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

详细信息    查看全文

• 作者：David L. Achat ; Mark R. Bakker ; Etienne Saur ; Sylvain Pellerin ; Laurent Augusto ; Christian Morel
• 关键词：Dead soil organic matter ; Gross mineralisation ; Homogeneous labelling ; Isotopic dilution method ; Phosphorus ; Specific activity
• 刊名：Geoderma
• 出版年：2010
• 出版时间：15 September 2010
• 年：2010
• 卷：158
• 期：3-4
• 页码：163-172
• 全文大小：911 K

文摘

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.