Chemical fractionation of phosphorus, sulphur, and molybdenum in Brazilian savannah Oxisols under different land use

In soils, P, S, and Mo occur organically bound or as oxyanions. In Oxisols, oxyanions may be strongly sorbed due to the high positive surface charge. The objective of this work was to compare chemical fractions of P, S, and Mo in differently used Oxisols with similar properties in an on-farm experiment. Soil samples (0–0.15 m) were taken from three replicate plots of each of conventional tillage (CT) and no-till (NT) maize–soybean and conventional tillage sugarcane (SC) cropping systems, degraded (DP) and productive pastures (PP), Eucalyptus (EU) and Pinus reforestations (PI), and native savannah (CE). The samples were sequentially extracted with (1) 0.5 M NaHCO3, (2) 0.1 M NaOH, (3) 1 M HCl, (4) hot concentrated HCl, and (5) concentrated HClO4/HNO3. In the extracts, inorganic (Pi) and total P, S, and Mo were determined. Organic P (Po) was calculated as total P−Pi. Total concentrations were 333–567 mg P kg−1, 231–284 mg S kg−1, and 3.2–3.9 mg Mo kg−1. The most important fractions in all studied systems were the NaOH fraction for P (38–49% of total P), the HClO4/HNO3 fraction for S (27–35% of total S) and the concentrated HCl fraction for Mo (86–90% of total Mo). The proportion of the more easily extracted NaHCO3+NaOH fractions decreased along the line S>P>Mo. Fertilisation increased plant-available P and S fractions in CT and NT whereas recalcitrant fraction (concentrated HCl and HClO4/HNO3) remained unchanged. The Pi/Po ratios in NT and CT were higher than in CE because fertiliser P mainly accumulated in inorganic P fractions. The pasture soil had lower P concentrations indicating export by grazing. Thus, 12–20 years of land use had marked effects on P, smaller ones on S, and almost none on Mo concentrations and chemical fractions.