Vertical patterns of phosphorus concentration and speciation in three forest soil profiles of contrasting climate

Abstract Phosphorus (P) availability in soils controls critical functions and properties of terrestrial ecosystems. Vertical distribution patterns of P concentration and speciation in soil profiles provide historical records of how pedogenic processes redistribute and transform P and thus change its availability in soils, which, however, remain poorly understood. We determined the patterns in three forest soil profiles of contrasting climate, using fine sampling intervals, P K-edge X-ray absorption near edge (XANES) spectroscopy and chemical extractions. The major features of the patterns persist under the contrasting climate. The total P concentration decreases from A to B horizons, reaches a minimum in the B horizons, and then increases towards the upper C horizons, but with little variations with depth in the lower C horizons. Both calcium-bound inorganic P (Ca–Pi) and organic P (Po) decrease and Fe- and Al-bound Pi [(Fe + Al)–Pi] increases in proportion downward in the A horizons because dust inputs and accumulation of organic matter both decline with increasing depth. Ca–Pi is negligible and (Fe + Al)–Pi is dominant in the B horizons due to strong weathering. There is a strong downward increase in Ca–Pi proportion and decrease in (Fe + Al)–Pi proportion from the lower B to the upper C horizons. New Ca–Pi seems to form in the upper C horizons where downward leaching Ca2+ and phosphate accumulate due to the low water permeability of the soils. In the lower C horizons, Ca–Pi increases and (Fe + Al)–Pi decreases with increasing depth due to decreasing chemical weathering. Regarding P bioavailability, the proportion of occluded P (Pocc) shows an increasing and decreasing trend with increasing depth, being the highest in the B horizons; however, there are no consistent trends for non-occluded P (Pn-occ). While the P vertical patterns can be understood by considering the relative importance of different pedogenic processes, climate affects the intensities of these processes and thus the details of the patterns. When depth-integrated, warmer/wetter climate results in decreases in the proportions of both Ca–Pi and Pn-occ but increases in the P loss and the proportions of Po, (Fe + Al)–Pi, and Pocc. Regardless of soil depth and climate, the Pi speciation, i.e., the relative proportions of Ca–Pi and (Fe + Al)–Pi over total Pi, correlates well with soil pH and weathering degree (Chemical Index of Alteration, CIA), and the Po concentration correlates with pedogenic Fe and Al and organic carbon concentration. The correlations suggest that the Pi speciation is primarily controlled by soil geochemistry/mineralogy, and the Po concentration by both soil geochemistry/mineralogy and biological activities. Pocc correlates with CIA, and thus is mainly controlled by soil mineralogy; but Pn-occ correlates weakly with soil properties, probably due to its susceptibility to combined influences of dust inputs, leaching, biological activities, and adsorption on minerals. The above quantitative relationships may help predict P speciation and availability in diverse soils. We further show that soil profiles, and climate and CIA gradients are useful tools for studying P transformations, particularly for the Pi pool, during pedogenesis. This study provides an integration and synthesis of controls of climatic and edaphic variables on P dynamics in forest soils.