Evaluation of hormone-like activity of the dissolved organic matter fraction (DOM) of compost and digestate

- Biomasses can have biostimulating effect for the presence of hormone-like molecules.
- No data have yet been reported on the bio-stimulating effects of digestate.
- Auxin-like activity was associated with digestate dissolved organic matter (DOM).
- DOM bio-stimulating activity was due to neutral hydrophobic DOM fraction.
- Hormone-activity was due to the auxin coming from aromatic amino acid biodegradation.

Biomasses are usually applied to soil for their agronomic properties (fertilization and amendment properties). Biomass can also have bio-stimulating effects on plants because of the presence of hormones and hormone-like molecules. Although compost has been the subject for studies of this aspect, no data have yet been reported on the extraction of this kind of molecule from digestate biomass. The aim of this work is to study the auxin- and gibberellin-like activity of pig slurry digestate in comparison with those of pruning and garden wastes compost's dissolved organic fraction (DOM). DOM (i.e., fraction < 0.45 μm) is the most reactive among the organic matter fractions readily available to microbial and plant metabolism. No gibberellin-like activities were found for either compost or digestate, whereas digestate showed auxin-like properties which were found to be located in its neutral hydrophobic (NHo) DOM fractions. Hormone activity was due principally to the presence of auxin coming from the anaerobic digestion of aromatic amino acids.