Urban waste composts enhance OC and N stocks after long-term amendment but do not alter organic matter composition

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.