Humic acid formation in artificial soils amended with compost at different stages of organic matter evolution

A composting process was conducted under optimal conditions for 150 d, obtaining three biornasses at different levels of maturity: raw material (RM), fresh compost obtained after 11 d of composting (FC), and evolved compost (EC) obtained after 150 d of composting. During the composting process, RAs were extracted and fully characterized by mass balance, DRIFT, and I H and 13C-nuclear magnetic resonance spectroscopy. Each compost sample was incubated for 180 d in an artificial soil, after which HA extraction was repeated and characterized. To compare composts containing different amounts of labile organic matter (OM), an equal amount of unhydrolyzable OM was added to the soils. Our results indicated that compost HAs consist of a biologically and chemically stable fraction (i.e., the unhydrolyzable HA [U-HA]) and a labile fraction, whose relative contents depended on the composting duration. HLLmic acid from more EC contained a higher amount of recalcitrant fraction (aromatic carbon) and a lesser a-mount of labile fraction (aliphatic carbon) than HA from RM and FC. These results suggest that the hurnification process during composting preserves the more recalcitrant fraction of the compost-alkali soluble/acid insoluble fraction (RA-fraction). Incubation of composts in soil showed that due to the higher labile fraction content, HAs from raw material were more degraded than those from EC. The abundance of labile carbon of soil a-mended with less-evolved compost (RM and FC) allowed the more recalcitrant fractions of U-H4A to be more presmed than in EC. These results suggest that lessevolved compost could contribute more than well evolved compost to the stable soil OM.