Stabilization of Organic Matter at Micropores (<2 nm) in Acid Forest Subsoils

Abstract

Binding of soil organic matter (OM) at micropore entrances within small mesopores (2–10 nm) has been suggested as a potential mechanism for the stabilization of OM against biodegradation. We hypothesized that the mineral‐associated fraction of stable OM [OM resisting treatment with 6% sodium hypochlorite (NaOCl) and subsequently extracted by 10% hydrofluoric acid] is associated with pores <10 nm in 12 acid subsoil horizons. To study the coverage of micropores by stable OM sorbed in mesopores, we assumed that most micropores have entrances of mesopore size. We compared the accessibility of CO2 at 273 K to micropores after NaOCl treatment with that of N2 at 77 K. In contrast to N2, diffusion of CO2 into micropores is little affected by OM and the difference in both micropore volumes (MIV) is taken as a measure of micropore clogging and thus of the association of stable OM with mesopores. The MIV measured by CO2 adsorption was corrected for CO2 sorption in OM (MIV‐CO2corr). In 7 out of 12 samples, the MIV‐CO2corr equaled the MIV‐N2, suggesting that micropore entrances are not blocked by stable OM. For four samples the results were ambiguous, whereas in an Eutric Hapludand Bw horizon, a threefold larger MIV‐CO2corr compared with the MIV‐N2 indicates that stable OM was associated with pores < 10 nm. Based on the findings that (i) mineral MIVs derived from CO2 and N2 adsorption were similar for most samples after exposure to NaOCl, (ii) the change of MIV‐N2 on NaOCl treatment was small in all samples, and (iii) no relationship existed of the MIV‐CO2corr and small mesopore volume with the content of mineral‐associated stable organic C (OC), we conclude that the association of OM with micropore entrances in small mesopores does not primarily control the stabilization of OM in these acid subsoils.