Phosphorus and soil development: does the Walker and Syers model apply to semiarid ecosystems?

The Walker and Syers model of phosphorus (P) transformations during pedogenesis is widely accepted for the development of humid ecosystems, but long-term P dynamics of more arid ecosystems remain poorly understood. We tested the Walker and Syers model in semiarid piñon-juniper woodlands by measuring soil P fractions under tree canopies and in intercanopy spaces along a well-constrained, approximately 3000 ka (1 ka = 1000 years) volcanic substrate age gradient in northern Arizona, USA. The various pools of soil P behaved largely as predicted; total soil P and primary mineral P declined consistently with substrate age, labile inorganic P increased early in soil development and then declined at later stages, and organic phosphorus increased consistently across the chronosequence. Within each site, soils under tree canopies tended to have higher concentrations of labile and intermediately available P fractions compared to intercanopy soils. However, the degree of spatial heterogeneity conferred by tree islands was moderated by the stage of soil development. In contrast, tree islands had no influence on within-site distribution of more recalcitrant soil P pools, which appear to be controlled solely by the stage of pedogenesis. Coincident with declines in total P, primary mineral P, and labile inorganic P, we found that phosphatase enzyme activity increased with substrate age; a result consistent with greater ecosystem-level P demand on older, more highly weathered substrates. Our results suggest that, compared to humid climates, reduced inputs of water, energy, and acidity to semiarid ecosystems slow the rate of change in P fractions during pedogenesis, but the overall pattern remains consistent with the Walker and Syers model. Furthermore, our data imply that pedogenic change may be an important factor controlling the spatial distribution of labile P pools in semiarid ecosystems. Taken together, these data should both broaden and unify terrestrial ecosystem development theory.