Soil Organic Matter Clogs Mineral Pores
- Christian Mikutta *,
- Friederike Lang and
- Martin Kaupenjohann
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 (SSABET) of AlOOH by 34 m2 g−1 (15%) and 77 m2 g−1 (36%). The reduction in SSABET 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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