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Effects of Black Soldier Fly Frass Containing Biodegradable Microplastics on the Plant-Soil-Microbial System
Summary
A pot experiment found that black soldier fly frass containing biodegradable polylactic acid microplastics altered soil respiration and root biomass in soybean and wheat, with effects concentrated in bulk soil rather than the rhizosphere. This raises concerns that using insect frass as a fertilizer—while otherwise sustainable—may inadvertently introduce biodegradable microplastics that disrupt soil microbial dynamics and nutrient cycling.
Utilization of black soldier fly to dispose of food waste stream bioplastics results in frass with biodegradable microplastics that may alter soil nutrient cycling and reduce the commercial value of frass. However, the impacts of frass with biodegradable microplastics on microbial community consumption of root exudates and plant growth are poorly characterized. Utilizing a pot experiment, Glycine max and Triticum aestivum were grown in soil amended with black soldier fly frass at two concentrations from flies fed solely food waste or food waste containing 25% (w/w) polylactic acid microplastics. Plants were grown in controlled conditions for eight weeks with sub-irrigation. At maturity, pots were harvested for plant biomass, and bulk plus rhizosphere soil was probed for changes in root exudate induced respiration. Frass with and without biodegradable microplastics generally lowered bulk and rhizosphere soil moisture and increased root biomass up to > 200%. Soil respiration rates were generally higher in the frass only and frass plus biodegradable microplastics treatments. Substrate induced respiration indicated the frass plus biodegradable microplastics treatments had the largest effect on bulk rather than rhizosphere soil for both species. Principal component analysis indicated frass concentration had the stronger influence on per treatment principal component scores and that frass plus biodegradable microplastics were statistically significant relative to frass without bioplastics of the same concentration. Biodegradable microplastics in black soldier fly frass altered the soil and induced positive responses in roots and the microbial community consumption of root exudates.