RESUMO
The larvae of the black soldier fly (BSFL, Hermetia illucens) efficiently close resource cycles. Next to the nutrient-rich insect biomass used as animal feed, the residues from the process are promising plant fertilizers. Besides a high nutrient content, the residues contain a diverse microbial community and application to soil can potentially promote soil fertility and agricultural production through the introduction of beneficial microbes. This research assessed the application of the residues on plant-associated bacterial and fungal communities in the rhizosphere of a grass-clover mix in a 42-day greenhouse pot study. Potted soil was amended with BSFL residues (BR+) or conventional compost (CC+) produced by Rwandan waste management companies in parallel to residues and compost sterilized (BR-, CC-) by high-energy electron beam (HEEB) as abiotic controls. The fertilizers were applied at a rate of 150 kg N ha-1. Soil bacterial and fungal communities in both fertilizer and soil were assessed by high-throughput sequencing of ribosomal markers at different times after fertilizer application. Additionally, indicators for soil fertility such as basal respiration, plant yield and soil physicochemical properties were analyzed. Results showed that the application of BSFL residues influenced the soil microbial communities, and especially fungi, stronger than CC fertilizers. These effects on the microbial community structure could partly be attributed to a potential introduction of microbes to the soil by BSFL residues (e.g., members of genus Bacillus) since untreated and sterilized BSFL residues promoted different microbial communities. With respect to the abiotic effects, we emphasize a potential driving role of particular classes of organic matter like fiber and chitin. Indeed, especially taxa associated with decomposition of organic matter (e.g., members of the fungal genus Mortierella) were promoted by the application of BSFL residues. Soil fertility with respect to plant yield (+17% increase compared to unamended control) and basal respiration (+16% increase compared to unamended control) tended to be improved with the addition of BSFL residues. Findings underline the versatile opportunities for soil fertility arising from the application of BSFL residues in plant production and point to further research on quantification of the described effects.
RESUMO
The disposal of feacal matter from Urine Diversion Dry Toilets is a significant challenge due to limited land availability, possible underground water contamination, and the risk of spreading diseases. The collected faecal matter can be fed to Black Soldier Fly Larvae to produce protein-rich larvae used as animal feed. The disposal of the leftover waste (BSFL residue) is still a problem due to the risk of residual pathogen contamination. The BSFL residue contains residual plant nutrients and can be further processed into biochar. Faecal matter biochar offers an exciting value proposition where the pyrolysis process guarantees a 100% pathogen elimination. It also results in significant waste reduction in transport, storage weight, and volume. A preliminary study was conducted to (i) optimise pyrolysis conditions (optimal temperature treatment and residence time) for biochar production using residue obtained after faecal matter from urine diversion dry toilets was fed to black soldier fly larvae as feedstock; and (ii) determine the physicochemical and morphological characteristics of biochar produced. The residue was pyrolysed at 300, 400, and 500 °C and characterised for chemical, biological and physical characteristics. Surface area (6.61 m2 g-1), pore size, and C: N (9.28) ratio increased at 500 °C for 30 min. Exchangeable bases, (Calcium) Ca, (Magnesium) Mg, (Potassium) K, and (Sodium) Na increased with increasing pyrolysis temperature. The increase in basic cations resulted in an increase in pH from 6.7 in the residue to 9.8 in biochar pyrolysed at 500 °C. Biochar pyrolysed at 500 °C can therefore be used to improve acidic soils. Phosphorus increased with increasing pyrolysis temperature to 3 148 mg kg-1 at 500 °C. Biochar produced at 500 °C for 30 min had desirable characteristics: surface area, exchangeable bases, and pH. Also, biochar can be used as a phosphorus source with potential for crop production, although an external nitrogen source is needed to meet crop nutrient requirements.
