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 FMDSOM) 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 FMDSOM 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 (SAW) and of microbial P (SAMB) 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 SAW and SAMB converged after 83 days and that the small reactions between solution and sorbed ionic P did not significantly affect values of SAW and SAMB. Beyond day 83 the homogeneous labelling of solution ionic P and microbial P was not strictly maintained (divergence between SAW and SAMB; homogeneous labelling assumption invalidated). However, the combined pool of the two remained at the same SA level. This alternative approach enabled us to evaluate FMDSOM 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.