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(NO 3 ) 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 13 C 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.

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A long-term field demonstration was initiated in 1995 to evaluate the effect of organic manures (FYM, poultry manure and pressmud) and mineral fertilizers on changes of pH, electrical conductivity (EC), organic C and nutrient contents under pearl millet -wheat cropping sequence. Continuous application of organic manures alone or in conjunction with NP fertilizer for 10 years decreased the soil pH. However, a reverse trend was observed in case of EC. Organic C content of the soil decreased from its initial value, when only NP fertilizers were applied and increased significantly with the application of organic manures applied alone or with NP fertilizers. The highest organic carbon content of the soil has approached to 0.99% in plot receiving 15 Mg FYM þ150 kg N þ30 kg P 2 O 5 ha 71 . The application of organic manures with or without NP fertilizers could not sustain the original level of N. However, their application increased the available P, K and DTPA extractable Zn, Fe, Mn and Cu content of soil. Application of P, K and micronutrients can be avoided with the application of organic manures. The build-up of organic C and nutrient contents was higher in surface soil as compare to subsurface soil.

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A long‐term field experiment, conducted since 1962 in Gumpenstein (Austria) on a Dystric Cambisol, was used for the present investigation. We combined a physical fractionation procedure with the determination of natural abundance of 13C and FT‐IR spectroscopy to study the influence of fertilizer amendments (organic manure and mineral fertilizers) and management practices (fallow vs. cropped) on changes in organic carbon (OC) associated with different particle‐size fractions. The OC content in bulk soil decreased or was not affected by slurry+straw, PK, and NPK treatments in both fallow and cropped plots after 28 and 38 yr of treatment. However, OC in plots receiving organic manures increased depending on the quality of the organic manures applied. The ranking among the different treatments under both fallow and cropped plots was: animal manure (liquid) > animal manure (solid) > cattle slurry = slurry+straw = PK = NPK. Results showed that the two types of management practices, fallow (non‐tilled) vs. cropped (tilled) had effects on OC concentrations. Comparing the OC contribution of particle‐size fractions to the total OC amount revealed the following ranking: silt > clay > fine sand > coarse sand except in the plots receiving solid or liquid animal manure. Size fractions within treatments showed larger variations of 13C abundances than bulk samples between treatments. The natural abundances of 13C increased especially in cropped (and tilled) plots. It was shown by cluster analysis that FT‐IR spectra differentiated between the different treatments originating from different land management practices. The present study revealed that below‐ground C deposition by agricultural plants can hardly compensate the C losses due to tillage.

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