Quantification and characterisation by FT-ICR MS of potential biostimulant fractions in territorial biogas digestates

Digestates, as co-products of anaerobic digestion plants, constitute not only a source of nutrients and residual carbon, but also a potential source of biostimulant molecules. These molecules play a crucial role in modern agriculture by enhancing plant growth, crop quality and tolerance to environmental stress. However, few data are available on quantities and characteristics of humic-like substances and biostimulant compounds in digestates from different origins. This study focused on fraction yields after alkaline-acid extraction of humic-like substances on raw, solid and liquid digestates from three different plants. Then chemical characterisation of one raw digestate and, for the first time, of its alkaline-acid fractions was conducted using FT-ICR MS in comparison with a commercial biostimulant. Yields of humic-like substances; humic-like acids and fulvic-like acids in the digestates varied from 11.1 to 26.4 g/100 g DM; 3.0 to 11.9 g/100 g DM and 3.5 to 19.7 g/100 g DM respectively. Raw and liquid digestates from food waste feedstocks showed the lowest content of humic-like substances and fulvic-like acids while the use of flocculants combined with centrifugation concentrated humic-like substances in the solid digestates. These findings confirm the significant influence of both the feedstock mix and the phase separation process on yields of humic-like substances. Biostimulant effects observed for the fulvic-like acids and water-soluble fraction in previous lab-scale agronomic trials could be attributed to their high content of oxy-aromatic and highly unsaturated compounds, as well as the presence of organobromides, inorganic bromide and silver, as revealed by FT-ICR MS analysis.