Evolution of humic acid-like and core-humic acid-like during high-rate composting of pig faeces amended with wheat straw

The changes in the absolute amounts and in the composition of humic acid-like (RA-like) and core-humic acid-like (core-RA-like) were monitored for the high-rate composting process (4 weeks) of wheat straw-amended pig faeces. Absolute amounts of HA-like and core-HA-like were obtained by extraction of RA-like and core-HA-like from the compost bed once a week. The decrease in the amount of RA-like (350 g kg(-1)) took place during the first week of composting and was due to the degradation of organic molecules co-extracted with HA-like, the so-called interference or coating material. The coating material was mainly composed of fatty acids. Core-HA-like, after an initial decrease, showed an increase in the total amount after 2 weeks of composting. This was due to the partial solubilization of the hamin fraction, which started when lignin-degrading fungi became active at the mesophilic stage of the composting process. We concluded that HA-like consisted of refractory organic molecules, probably lignin and biopolymers, which formed a stable structure (core-HA-like) coated with the degradable material associated with the core by weak physical association, ether or ester bounds. The ratio of HA-like and core-HA-like, the so-called Organic Matter Evolution Index (OMEI), was found to be useful to quantify the evolution of organic matter during composting, as it takes into account the biodegradability of all types of organic compounds and humification processes. OMEI increased from 0.26 to 0.41 during the high-rate composting process. This value was lower than that for well-developed composts (OMEI > 0.6) but the presence of degradable organic matter could promote the stabilization of soil aggregates and soil biological activity.