Effect of land use on the composition of soil organic matter in density and aggregate fractions as revealed by solid-state 13C NMR spectroscopy

Helfrich, M., Ludwig, B., Buurman, P. and H. Flessa

Geoderma 136:331-341


Changes in soil organic carbon (SOC) contents with soil cultivation have been investigated extensively, but information on the influence of land use changes on the chemistry of soil organic matter (SOM) and SOM fractions is scarce. To make a contribution in this context, we sampled silty soils under different land use (spruce forest (Ah horizon, 0-7 cm), continuous maize cropping (Ap horizon, 0-30 cm), and grassland (Ah horizon, 0-10 cm)) as well as the organic layer (L, Of, Oh) of the forest floor and collected plant materials from the three sites. Separation of SOM density fractions (free particulate organic matter, occluded particulate organic matter, mineral-associated organic matter) and water-stable aggregate size fractions of the surface soils was performed in order to follow changes in the chemical composition of SOM in physical soil fractions using CPMAS 13C NMR spectroscopy and to determine land use effects on the chemistry of SOM fractions.

The CPMAS 13C NMR spectra showed that maize litter had the highest content of O-alkyl-C and the lowest content of alkyl-C, aryl-C and carbonyl-C compared with the plant material from the spruce stand and the grassland. The forest litter had the largest content of aromatic and alkyl-C. Decomposition of spruce litter in the humus layer resulted in a decreasing O-alkyl-C content and an increasing alkyl-C content. The SOM of the acid forest soil consisted mainly of particulate organic matter (POM) with a high content of spruce litter-derived alkyl-C. The SOC stocks in the grassland and maize soil were dominated by mineral-associated SOM which contained relatively larger proportions of aryl and carbonyl-C. The decrease of the SOC concentration induced by cultivation resulted in a relative accumulation of aromatic C structures in the mineral-bound SOM. In all soils, the free POM had a smaller proportion of alkyl-C and a larger proportion of O-alkyl-C than the POM occluded in aggregates. The mean age of the SOM in the density fractions of the maize soil increased with increasing aromaticity in the order free POM < occluded POM < mineral-associated organic matter.