Effect of Tillage On the Dynamics of Diffusive Phosphate Ions in the Top-Soil.

The dynamics of phosphate ions (Pi) in the rhizosphere of cultivated plants is well described by a mechanistic model that couples the concentration of Pi in solution (Cp) and the diffusive ions of the solid phase (Pr). In the top-soil of no till (NT), there is an accumulation of Pi in solution but less is known on how this accumulation affects the status of P onto the solid phase and the subsequent dynamics of P transfer at the solid-solution interface. Our objective was to determine the effect of tillage on the dynamics of Pr at the solid-solution interface in the topsoil. Soils were sampled (0-5 cm) during fall 2009 in a long term corn/soybean rotation under NT and mouldboard plough (MP) established since 1992. A 40 hour sorption/desorption experiment coupled with isotopic dilution labelling were conducted on soil suspensions (M/V:1/10) at steady state to describe the transfer of Pr. The kinetic Freundlich equation described precisely the transfer of Pr at the solid-solution interface but was influenced by the tillage system, yielding two distinct parameterizations: Pr=23.20Cp^0.43t^0.27 for the NT (r²=0.97), and Pr=19.90Cp^0.42t^0.30 for the MP (r²=0.99). These equations showed that the effect of NT on Cp after 17-years, induced higher quantities of Pr under NT compare to MP. These results are supported by values of the degree of P saturation of the top-soils which are higher under NT (10.15 ± 1.01 mmol kg^–1. mmol kg^–1) compare to MP (7.83 ± 0.89 mmol kg^–1. mmol kg^–1). The characteristics of the diffusive P supply derived from the models showed a modification of the dynamics of Pr at the solid-solution interface in the NT. The NT soil has a greater ability to supply P to the solution and therefore to maintain higher levels of Pi in the top-soil.