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a ENITA de Bordeaux, UMR 1220 TCEM (INRA-ENITAB), 1 cours du Général de Gaulle, F-33175 Gradignan, France
b INRA, UMR 1220 TCEM (INRA-ENITAB), 71 avenue E Bourlaux, F-33883 Villenave d'Ornon, France
* Corresponding author (dachat{at}bordeaux.inra.fr).
Our objective was to evaluate the relative contribution of physicochemical (diffusion) and biological (mineralization) processes to the supply of ionic P (iP) in solution and potential P availability to plants in a low P sorbing forest soil. To this end, we quantified the gross amount of diffusive iP (ionic P species that can be transferred from the solid phase to the soil solution due to a gradient of concentration, FDIFF), P remineralization (gross mineralization of microbial P, FREM), and gross mineralization of P in dead soil organic matter (FMDSOM) using isotopic dilution techniques during a long-term (154 d) incubation experiment. Initial pools of P in dead soil organic matter and of microbial P represented high proportions (77 and 17%, respectively) of total P (31 µg g–1). The FMDSOM value (1.0 µg g–1) was lower than the FDIFF value (1.2 µg g–1) during the 154-d period of incubation. The FMDSOM and FDIFF values were quantitatively very low compared with FREM (13.7 µg g–1). The FREM value contributed largely to the total pool of isotopically exchangeable iP (89%), suggesting that microorganisms play a crucial role in P availability and cycling. Net organic P mineralization (FMDSOM + FREM – P immobilization by microorganisms) caused a large increase (340%) in readily available P (iP in solution). Extrapolated to longer time scales (1 yr or more), FMDSOM appeared to be higher than FDIFF since FDIFF rapidly reaches its theoretical maximum value. We conclude that, in our low P sorbing sandy forest soil, inorganic P sorbed to the solid phase represented a small but rapidly available pool, and P in dead soil organic matter a larger but slowly available pool. Our work showed that the relative contribution of physicochemical or biological processes to plant available P depends on the length of the observation period. Limits of the isotopic dilution approaches to quantify gross or net organic P mineralization are discussed.
Abbreviations: Ebatch, isotopically exchangeable ionic P pool assessed during the batch experiment • Eincub, isotopically exchangeable ionic P pool assessed during the incubation experiment • F, flux • FDIFF, diffusion of ionic P from the solid constituents to the soil solution or from the soil solution to the solid constituents (two gross fluxes equal in the operational conditions) • FIMM, immobilization by microbes of ionic P from the solution • FMDSOM, gross mineralization of P in dead soil organic matter under basal (constant) condition for soil respiration, basal potential of a soil to deliver ionic P to solution from soil organic matter in the absence of remineralization and of flush effects due to drying-wetting or freezing-thawing sequences • FMTO, gross mineralization of total organic P • FNMTO, net mineralization of total organic P • FREM, remineralization or gross mineralization of microbial biomass P, mineralization of recently synthesized organic P • MDSOM, mass of P in dead soil organic matter or non-microbial organic P • MIS, mass of inorganic P sorbed to the inorganic and organic solid constituents, theoretical maximum value of diffusive ionic P • MMB, mass of microbial biomass P • MT, mass of total P • MTI, mass of total inorganic P • MTO, mass of total organic P • MW, mass of water soluble ionic P or ionic P content in soil solution
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