Open Access

Conor Watson, Pauline Rummel, Lukas Beule, Charlotte Schlösser, Michael Hemkemeyer, Sanja Schwalb, Timo Preißing, Jakob Vögerl, Florian Wichern

• Insect rearing is a rapidly growing sector in Europe, with larval protein production projected to exceed 1 Mt in 2025. The by-product frass is a mix of larval faeces, exoskeletons and undigested substrate. Given its potential as a soil improver, frass could make insect production a “zero waste” industry. However, its positive reputation has not been thoroughly scrutinized with regard to environmental impacts. We therefore conducted experiments to investigate the effects of frass application on the soil microbial community, release of greenhouse gases and heavy metal availability. The main conclusions were: 1) Frass is nutritious and can benefit the soil microbial biomass, but rapid loss of C and N reduces its fertiliser and organic amendment potential 2) Co-application of frass with urease inhibitors could allow it to be utilised as a slow-release N fertiliser 3) Concurrent soil amendment of frass with nitrification inhibitors prevents soil nitrite accumulation 4) All frass types tested are suitable as ameliorative amendments to heavy-metal contaminated soils