Micropore characteristics of organic matter pools in cemented and non-cemented podzolic horizons

In Podzols, organic matter (OM) is stabilized mainly by interaction with minerals, as a direct consequence of pedogenic processes. Metal–organic associations strongly affect OMsurface features, particularly microporosity. Cemented ortstein horizons (CM) may form during podzolization, accompanied by a spatial arrangement of OM on mineral surfaces, which differs from that in non-cemented horizons (N-CM). To investigate the metal–organic associations and their changes during pedogenesis, we selected both N-CM and CM podzolic horizons, isolated NaClO-resistant OM and compared the specific surface area (SSA) before and after OM oxidation. The SSA was assessed by using N2, to detect the pores in the range of micropores (<2 nm) and mesopores (2–50 nm), and CO2, to measure a smaller microporosity (<0.5 nm), which is not accessible to N2. Only the N-CM samples showed the typical increase in N2-SSA after the removal of labile OM, while a decrease was found in all CM horizons. The CO2-SSA revealed a large number of small micropores characterizing OM, both before and after oxidation. The smallest micropore classes (<0.5 nm) were, however, more abundant in NaClO-resistant OM, which had therefore a larger number of N2-inaccessible surfaces than the labile pool. The N2-SSA data thus indicated a more homogeneous coverage of mineral surfaces by stabilized OM in CM samples. Because of the abundance of small micropores, OM in these podzolic B horizons had extremely large CO2-SSA values (about 800m2 g−1), with sharp differences between the NaClO-labile OM (290–380m2 g−1) and the NaClO-stabilized pool (1380–1860m2 g−1), thus indicating very reactive illuvial organic materials.