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We examined the short-term effect of five organic amendments and compared them to plots fertilized with inorganic fertilizer and unfertilized plots on aggregate stability and hydraulic conductivity, and on the OC and ON distribution in physically separated SOM fractions. After less than 1 year, the addition of organic amendments significantly increased (P < 0.01) the aggregate stability and hydraulic conductivity. The stability index ranged between 0.97 and 1.76 and the hydraulic conductivity between 1.23 and 2.80 x 10(-3) m/s for the plots receiving organic amendments, compared with 0.34-0.43, and 0.42-0.64 x 10(-3) m/s, respectively, for the unamended plots. There were significant differences between the organic amendments (P < 0.01), although these results were not unequivocal for both soil physical parameters. The total OC and ON content were significantly increased ( P < 0.05) by only two applications of organic fertilizers: between 1.10 and 1.51% OC for the amended plots versus 0.98-1.08% for the unamended and between 0.092 and 0.131% ON versus 0.092-0.098% respectively. The amount of OC and ON in the free particulate organic matter fraction was also significantly increased (P < 0.05), but there were no significant differences (P < 0.05) in the OC and ON content in the POM occluded in micro-aggregates and in the silt + clay-sized organic matter fraction. The results showed that even in less than 1 year pronounced effects on soil physical properties and on the distribution of OC and ON in the SOM fractions occurred

There is a need for further studies to compare the decomposition of biochar to that of the original feedstock and determine how these amendments affect the cycling of native organic matter (NOM) of different soils to improve our understanding of the resulting net C sequestration potential. A 510-days incubation experiment was conducted (i) to investigate the evolution of CO 2 from soils amended with either fresh corn stover (CS) or with biochars produced from fresh CS at either 350 (CS-350) or 550°C (CS-550), and (ii) to evaluate the priming effect of these amendments on NOM decomposition. Two soil types were studied: an Alfisol and an Andisol, with organic C contents of 4% and 10%, respectively. Except for the controls (with no C addition), all treatments received 7.18 t C ha À1 . We measured C efflux in short-term intervals and its isotopic signature to distinguish between C evolved from C 4 amendments and C 3 -dominated NOM. Emission rates were then integrated for the whole time period to cover total emissions. Total CO 2 -C evolved from the original C in fresh CS, CS-350 and CS-550 was greater in the Andisol (78%, 13% and 14%) than in the Alfisol (66%, 8% and 7%). For both soils, (i) no significant differences (P > 0.05) were observed in the rate of CO 2 evolution between controls and biochar treatments; and (ii) total accumulated CO 2 evolved from the uncharred amendment was significantly higher (P < 0.05) than that from the other treatments. In the Alfisol, a significant (P < 0.05) net positive priming effect on NOM decomposition was observed when amended with fresh CS, while the opposite was detected in biochar treatments. In the Andisol, no significant (P > 0.05) net priming effect was observed. A C balance indicated that the C lost from both biochar production and decomposition 'broke even' with that lost from fresh residue decomposition after <35 weeks. The 'break-even' point was reached earlier in the Andisol, in which the fresh CS mineralizes faster. These results provided experimental evidence for the potential of biochar to sequester C and avoid CO 2 emissions from original feedstock while protecting native soil organic matter.