Multi-waste filters for nitrogen gas capture from slurry and reuse as soil amendment

The valorization of agro-industrial and domestic wastes has gained increasing attention as a strategy to mitigate the environmental impact of agriculture while promoting circular economy principles. This study evaluates the nitrogen gases capture potential of various wastes used as biofilters for pig slurry emissions and their subsequent application as inorganic amendments in soil. Three main questions were addressed: (1) Nitrogen gas captured by each waste; (2) Existence of synergy effects by combining wastes (mixed or in layers), and (3) Improvements in crop yield by nitrogen captured without affecting nutritional quality. A total of 28 wastes were characterized and tested for their nitrogen gas retention capacity. Cork and bentonitic sand were the most efficient materials. Synergistic effects were observed with bentonitic sand and orange peel, which improved N retention by 18%. Additionally, layered configurations of wastes significantly enhanced N capture efficacy relative to mixed arrangement (67% and 42% respectively). Finally, three formulated inorganic amendments (N-enriched and non-enriched) were tested on lettuce and ryegrass. Amendments increased lettuce biomass by up to 250% and nitrogen uptake by 100% versus the unfertilized control, whereas smaller effects were observed in ryegrass. No significant differences were found between enriched and non-enriched amendments. No significant accumulation of potentially toxic elements (Aluminium, Arsenic, Barium, Cadmium, Chromium, Nickel) was detected in the vegetal tissue, confirming the safety of the formulated amendments. These findings support the development of cost-effective and environmentally sustainable solutions for reducing nitrogen emissions while enhancing soil fertility. • Cork, Bentonitic sand and orange peel residues showed the highest capture capability • pH, specific surface and zinc extractable were correlated with nitrogen-gas capturing • The combination of residues showed synergy effects relative to individual residues • The arrangement of residues in layers was more efficient than mixed • Double formulation as filter and amendment is key to waste recovery