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 P-32 With P-31 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 51d experiment could not be completely explained by P-32 exchange with P-31 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 mu g P kg(-1) d(-1)) were similar to values in the literature that were obtained by isotopic and non-isotopic methods. (C) 1998 Elsevier Science Ltd. All rights reserved.