Interactive bioclimatic and lithological impacts on soil phosphorus revealed by X-ray absorption structure spectroscopy

Phosphorus (P) is a key constraint to terrestrial ecosystem primary productivity, the biogeochemical cycling of which is significantly influenced by soil-forming factors. While previous studies have largely focused on the impacts of a single soil forming factor, generally time using chronosequences (i.e., Walker-Syers model), this offers only a partial understanding of soil P since multiple soil-forming factors operate simultaneously. We evaluated the interactive influences of bioclimate and lithology on soil P pools in A horizons across a full factorial of four bioclimatic zones and three bedrock lithologies (granite, basalt, andesite) using P K-edge X-ray absorption structure (XANES) spectroscopy. Soil P speciation across diverse lithologies and bioclimates was dominated (65–100%) by Fe-bound P (Fe-P) and/or Al-bound P (Al-P), with remaining contributions varying between organic P (P o ) and Ca-bound P (Ca-P). Though the trend of soil P speciation was overall similar across lithologies, we find evidence for lithology-specific modulation of relative P speciation by bioclimate. Unexpectedly, soils developed on felsic lithology did not necessarily have a lower proportion of total P as Fe/Al-P than soils developed on mafic lithology, despite having 4-fold lower Fe content. Moreover, Al-P was generally more dominant than Fe-P depending on bioclimate. Despite appreciable variation in P t concentration as a function of lithology, Ca-P and P o exhibited overall similar trends with increasing elevation when expressed as a proportion of P t . Surface mineral horizons appeared to inherit lithological characteristics despite the A horizon generally thought to be most sensitive to bioclimatic impacts. This study demonstrates that the need to expand the classic Walker-Syers time-based model of soil P development to additional soil-forming state factors.