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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

View larger version (24K): [in a new window]   Fig. 1. Nitrogen adsorption–desorption isotherms at 77 K of selected subsoil samples used in our study showing a range of observed hystereses. The adsorption–desorption isotherms of Samples 7 and 11 are representative whereas Sample 6 exhibits the largest hysteresis among the samples due to the prevalence of smectite. Note that the isotherms are stacked by a value of 20 cm3 g–1 for better visualization.

View larger version (18K): [in a new window]   Fig. 2. Micropore volumes of ferrihydrite, Na-bentonite, and Na-vermiculite determined by N2 adsorption at 77 K and CO2 adsorption at 273 K.

View larger version (17K): [in a new window]   Fig. 3. Nitrogen micropore volume (MIV-N2) after NaOCl treatment versus the CO2 micropore volume after NaOCl treatment corrected for CO2 sorption to stable OM (MIV-CO2corr) (a); plot of the N2–MIVs determined before and after NaOCl treatment (b).

View larger version (26K): [in a new window]   Fig. 4. Small mesopore volume (2–10 nm) corrected for the weight of residual OM after NaOCl treatment versus the average volume of the mineral-associated stable OM assuming a minimal and maximal OM density of 1.4 and 1.9 g cm–3 (vertical error bars). The amount of mineral-associated stable OM was determined as the fraction that was released by 10% HF following the NaOCl treatment.

View larger version (87K): [in a new window]   Fig. 5. Transmission electron images of a small aggregate in the fine clay fraction (<0.2 µm) of the Eutric Hapludand (Sample 11) after treatment with NaOCl. The A-B line represents the spatial distribution of Si, Al, Fe, and stable OC as analyzed by energy-dispersive X-ray spectroscopy.