Phosphorus speciation and elemental associations in well-drained soils of temperate forests

Phosphorus (P) dynamics in the soil subsurface are relatively unstudied, especially in forest soils, and may be important in areas with groundwater-fed surface water bodies. Here, we use X-ray absorption near-edge P spectroscopy (P-XANES) and X-ray fluorescence microprobe mapping (µ-XRF) to examine P speciation and co-localization with Fe and Al as a function of depth in soils from coniferous and deciduous forests. P-XANES revealed similar trends in P speciation with depth for both forest types, with Al-P and Fe-P being dominant, whereas Ca-P and organic P were minor constituents. Correlation plots derived from µ-XRF mapping revealed a strong correlation between P and both Fe and Al at all depths. At depths of up to 378 cm, a range that includes A, E, and B horizons, the relationships between P and Al/Fe were remarkably consistent, consisting of a population of P diffusely associated with Al/Fe, consistent with Fe- or Al-adsorbed P, as well as P linearly correlated with Al/Fe in several discreet groupings, indicative of the presence of specific P compounds. At depths below 378 cm, the relationships between P and Al/Fe changed dramatically, with correlation plotting indicating the disappearance of any diffuse P:Al/Fe relationships. This change occurred concurrently with a decrease in oxalate extractable P, suggesting that the P present in this deep soil horizon may be present in a more stable form that is less available for transport, as well as potentially indicating that P adsorbed to Fe/Al did not readily pass through the overlaying Bw soil horizon.