Phosphorus pools in bulk soil and aggregates of differently textured oxisols under different land-use systems in the Brazilian cerrados

This study assessed the influence of land use (continuous cropping, CC; tree plantations, F; pasture, PG; and native savanna, NS) on P concentrations and partitioning in bulk soil and two aggregate size fractions of two Oxisols, one loamy and one clayey. The quantity and quality of physically protected P within aggregates were also determined. Total P in bulk soil and macroaggregates (0.25-2 mm and 2-8 mm) was partitioned into inorganic and organic P fractions (P, and P., respectively) after sequential extraction, using NaHCO, (Olsen), NaOH, HCl, and H2SO4 (residual). Additionally, P binding was examined with 31P nuclear magnetic resonance spectroscopy. Total P concentrations were 70-170 mg/kg in the loamy and 300-450 mg/kg in the clayey soil. The largest P fraction was the NaOH-soluble P (33%-55% of total P concentration). The proportion of easily extractable P was higher in the loamy than in the clayey soil. Because of fertilization, CC and F had higher total and, particularly, higher easily available P concentrations than NS and PG. In the loamy soil, Pi and, in the clayey soil, both Pi and P., were higher in CC and F than in NS and PG. In the loamy soil, P concentrations in macroaggregates (>2 mm) were higher than in bulk soil. Higher monoester P proportions within the large macroaggregates in unplowed systems indicated older intra-aggregate soil organic matter than at the aggregate surfaces. In the plowed system, the monoester/diester ratio was not different between bulk soil and aggregate fractions, indicating faster aggregate turnover.