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Differentiation of biochar and native soil organic matter (SOM) is required to assess the effect of biochar amendment on in-situ changes of SOM. Therefore, we used C 4 biochar produced at high temperature (1200 °C) by gasification (BC GS ) and measured the 13 C abundance of density and particle size fractions. We quantified the BC GS effects on distinct native C 3 -SOM pools of a grassland topsoil one year after BC GS amendment. The chemical composition was analyzed with solid-state 13 C CPMAS NMR, whereas information on the nanostructure of BC GS were obtained by Raman microspectroscopy measurements. Our aim was to assess BC GS induced chemical changes of SOM and physical fractions and to validate the accuracy of BC GS detection by 13 C NMR spectroscopy. Quantification by isotopic measurements and 13 C NMR spectroscopy for aromatic C yielded similar estimates of BC in soils. Of the total BC GS , 52% were recovered as free particulate organic matter (POM) and 33% were located in aggregated soil structures isolated as occluded POM particles. Around 4% of the total BC GS was detected in the clay fraction. After one year of field exposure, the surface of the BC GS particles decreased in unordered graphitic-like structures. The higher ordered BC residue is supposed to be more recalcitrant. The native SOC stock increase ( p = 0.06, n = 4) in the clay fraction indicated increased sequestration of organic matter as mineral-bound SOM due to BC GS amendment. With respect to soil functionality, the BC GS amendment induced a tremendous shift from a soil system dominated by organo-mineral associations to POM−dominated OC storage, resulting in increased soil air capacity. [ABSTRACT FROM AUTHOR]

Organic matter (OM) amendments, originating from waste materials, could be used to enhance soil organic carbon (SOC) storage and fertility of cropland soils. However, there is a limited understanding about the long-term effect of different urban waste compost amendments on soil organic matter (SOM) formation processes and their impact on SOC storage. Accordingly, the long-term effects of different OM amendments on the amount and composition of particulate and mineral associated SOM were investigated. Surface soils were sampled from a Luvisol under cropping rotation, which received biannually 0.4 kg organic carbon m −2 in form of three different urban composts or cattle manure for a period of 15 years. Despite similar C input, different urban waste compost amendments resulted in contrasting C storage. While there were no C stock changes for municipal solid waste compost amended soils, composts from organic waste and green waste and sewage sludge increased SOC stocks in a similar range as conventional farmyard manure. In bulk soils, SOC stocks were increased by approximately 30%, in occluded particulate OM <20 μm by 155% (organic waste compost) and 71% (green waste and sewage sludge compost). Carbon storage in clay fractions showed approximately 20% higher values in all treatments. Organic matter amendments result in C–N coupling as for N proportional stock increases were recorded. The high variability of the composition of different amendment types was reflected in the particulate OM fractions but not in the highly uniform composition of the mineral-associated OM. Bulk soil wettability was not affected by the amendments, as only POM fractions showed increased hydrophobicity, but not clay fractions. Clay fractions showed high alkyl/O-alkyl and low C/N ratios, characteristic for microbial material. The OM composition in the clay fraction is determined by the microbial residues that are independent in their composition from the input material. The increased C storage in the clay fractions of the soil amended with organic wastes and green waste and sewage sludge compost and farmyard manure might be promoted by a better microbial use efficiency leading to C sequestration as microbial compounds. [ABSTRACT FROM AUTHOR]