Phosphorus pools in Oxisols under shaded and unshaded coffee systems on farmers' fields in Brazil

Phosphorus (P) is a primary limiting nutrient for crop production in weathered tropical soils. The deficiency is mainly caused by sorption of phosphate onto Al- and Fe- (hydr)oxides. We hypothesise that the distribution of soil P among various pools is influenced by land use. Our objective was to characterise the soil inorganic (Pi) and organic P (Po) pools and to compare the various pools at different depths in agroforestry (shaded) and monocultural (unshaded) coffee cultivation systems. The study was carried out in the Atlantic Coastal Rainforest domain, Brazil, with Oxisols as the dominant soil type. Soils were collected from four farmers' coffee (Coffea arabica L.) fields, two agroforestry and two monocultural systems. Three profiles were sampled per field, at depths of 2-3, 10-15 and 40-60 cm. A simplified sequential P fractionation was carried out, using resin, 0.5M NaHCO3, 0.1 M NaOH, 1 M HCl and concentrated HCl as extractants. Sum-P (resin, NaHCO3, NaOH, 1 M HCl and concentrated HCl) ranged from 370 to 830 mg kg(-1). Concentrated HCl extracted the largest portion (74%), followed by NaOH (22.5%). Labile (sum of resin, NaHCO3 and NaOH) P ranged from 13 to 40% of Sum-P. The major part (62%) of the labile fraction was Po. In the agroforestry fields, the amount of Po decreased less with depth and the percentage of Po in labile pools was higher than in monocultural fields. This suggests that agroforestry maintains larger fractions of P available to agricultural crops by influencing the dynamics of P through the conversion of part of the Pi into Po, thereby reducing P losses to the unavailable pools.