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Soil Chemistry-Note

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

^a Institut für Bodenkunde und Pflanzenernährung, Martin Luther Universität Halle-Wittenberg, Weidenplan 14, D-06108 Halle, Germany
^b Institut für Ökologie, Technische Universität Berlin, Salzufer 12, D-10587 Berlin, Germany

^* Corresponding author (robert.mikutta{at}landw.uni-halle.de)

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-CO₂^corr). In 7 out of 12 samples, the MIV-CO₂^corr 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-CO₂^corr 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-CO₂^corr 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.

Abbreviations: MIV, micropore volume • OC, organic carbon • OM, organic matter • SSA, specific surface area