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

Summary In P odzols, organic matter ( OM ) is stabilized mainly by interaction with minerals, as a direct consequence of pedogenic processes. Metal–organic associations strongly affect OM surface 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 N 2 , to detect the pores in the range of micropores (< 2 nm) and mesopores (2–50 nm), and CO 2 , to measure a smaller microporosity (< 0.5 nm), which is not accessible to N 2 . Only the N‐CM samples showed the typical increase in N 2 ‐SSA after the removal of labile OM , while a decrease was found in all CM horizons. The CO 2 ‐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 N 2 ‐inaccessible surfaces than the labile pool. The N 2 ‐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 CO 2 ‐SSA values (about 800 m 2 g −1 ), with sharp differences between the NaClO ‐labile OM (290–380 m 2 g −1 ) and the NaClO ‐stabilized pool (1380–1860 m 2 g −1 ), thus indicating very reactive illuvial organic materials.