Partitioning of P Retention between Al-Saturated Montmorillonite and Ferrihydrite Is pH-Dependent and Differs for Organic and Inorganic P.

Pedogenic secondary minerals such Fe and Al oxyhydroxides and clay minerals play a crucial role for the retention of inorganic and organic phosphorus (P) in soils. For the first time, we directly compared the relative contribution of Fe oxyhydroxide (ferrihydrite) and an Al-saturated high-activity clay mineral (montmorillonite) to the retention of orthophosphate (oPO4) and inositol hexakis phosphate (IHP) at different pH values (range: 3 – 6). In our study, we combined analysis of solution P concentration by ICP-OES with a quantification of P bound to solid phase Fe and Al, respectively, using synchrotron-based P K-edge XANES spectroscopy. The results of solution and solid phase analyses agreed very well with each other: Orthophosphate is preferentially retained by ferrihydrite compared to Al-saturated montmorillonite in the entire pH range 3-6. The contribution of Al-saturated montmorillonite to overall oPO4 retention is considerable (30%) at pH 5 and 6, smaller (20%) at pH 4, and negligible (10%) at pH 3. The relative and absolute decrease of oPO4 retention by Al-saturated montmorillonite with decreasing pH is probably due to a speciation change of the Al sorbed to the montmorillonite from surface and interlayer Al(OH)3 at pH 6 to polymer and monomer Al hydroxy cations at pH 5 and 4, respectively, and to Al3+ at pH 3. In contrast, IHP was mostly retained by Al-saturated montmorillonite rather than ferrihydrite in the entire pH range, particularly at low P concentrations, indicating a retention mechanism different to that of oPO4. Our results suggest that soil acidification is associated with a shift of oPO4 retention by Al-saturated high-activity clay minerals (e.g. montmorillonite) to Fe oxyhydroxides (e.g. ferrihydrite), which is absent for IHP. According to our results soils with low Fe oxyhydroxide contents, e.g. due to Fe-poor parent material and/or reduced soil conditions, are prone to oPO₄ mobilization and loss with the seepage water at pH values <5.