Greenhouse gas and ammonia emissions from duckweed cultivation systems using diluted liquid manure

Protein from duckweed (Araceae, subfamily Lemnoideae) grown on diluted animal slurries for nutrient upcycling could potentially replace plant-derived feed proteins which would be more efficiently used as human food. However, little information is available on greenhouse gas (GHG) emissions from slurry-grown duckweed, and previous studies have not reported methane emissions from a similar system. Here, we report on GHG (methane, carbon dioxide and nitrous oxide) and ammonia emissions from duckweed grown on diluted cattle slurry measured in daylight and darkness, compared with emissions from diluted slurry without duckweed. We observed (i) initially high but rapidly declining methane emissions, independent of lighting or treatment, (ii) a net carbon dioxide fixation by duckweed, independent of lighting, (iii) high nitrous oxide emissions, independent of lighting, and (iv) a > 80% reduction of ammonia emissions by duckweed, independent of lighting. Our data shows potential of duckweed protein as a sustainable protein with 3.54 to 6.54 CO₂eq kg^- 1 protein, compared to faba bean (3.61 kg CO₂eq kg^- 1 protein) or barley protein (5.35 CO₂eq kg^- 1 protein). But despite the potential of slurry-grown duckweed as sustainable protein source, swapping ammonia volatilization for nitrous oxide emissions represents a limitation of the current system and mitigation strategies are needed.