P fractionation in sodium bicarbonate extracts of andic soils

The hypothesis of stabilization of organic phosphate by Al(Fe) in andic soils was investigated. P fractionation in soil bicarbonate extracts was studied in relation to soluble C (DOC), Al and Fe. Phosphatase-hydrolysable P was determined in order to assess the bioavailable P forms in the extractable organo-mineral compounds. The centrifugation temperature of the bicarbonate extracts had an important influence on the extracted compounds, especially DOC and P; the results pointed to the existence of colloidal minerals together with Al(Fe)-humus complexes. The results questioned the conventional classification of Pi and Po by fractionation in this type of soils, Po being underestimated in the acid solution. Phosphatase-hydrolysable P (Pen) ranged from 33 to 93% of Po and decreased with decreasing centrifugation temperature. The results have been interpreted in terms of soluble ternary complexes of organic or inorganic P with Al(Fe)-humus complexes.