Content and composition of free and occluded particulate organic matter in a differently textured arable Cambisol as revealed by solid‐state 13C NMR spectroscopy

The composition of functional light soil organic matter pools of arable Cambisols with a gradient in clay content was investigated. Soil texture differences originate from increasing loess admixture to the parent material (coarse-grained tertiary sediments). Using density fractionation in combination with ultrasonic dispersion, two types of particulate organic matter (POM) were obtained: (1) free POM and (2) POM occluded in soil aggregates. Both POM fractions were analyzed by elemental analysis (C, N) and CPMAS 13C NMR spectroscopy. With increasing clay content the amount of organic carbon stored in the occluded POM fraction increased considerably, whereas the amounts of free POM were not related to the soil clay content. With increasing soil clay contents increasing proportions of O-alkyl C and decreasing proportions of aryl C were found for both POM fractions. The occluded POM fraction showed a higher degree of degradation as indicated by lower amounts in O-alkyl carbon. A lower degree of POM degradation was associated with higher clay contents. Higher soil clay contents promoted the conservation of POM with a low degree of alteration. This effect of soil texture was found to be highly significant when the aryl C : O-alkyl C ratio was used as indicator for POM decomposition rather than the alkyl C : O-alkyl C ratio. Menge und Zusammensetzung freier und okkludierter partikularer organischer Substanz in ackerbaulich genutzten Braunerden mit unterschiedlicher Bodentextur, aufgezeigt mittels Festkorper-13C-NMR-Spektroskopie Es wurde die Zusammensetzung funktionaler, leichter Pools der organischen Bodensubstanz in ackerbaulich genutzten Braunerden entlang eines Tongehaltsgradienten untersucht. Die Unterschiede in der Bodentextur liesen sich auf unterschiedliche Ausgangsmaterialien zuruckfuhren (grobkornige tertiare Sedimente mit zunehmenden Losanteilen). Mittels Dichtefraktionierung und Ultraschallbehandlung wurden zwei Arten von partikularem organischen Material (POM) gewonnen: (1) freies POM und (2) in Aggregaten okkludiertes POM. An beiden POM-Fraktionen wurden Elementaranalysen (C, N) sowie CPMAS-13C-NMR-spektroskopische Untersuchungen durchgefuhrt. Die okkludierte POM-Fraktion weist einen hoheren Abbaugrad auf, wie die geringeren Anteile an O-Alkyl-Kohlenstoff zeigen. Mit zunehmendem Tongehalt nimmt der Gehalt an organischem Kohlenstoff, der als okkludierte POM-Fraktion gespeichert wird, deutlich zu. Die Menge des freien POM ist dagegen nicht mit dem Tongehalt des Bodens korreliert. Beide POM-Fraktionen zeigen mit steigendem Tongehalt zunehmende Anteile an O-Alkyl-C, aber abnehmende Anteile an Aryl-C. Mit dem hoheren Tongehalt im Boden ist ein geringerer Abbaugrad des POM verbunden. Hohere Tongehalte fordern somit die Erhaltung weniger umgewandelter POM-Fraktionen. Dieser Effekt der Bodentextur wird nur dann hochsignifikant sichtbar, wenn das Aryl-C:O-Alkyl-C-Verhaltnis und nicht das Alkyl-C:O-Alkyl-C-Verhaltnis als Indikator fur den Abbaugrad der POM-Fraktionen herangezogen wird.

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