We studied the quantitative and qualitative changes of soil organic matter (SOM) due to different land
uses (arable versus grassland) and treatments (organic manure and mineral fertilizer) within an agricultural crop rotation in a long-term field experiment, conducted since 1956 at Ultuna, Sweden, on a Eutric
Cambisol. The organic carbon (OC) content of the grassland plot was 1.8 times greater than that of the
similarly fertilized Ca(NO3)
2 treated cropped plots. The comparison of two dispersion techniques (a lowenergy sonication and a chemical dispersion which yield inherent soil aggregates) showed that increasing
OC contents of the silt-sized fractions were not matched by a linear increase of silt-sized aggregates. This
indicated saturation of the aggregates with OC and a limited capacity of particles to protect OC
physically. Thermogravimetric analyses suggested an increase of free organic matter with increasing
OC contents. Transmission FT-IR spectroscopy showed relative enrichment of carboxylic, aromatic,
CH and NH groups in plots with increasing OC contents. The silt-sized fractions contained the largest
SOM pool and, as revealed by 13C NMR spectroscopy, were qualitatively more influenced by the plant
residue versus manure input than the clay fractions. Alkyl and O-alkyl C in the silt-sized fractions
amounted to 57.4% of organic carbon in the animal manure treated plots and 50–53% in the other
treatments.We thank the Austrian Science Fund (Fonds zur FÖrderung
der wissenschaftlichen Forschung) for funding this bilateral
project.Peer reviewe
