The formation of non-labile from labile P forms, adsorbed onto Fe and Al oxyhydroxides, is the reason for low residual of phosphate fertilization in tropical soils. The reversibility of non-labile P to labile forms in these soils may be favored by the reduction of Fe3+ and, or,by the reduced activity of Fe and Al oxyhydroxides. This study aimed to evaluate P adsorption and desorption, the transformation into non-labile forms and reversibility of these forms in soil samples from environments of microbial or chemical reduction. Samples of 11 soils were homogenized with 300 mg kg-1 P (NaH2PO4) in solution and incubated for 30 days. P was extracted in 10 successive extractions with anion-exchange resin (AER) (treatment without reduction). In a second experiment, the soil samples were treated with 0.1 mol L-1 sucrose solution to create a reduction enrviroment and subsequent application of P dose (300 mg kg-1) and 10 successive P extractions (treatment microbial reduction). In a third experiment, the samples were complexed/reduced with ammonium oxalate (Ox) or with citrate-dithionite-bicarbonate (CDB). The soils/residues were treated with the above P dose, incubated for 30 days and subjected to successive extractions with AER (chemical reduction). It was observed that maximum P adsorption capacity (MPAC) of the soils was closer related with goethite, (which accounted for 70.8 % of the value), than gibbsite (with a contribution of 29.2 %). The negative correlation between the P contents in the second AER extraction for the soils in natural condition without reduction, and the contents of gibbsite only, indicated that gibbsite and not goethite is the oxyhydroxide that restricts P desorption most. The MPAC values estimated by equilibrium P showed, in the samples treated with microbial reduction, a slight alteration for the group of the less oxidic soils, with lower MPAC. But for more oxidic soils with higher MPAC, the previous effect of sucrose, which increased equilibrium P (reducing the MPAC), was high, about 10 times higher than in soils of the treatment without reduction. However, the reduction caused by sucrose did not alter the desorption of previously adsorbed P. The expectation of a significant reversibility of non-labile P due to microbial or chemical reduction of the soils was not confirmed, demonstrating the great stability of this form.
Redox potential; P-desorption; gibssite; goethite; oxalate; CDB; residual P
