Sequential chemical extraction has been widely used to study soil phosphorus (P) dynamics and inform nutrient management, but its efficacy for assigning P into biologically meaningful pools remains unknown. Here, we evaluated the accuracy of the modified Hedley extraction scheme using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy for nine carbonate-free soil samples with diverse chemical and mineralogical properties resulting from different degrees of soil development. For most samples, the extraction markedly overestimated the pool size of calcium-bound P (Ca-P, extracted by 1 M HCl) due to (1) P redistribution during the alkaline extractions (0.5 M NaHCO 3 and then 0.1 M NaOH), creating new CaP via formation of Ca phosphates between NaOH-desorbed phosphate and exchangeable Ca 2+ and/or (2) dissolution of poorly crystalline Fe and Al oxides by 1 M HCl, releasing P occluded by these oxides into solution. The first mechanism may occur in soils rich in well-crystallized minerals and exchangeable Ca 2+ regardless of the presence or absence of CaCO 3 , whereas the second mechanism likely operates in soils rich in poorly crystalline Fe and Al minerals. The overestimation of CaP simultaneously caused underestimation of the pools extracted by the alkaline solutions. Our findings identify key edaphic parameters that remarkably influenced the extractions, which will strengthen our understanding of soil P dynamics using this widely accepted procedure.
Saharan
dust is an important phosphorus (P) supply to remote and
oligotrophic parts of the oceans and American lowland tropical rainforests.
Phosphorus speciation in aeolian dust ultimately controls the release
and bioavailability of P after dust deposition, but the speciation
in Saharan dust and its change during the trans-Atlantic transport
remains unclear. Using P K-edge X-ray absorption near edge structure
(XANES) spectroscopy, we showed that with increasing dust traveling
distance from the Sahara Desert to Cape Verde and to Puerto Rico,
about 570 and 4000 km, respectively, the proportion of Ca-bound P
(Ca-P), including both apatite and non-apatite forms, decreased from
68–73% to 50–71% and to 21–37%. The changes were
accompanied by increased iron/aluminum-bound P proportion from 14–25%
to 23–46% and to 44–73%, correspondingly. Laboratory
simulation experiments suggest that the changes in P speciation can
be ascribed to increasing degrees of particle sorting and atmospheric
acidification during dust transport. The presence of relatively soluble
non-apatite Ca-P in the Cape Verde dust but not in the Puerto Rico
dust is consistent with the higher P water solubility of the former
than the latter. Our findings provide insights into the controls of
atmospheric processes on P speciation, solubility, and stability in
Saharan dust.
Summary
Aggregation properties of iron oxide particles have received much attention because of their environmental importance in soil and aquatic environments. The effects of silicic acid on the pH‐dependent surface charges that govern the aggregation of goethite have received little attention. In this study we determined the effect of silicic acid as a function of pH and ionic strength (IS) on the aggregation of a synthetic goethite by a combination of particle‐size analysis using dynamic light scattering and test tube experiments to quantify particles in suspension. We investigated silicic acid adsorption on goethite quantitatively by batch adsorption isotherms, and the changes in surface charge on the adsorption of silicic acid were identified by the zeta potential (ζ). Our results revealed that silicic acid can lead to a significant decrease in ζ. A change in the pHiep (iep: isoelectric point) from 8.5 to lower pH values, typically pH 4–5.5, in the presence of silicic acid suggests enhanced aggregation of goethite in the acidic pH range. At alkaline pH, both goethite and silicic acid were negatively charged because of strong deprotonation, and silicic acid adsorption promoted goethite dispersion. Ionic strength affected the aggregation of goethite by shifting ζ towards the iep and it can also obscure the role of silicic acid.
Highlights
Effect of silicic acid on surface charge and aggregation of goethite was investigated.
Adsorption of silicic acid reduces ζ and allows goethite to aggregate at lower pH.
Ionic strength can also affect aggregation of goethite by shifting ζ towards the isoelectric point.
Silicic acid in soil might maintain goethite particles in an aggregated state.
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