes deeper with substrate age and follows the relative availability of P as P‐limitation develops and (c) soil P transformations in the oldest soil profiles resemble the theoretical changes with age to produce a depth gradient of relative N and P availability. Synthesis . The expanding role of deep roots in this model system is tightly linked to phases of ecosystem development and relative nutrient availability. The inclusion of whole soil profiles is vital to investigating the intersections of biota, soil and geologic substrate and developing a more complete understanding of ecosystem structure and function.