Phosphorus speciation and iron mineralogy in an oxisol after 11 years of pig slurry application

Application of phosphate fertilizers beyond plants needs favors phosphorus (P) accumulation in soils, which may alter its reactivity and chemical speciation. The objective of this study was to assess the changes in P speciation in a Brazilian oxisol that received consecutive applications of varying rates of pig slurry (PS) over 11 years. The soils were treated with PS at rates of 50, 100 and 200 m³ ha^-1 year^-1, whereas a control plot received P and potassium (K) to replenish the amounts removed by harvest. The soils were sampled and characterized for its P sorption capacity (PSC) as determined by Langmuir sorption isotherms, P partitioning by sequential chemical fractionation (SCF), P chemical speciation via P K-edge XANES and iron (Fe) mineralogy via Fe K-edge EXAFS spectroscopies. Increases in applied PS rates were accompanied by increases in PSC at the 0 to 2.5 and 0 to 10 cm soil layers. P accumulation was observed to be restricted up to the depth of 20 cm, regardless of the PS rate applied. The P K-edge XANES analysis indicated that P accumulation in the topmost soil layers, occurred predominantly associated with Fe-(hydr)oxide minerals. In this soil layer (0 to 2.5 cm), the organic P pool was of particular importance likely due to no-tillage. A dramatic change in Fe mineralogy in the topmost soil layer was observed across the studied soils, with the predominance of hematite in the reference soil and in the control plot, whereas the occurrence of goethite and ferrihydrite was followed by the application of PS.