Quantifying gross mineralisation of P in dead soil organic matter: Testing an isotopic dilution method

Gross mineralisation of organic phosphorus (P) may play a key role in soil P availability to plants and in P cycling. The challenge for studying P availability is to accurately quantify the two main biological processes involved (mineralisation of microbial P and gross mineralisation of P in dead soil organic matter F MDSOM) separately. However, distinguishing between the two processes can be extremely difficult using the usual isotopic dilution methods. Our objective was to test the basic assumption of another isotopic method — homogeneous labelling of all exchangeable pools of phosphate ions (iP in soil solution, microbial biomass P, and iP sorbed to the solid phase) — that would allow direct quantification of F MDSOM separately from the mineralisation of microbial P. To favour homogeneous labelling, we incubated for a long period a low P-sorbing soil with a low fraction of inorganic P (6% of total P). The soil was labelled with 33P at constant soil respiration in an incubator at 20 °C, and then specific activities of solution ionic P ( SA W) and of microbial P ( SA MB) were monitored for 154 days. A batch experiment with 32P-labelled soil was used to model the exchange reactions with the solid constituents. The results showed that SA W and SA MB converged after 83 days and that the small reactions between solution and sorbed ionic P did not significantly affect values of SA W and SA MB. Beyond day 83 the homogeneous labelling of solution ionic P and microbial P was not strictly maintained (divergence between SA W and SA MB; homogeneous labelling assumption invalidated). However, the combined pool of the two remained at the same SA level. This alternative approach enabled us to evaluate F MDSOM through both pools of ionic P in solution and microbial P. Comparing this result with previous measurements of the biological processes on the same soil showed that gross fluxes of mineralised P were likely to include a larger proportion of what was mineralised P from microbial biomass rather than mineralised P from dead soil organic matter. Thus, the method tested here could avoid any erroneous interpretations when attributing the gross organic P mineralisation flux to any defined biological process. Moreover, quantifying the mineralisation fluxes correctly and separately would enable a better understanding of the biological processes and possible assessment of the changes in P cycling in a changing environment.