Response of different soil organic matter pools to biochar and organic fertilizers

•Biochar enhances C stabilization in soils treated with organic fertilizers.•Biochar promotes sorption of C on mineral surfaces in organically-fertilized soils.•Biochar accumulates in the free organic matter pool not protected by the soil mineral matrix.•Biochar has little impact on mineral-associated organic matter composition. A full understanding of the agronomic and environmental potential of biochar, and especially its potential as a C sequestration strategy, requires a full comprehension of its effects on native soil organic matter (SOM), as well as of its interactions with other organic amendments co-applied to the soil. In a field experiment, we investigated changes in quantity and quality of SOM pools characterized by different protection mechanisms as affected by 20tha−1 of biochar added alone or combined with two different organic fertilizers (i.e., municipal solid waste compost and sewage sludge) 8 months after application. In particular, free, intra-macroaggregate, intra-microaggregate, and mineral-associated SOM fractions were separated and analyzed for organic C, total N, and stable isotopic ratios; further, diffuse reflectance infrared Fourier transform spectroscopy was used to examine functional group composition. Soil biomass C content, basal respiration, and metabolic quotient were also determined. Biochar and organic fertilizer application increased significantly SOM content. Biochar accumulated mainly in the free SOM pool, not protected by the soil mineral matrix. Especially noteworthy was the significant interaction effect found between the biochar and organic fertilizer factors on mineral-associated organic C content. This suggested a promoting action of biochar on C stabilization in organically-fertilized soils through the formation of organo-mineral complexes. Organically-fertilized soils had higher microbial C than unfertilized soils. Basal respiration and metabolic quotient, however, were not affected by any of the treatments. As a whole, our results support the potential of biochar application as a strategy to sequester C in soils.