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

Abstract

SummaryIn Podzols, 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 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 800 m2 g−1), with sharp differences between the NaClO‐labile OM (290–380 m2 g−1) and the NaClO‐stabilized pool (1380–1860 m2 g−1), thus indicating very reactive illuvial organic materials.