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 (13)C 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.