Gross organic phosphorus mineralization rates can be assessed in a Ferralsol using an isotopic dilution method

• We studied a high-P-sorbing Ferralsol with very low available phosphate (Pi_water).
• We used an isotopic dilution method together with sensitive detection of Pi_water.
• The gross organic P mineralization rate was 0.8 ± 0.5 mg P kg− 1 soil day− 1.
• Previous residue amendment increased this rate to 1.7 ± 0.2 mg P kg− 1 soil day− 1.
• This is the first measurement of gross organic P mineralization in a Ferralsol.

Mineralization of organic phosphorus (Po) may be of great importance for plant nutrition in soils containing very little available inorganic phosphorus (Pi). Gross organic P mineralization rates can be quantified by an isotopic dilution method using 33P labeling of soil. However, its application remains a challenge in tropical soils in which the concentration of phosphate ions in the soil solution is below the detection limit of traditional colorimetric methods. This limitation can potentially be overcome by the hexanol concentration method, which uses hexanol to concentrate the blue-colored phosphomolybdate complex from large volumes. We applied the isotopic dilution method in combination with the hexanol concentration method to a Ferralsol from the highlands of Madagascar which had been preincubated in the presence or absence of plant residues for 90 days before the start of the experiment. The limits of detection (DL) and quantification (QL) of the gross Po mineralization rate were 0.2 and 0.7 mg P kg− 1 soil day− 1, respectively. Basal gross Po mineralization rates after 7 days of incubation were 0.8 ± 0.5 and 1.7 ± 0.2 mg P kg− 1 soil day− 1 in non-amended and residue-amended soils, respectively. These rates are plausible, suggesting that the isotopic dilution method is applicable in highly weathered tropical soils with Pi concentrations in the soil solution below the detection limit of traditional colorimetric methods. Net Po mineralization which sustains the plant available P pool remains to be quantified. Gross and net Po mineralization rates should now be assessed in highly weathered soils under a range of land uses.