Evolution of surface properties and organic matter stabilisation in podzolic B horizons as assessed by nitrogen and phosphate sorption

In podzolic B horizons illuviated Al, Fe and organic matter (OM) increase with the ongoing of the pedogenic process. Depending on OM load on mineral surfaces, modifications of the soil surface properties are expected and may influence OM stabilisation. The proportion of labile organic pools should thus vary depending on the type of podzolic horizon. In this work, we selected B horizons at increasing intensity of podzolisation, evaluated the labile OM pools through oxidation with 2 % NaClO and characterised surface properties with N2 and phosphate sorption. Before and after oxidation, we assessed the NaOH-extractable OM fractions. Oxidation was more effective on the least polar organic compounds and led to an increase in the fulvic to humic acid ratio. Specific surface area (SSA) increased after oxidation only in the least podzolised horizons, while selectively preserved OM induced a decrease in SSA in the more developed Bs, Bsm and Bhs. Phosphate sorption induced a release of OM and always decreased after oxidation, although variations in P affinity for the surfaces were observed. The effect of oxidation on surface parameters pointed to a specific association between organics and minerals that changed during soil development. At the very beginning of podzolisation, the dominant forms seemed related to organo-metallic complexes with little interaction with surfaces. With Bs development, weak interactions between mineral surfaces and OM appeared, while at a later stage OM differentiated into bulky structures and tightly bound, rigid ones, with extremely low N2 accessibility. The latter were not sensitive to low concentration NaClO while the former were easily oxidised.