P-Isotopic exchange values in relation to Po mineralisation in soils with very low P-sorbing capacities

The productivity of many agricultural systems is determined by the quantity of inorganic P (Pi) made available through mineralisation of organic P (Po) during the growing season. Because of the adsorptive nature of the soil mineral fraction, any recently-mineralised Po may be rapidly partitioned to the solid phase. Attempts to quantify Po mineralisation, have been unsuccessful due to the difficulties in separating recently-released Pi from labile Pi forms already existing in the soil. We have investigated the use of an isotopic dilution technique to measure the increase of isotopically-exchangeable P with time (E(t)) in: (i) A short-term experiment (1–100 min) where there was only the physico-chemical process of exchange of solution 32P with 31P located on the solid phase; and (ii) an incubation experiment (0–51 d) where there were both physico-chemical and biological (immobilisation and mineralisation) processes. Although the E(t) values were obtained by different means (water- and resin-Pi extractions), the marked difference between both values during the 51 d experiment could not be completely explained by 32P exchange with 31P in the solid phase. The strong association between E(t) increment with Po and CO2 respiration values during incubation suggest biological participation. Estimates of Po mineralisation rates for the low P-sorbing soils (220–900 μg P kg−1 d−1) were similar to values in the literature that were obtained by isotopic and non-isotopic methods.