Soil Phosphorus Stock and Speciation with Regolith Development: Does the Walker and Syers Model Apply to a Climatic Weathering Gradient?

Abstract:

Global environmental change models state that temperate ecosystems will serve as a carbon (C) sink, however any realized increase in C storage will also require increases in the availabilities of key nutrients. Phosphorus (P) has been described as the ultimately limiting nutrient in terrestrial ecosystems due to its unique biogeochemical cycle. The conceptual model of Walker and Syers (W&S) is widely accepted along soil chronosequences. It describes the evolution of P with ecosystem development where primary mineral (i.e., Ca-bound) P is increasingly incorporated into bioavailable, organic, and occluded pools. During late stages of pedogenesis, total P declines and the remaining P is tightly cycled between the vegetation and soil principally through the organic, and to a lesser degree the occluded pools. Our understanding of P transformations in soil, especially with ecosystem development and global climate change, is complicated due to the complexity of P species and their range of bioavailability. We used a space for time substitution approach along Mediterranean and arid elevational gradients to study how soil P biogeochemistry is altered by changes in climate. Although weathering intensity increases with changes in climate from the arid to Mediterranean sites, the total P stock also increased and therefore did not follow the W&S model. Alternatively, chemical speciation as identified by ³¹P nuclear magnetic resonance spectroscopy (NMR) found extractable organic P monoesters and diesters decreased (R²=0.53, P<0.05) and inorganic P increased (R²=0.42, P<0.05) with weathering. X-ray absorption near edge structure (XANES) spectroscopy of bulk soil identified inorganic P transitions from proportionally more Ca-P (R²=0.88, P<0.05) to more Al- (R²=0.50, P<0.05) and Fe-P (R²=0.37, P<0.05) with increases in weathering supporting the W&S model. Ultimately, more weathered locations may be relying on faster cycling organic species rather than primary mineral, precipitated secondary minerals, and sorbed species to support ecosystem development.