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Division S-2—Soil Chemistry

Soil Organic Matter Clogs Mineral Pores

Evidence from ¹H-NMR and N₂ Adsorption

Dep. of Soil Science, Institute of Ecology, Berlin Univ. of Technology, Salzufer 11-12, D-10587 Berlin, Germany

^* Corresponding author (christian.mikutta{at}tu-berlin.de)

Recent N₂ adsorption studies have suggested a ‘pore clogging’ effect on mineral soil phases caused by organic matter coatings. For methodological reasons, this pore clogging effect has been studied only after drying. Our hypothesis was that pore clogging is affected by drying of organic coatings. In our study, we used AlOOH, which has been equilibrated with dissolved organic matter (DOM) and polygalacturonic acid [PGA; (C₆H₈O₆)_n]. To test our hypothesis, we determined the porosity of moist and freeze-dried AlOOH samples. Freeze-dried samples were analyzed by N₂ adsorption, moist samples by ¹H-nuclear magnetic resonance (NMR). In addition, the samples were characterized by environmental scanning electron microscopy—energy dispersive x-ray spectroscopy (ESEM-EDX). Both, DOM and PGA significantly reduced specific surface area (SSA_BET) of AlOOH by 34 m² g^–1 (15%) and 77 m² g^–1 (36%). The reduction in SSA_BET normalized to the amount of C sorbed was 1.0 m² mg^–1 DOM-C and 5.9 m² mg^–1 PGA-C. Dissolved OM reduced the pore volume of micro- and small mesopores <3 nm whereas PGA also reduced the volume of larger pores. The ¹H-NMR results of moist samples showed that PGA sorption reduced the amount of water in pores <4 nm. In addition, the pore size maximum of AlOOH increased by 150%. Polygalacturonic acid coatings created new interparticle pores of about 10- to 70-nm size that are not stable upon freeze-drying. Porosity changes upon DOM-treatment were not commensurable by ¹H-NMR. Our results indicate that clogging of micro- and small mesopores is not an artifact of freeze-drying. Polygalacturonic acid seems not only to cover the mouth of AlOOH-nanometer pores but also to fill them.

Abbreviations: BET, Brunauer, Emmett, and Teller • DOM, dissolved organic matter • ESEM-EDX, environmental scanning electron microscopy–energy dispersive x-ray spectroscopy • OM, organic matter • PGA, polygalacturonic acid • SSA_BET, specific surface area determined after Brunauer, Emmett, and Teller (1938a) • SSA_MP, micropore surface area • TOC, total organic carbon

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