Black Soldier Fly Diet Impacts Soil Greenhouse Gas Emissions From Frass Applied as Fertilizer

Abstract

Increased global production of animal-based protein results in high greenhouse gas (GHG) emissions and other adverse consequences for human and planetary health. Recently, commercial insect rearing has been claimed a more sustainable source of animal protein. However, this system also leaves residues called frass, which—depending on the insect diet—is rich in carbon (C) and nitrogen (N), and could thus be used as fertilizer in agriculture. The impact of this kind of fertilizer on soil GHG emissions is yet unknown. Therefore, we investigated the effect of black soldier fly (Hermetia illucens L.) frass derived from a carbohydrate (Carb-) or a protein (Prot-) based diet applied at two different application rates to an arable soil on C and N fluxes and microbial properties in a 40-day incubation experiment. CO2, N2O, NO, N2, CH4, water extractable organic C (WEOC), and inorganic N were continuously measured quantitatively. At the end of the incubation, microbial biomass (MB), stoichiometry, community composition, and abundance of functional genes were assessed. Along with a strong increase in WEOC and CO2, Carb-frass caused strong initial N2O emissions associated with high N and C availability. In contrast, Prot-frass showed lower CO2 emissions and N2O release, although soil nitrate levels were higher. At the end of incubation, MB was significantly increased, which was more pronounced following Carb-frass as compared to Prot-frass application, and at higher amendment rates. Fungal abundance increased most from both frass types with an even stronger response at higher application rates, whereas bacterial abundance rose following Carb-frass as compared to Prot-application. Abundance of functional genes related to ammonia-oxidizing bacteria and archaea were enhanced by high frass application but did not clearly differ between frass types. C use efficiency of microorganisms, as revealed by the metabolic quotient, was most strongly reduced in the high Prot-frass application rate. Overall, insect diet influenced available C and N in frass and thus affected mineralization dynamics, GHG emissions, and microbial growth. Overall, emissions were very high undermining the potential environmental benefit of insect based protein production and calling for more detailed analyses before frass is widely applied in agriculture.

Residues from black soldier fly (Hermetia illucens) larvae rearing influence the plant-associated soil microbiome in the short term

“…Whether our results are generalizable might therefore depend on a generally high fiber content in BSFL residues compared to other organic fertilizers, which to our knowledge has not yet been assessed. The assumption is supported by recurrently highlighted stimulations of soil fungi after application of BSFL residues ( Temple et al, 2013 ; Rummel et al, 2021 ; Watson et al, 2021 ).…”

Farmed insects for sustainable food systems

“…d Calculated using a N fertilizer replacement value of 68% and a N to NH 4 NO 3 conversion factor of 2.9. e Calculated using a P to P 2 O 5 conversion factor of 2.3. f Calculated using a K to K 2 O conversion factor of 1.2. in larvae and frass, were obtained from and . Emissions from frass management are scarce in the literature, but we used those recently reported by Rummel et al (2021), who reported that on average 5.38% of the N in frass directly applied to soils is lost as N 2 O. It is worth to mention that frass can be composted prior to its use as a soil amendment, or directly applied to the soil without any prior composting.…”

“…It is worth to mention that frass can be composted prior to its use as a soil amendment, or directly applied to the soil without any prior composting. For our system, we assumed the later as it represented the measurements reported by Rummel et al (2021). We assumed that frass replaced the mineral fertilizer ammonium nitrate (NH 4 NO 3 for N, triple superphosphate for phosphorus pentoxide (P 2 O 5 ), and potassium chloride for potassium oxide (K 2 O).…”

“…To increase the understanding of how the overall GWP is affected with methodological assumptions for frass-related emissions and electricity impacts, we estimated the GWP using different input parameters for these two processes. In addition to the assumed 5.38 ± 2.8 % of the N contained in frass lost as N 2 O when frass was directly applied to agricultural soils (Rummel et al, 2021), the overall GWP was estimated using two different values for frass-related emissions. One was based on the latest IPCC emission factor for N 2 O emissions resulting from the application of organic amendments to agricultural soils (i.e., 1% of N lost as N 2 O) (Hergoualc'h et al, 2019).…”

“…The LCA developed in chapter 7 showed that frass-related emissions are one of the main contributors to the overall GHG emissions of BSFL production. Up to date, only one study experimentally quantified frass emissions (Rummel et al, 2021). To account for frass emissions, LCAs (Guo et al, 2021;Mertenat et al, 2019) have relied on standard emission factors for food waste composting (UNFCC, 2017), while other LCAs apparently omitted them (Bosch et al, 2019;Smetana et al, 2019).…”