Process and quality evaluation of different improved composts made with a smart laboratory pilot plant

This study monitored the process and investigated the quality of compost obtained from different biomasses. Five blends of agri-food waste were composted by a laboratory pilot plant named COMPOSTER, that is designed to optimize biodegradation, and produce compost efficiently. The COMPOSTER consists of two 35-liter nearly adiabatic, aerated bioreactors that simulate an industrial process involving the typical sequence of mesophilic-thermophilic-mesophilic phases. It continuously monitors and records temperature, internal pressure, and biomass weight, while controlling and quantifying oxygen consumption and carbon dioxide emissions resulting from aerobic biodegradation. All composts were characterized for their main chemical, physical, and molecular features, as well as their suppressiveness against Fusarium oxysporum f.sp. lycopersici (FOL), tested on tomato seedlings. Optimized biodegradation yielded 50-60 % mature compost with a cumulative oxygen consumption ranging from 282 to 456 gO₂ per kg of dry matter, with peaks of 2.55 gO₂ per kg of volatile solids per hour, and carbon dioxide emissions of 22-36 % of the initial carbon content, with peaks of 5.89 g CO₂ per kg of volatile solids per hour. Blends containing more ligno-cellulosic ingredients showed higher yields and lower CO₂ emissions. Most of the nitrogen present initially was retained in the final compost; indeed, all mixtures exhibited an apparent nitrogen concentration increase due to carbon loss. Composting determined deep modifications in the molecular structure of the organic matter. ¹³C CPMAS-NMR and off-line thermochemolysis GC-MS analyses highlighted decomposition degree of polysaccharides and peptidic moieties, selective preservation of aliphatic and aromatic recalcitrant compounds, and optimal ongoing humification. All composts were non-phytotoxic, except for that including pepper crop residues, and all resulted rich in macro- and micro-elements for plant nutrition and proved to be active in controlling FOL disease. Compost comprising 81.2 % tomato crop waste exhibited the best growth performance and pathogen control on tomato. Mature, non-phytotoxic, nutrient-rich, and suppressive composts represent promising by-products that can be successfully recycled in agriculture, including high-value applications, leading to lower use of fertilizers and pesticides.