Measuring rates of gross and net mineralisation of organic phosphorus in soils

The isotopic dilution method developed by Oehl et al. [2001b. Organic phosphorus mineralisation studies using isotopic dilution techniques. Soil Science Society of America Journal 65, 780–787] to measure gross mineralisation of soil organic phosphorus (P) was tested on a range of low-P sorbing soils. This isotopic dilution method relies on accurate prediction of radiolabel behaviour due to soil physicochemical processes. Based on experimental validation of the extrapolation for isotopic dilution due to physicochemical processes using autoclaved soils, a simple power function was used for extrapolation rather than the more complex equation used in the original method. For several soils, however, a potential overestimation of gross mineralisation by 0.1–2.0 mg P kg−1 d−1 was revealed. In addition, the detection limit of P mineralisation ranged between 0.6 and 2.6 mg P kg−1 d−1. The method is likely to be at the detection limit for soils that are high in available P and low in biological activity. The method was modified with respect to the extrapolation and successfully applied to a soil with relatively high microbial P (18 mg P kg−1) and soil respiration rates (29 mg C kg−1 d−1), revealing gross mineralisation rates of organic P of 0.9–1.2 mg P kg−1 d−1. Measurement of uptake of 32P by the microbial biomass allowed derivation of a net organic P mineralisation rate of 0.5–0.9 mg P kg−1 d−1.