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.

Long-term effects of alum-treated litter, untreated litter and NH4NO3 application on phosphorus speciation, distribution and reactivity in soils using K-edge XANES and chemical fractionation.

Whereas soil test information on the fertility and chemistry of soils has been important to elaborate safe and sound agricultural practices, micro-scale information can give a whole extra dimension to understand the chemical processes occurring in soils. The objective of this study was to evaluate the effects that the consecutive application of untreated poultry litter, alum-treated litter or ammonium nitrate (NH4NO3) had on P solubility in soils over 20 years. For this, we used soil test data, sequential chemical fractionation (SCF) of P, and P K-edge XANES and µ-fluorescence spectroscopies. Water extractable P data indicated that application of alum to poultry litter was a very effective treatment for reducing P solubility. On the basis of our SCF of P data, P was primarily found within the 0.1 M NaOH pool across the applied rates and regardless of the treatment, where application of alum-treated litter accounted for as much as 59 ±â€¯2% of the total, followed by NH4NO3, 49 ±â€¯4%, and untreated litter, 40 ±â€¯2%. It was also shown that in soils where alum-treated litter was applied, the Resin pool accounted for 10 ±â€¯1% of the total, followed by NH4NO3, 13 ±â€¯4%, and untreated litter, 18 ±â€¯2%, indicating that P was less readily available in soils where alum-treated litter was applied. Phosphorus XANES indicated that P was predominantly associated to Fe > Al > Ca > organic molecules, regardless of the treatment or applied rates, though the formation of PoAl complexes was only found in soils that received application of alum-treated litter and was positively related to the applied rates. The combination of P-XANES with SCF or µ-fluorescence data was shown to provide valuable information about P reactivity and distribution in soils and should thus be used to address the fate of applied P amendments in soils.