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

Publication date: November 2015
Source:Geoderma, Volumes 257–258
Author(s): Lalajaona Randriamanantsoa , Emmanuel Frossard , Astrid Oberson , Else K. Bünemann
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 90days before the start of the experiment. The limits of detection (DL) and quantification (QL) of the gross Po mineralization rate were 0.2 and 0.7mgPkg⁻¹ soilday⁻¹, respectively. Basal gross Po mineralization rates after 7days of incubation were 0.8±0.5 and 1.7±0.2mgPkg⁻¹ soilday⁻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.