We studied the quantitative and qualitative changes of soil organic matter (SOM) due to different land\uduses (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\udCambisol. The organic carbon (OC) content of the grassland plot was 1.8 times greater than that of the\udsimilarly fertilized Ca(NO3)\ud2 treated cropped plots. The comparison of two dispersion techniques (a lowenergy sonication and a chemical dispersion which yield inherent soil aggregates) showed that increasing\udOC contents of the silt-sized fractions were not matched by a linear increase of silt-sized aggregates. This\udindicated saturation of the aggregates with OC and a limited capacity of particles to protect OC\udphysically. Thermogravimetric analyses suggested an increase of free organic matter with increasing\udOC contents. Transmission FT-IR spectroscopy showed relative enrichment of carboxylic, aromatic,\udCH and NH groups in plots with increasing OC contents. The silt-sized fractions contained the largest\udSOM pool and, as revealed by 13C NMR spectroscopy, were qualitatively more influenced by the plant\udresidue versus manure input than the clay fractions. Alkyl and O-alkyl C in the silt-sized fractions\udamounted to 57.4% of organic carbon in the animal manure treated plots and 50–53% in the other\udtreatments.
展开▼
