Abstract
Insect protein production is considered a sustainable alternative to livestock protein which furthermore utilizes waste streams. Its production can have positive but also potentially negative environmental effects, which require evaluation. Frass, the byproduct of insect production, is regarded an efficient organic fertilizer or soil amendment. However, several studies report negative frass effects on plant growth and nitrogen (N) cycling. Therefore, a pot trial was carried out which sought to understand N release from frass and subsequent growth and nutrient uptake of Italian ryegrass. Mealworm frass (MWF) or buffalo worm frass (BFW) was applied at two rates (1.5 and 3% w/w) to a soil-sand mix. To evaluate N release processes, frass was applied alone, with a nitrification inhibitor (NI), a urease inhibitor (UI), or both (NI+UI). Plant N, nutrient uptake and soil inorganic N were measured at the experiment's end. To gauge whether altered N fluxes induced changes in the microbial community, soil microbial biomass, bacterial/archaeal abundances and ergosterol content as a fungal biomarker, were determined. Both frass types and application rates stimulated microbial growth and N mineralization. The 3% rate inhibited seed germination, possibly due to salinity or ammonia toxicity. At the 1.5% rate, both frass types were effective fertilizers. MWF led to higher biomass and nutrient uptake, owing to its higher extractable nutrient concentrations. The 3% rate caused nitrite accumulation in the absence of NI. NI improved plant biomass, nutrient uptake, stimulated archaeal and bacterial abundances and prevented nitrite accumulation. UI reduced N mineralization, showing that a substantial fraction of frass organic N is ureic. UI enhanced fungal contribution to the microbial biomass, revealing the importance of bacteria in frass N mineralization processes when UI is not applied. NI and UI combined, induced greater N release from frass than UI or NI alone. Our study demonstrated the usefulness of NI and UI in studying N release from frass. NI can improve plant N uptake and minimize N losses following frass application, reducing its potentially negative effects. UI can retard N release from frass, allowing its application as a slow-release fertilizer, but should not be used concurrently with NI.
Highlights
Global demand for protein such as meat and dairy products is increasing, but with growing environmental awareness fueling demand for food with lower environmental footprints
The higher cumulated biomass of MWF plants was accompanied by slightly higher uptake of N (Table 2), FIGURE 2 | N uptake in Italian ryegrass shoot biomass at four different harvest dates in replicates fertilized with 1.5% (w/w) buffalo worm frass, BWF (A) or mealworm frass, MWF (B)
Our results show the high potential of insect frass, both as an amendment which can benefit the soil microbial biomass and as an organic fertilizer which can provide adequate nutrients to allow plant growth in a substrate of low fertility
Summary
Global demand for protein such as meat and dairy products is increasing, but with growing environmental awareness fueling demand for food with lower environmental footprints. Eutrophication of groundwater and surface water bodies became a problem in Europe such as in parts of France, Belgium, the Netherlands and Germany (van der Wiel et al, 2020) To this end, insect rearing, which has a more efficient feed to protein conversion rate and can potentially utilize agricultural waste or side-products, may provide a technological solution to nutrient excess and be an alternative protein source for feed and food (Smetana et al, 2019; Schmitt and de Vries, 2020). With lower emissions per unit of protein and contribution to a regional nutrient circularity it may reduce negative consequences for the environment This needs to be verified for the whole production chain and starts with evaluating the different components from feed provision, insect rearing, processing to waste management
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