Co-composting of seaweed with lignocellulosic substratum: Effect of the compost composition on compost humic substances

Seaweed is one of Latvia’s underexploited natural resources. Green and brown algae collected after stranding on beaches were co-composted with lignocellulosic substratum – grass and/or hardwood sawdust. Six different compost compositions were prepared. Composting was carried out indoors in the autumn-winter season. The correspondence of the obtained composts to the national and EU legislation was evaluated. The best results were obtained for compost prepared from green algae, grass and sawdust mixture. Compost raw material, compost and compost humic acids were characterised by elemental analysis, UV and Fourier transform infrared spectroscopy and monosaccharides analyses. The compost humic substance composition was found to be dependent on the compost raw materials. Compost humic acids constituted of recognisable algae and plants biomolecules structures, including lignin-derived structures and carbohydrates. Humic acids of the obtained compost contained around 8% monosaccharides. Fucose is a deoxy sugar typical for algae and not for higher plants, and can be used as a chemical indicator of algae biomass in compost.