Excellent excrement? Frass impacts on a soil's microbial community, processes and metal bioavailability

Abstract

The commercial rearing of insects is a growing economic sector. Therefore, an assessment was made of the potential of its by-product, frass, to be a soil improver. Essential plant nutrients were extracted (using 0.01 M CaCl2 or Mehlich 3) from frass of mealworms (Tenebrio molitor), black soldier flies (Hermetia illucens) and buffalo worms (Alphitobius diaperinus). A 28-day incubation, in which frass was added to a sandy loam soil at application rates of 2.5% or 5% (w/w), assessed its effects on soil microbial biomass, abundance of bacteria, archaea and fungi, carbon mineralisation and nitrification. In a separate 56-day incubation, the impact of frass on heavy metal bioavailability in an artificially contaminated, carbon-poor substrate was tested. All frass types featured high electrical conductivity, a mildly acidic to neutral pH and C: N ratios between 11 and 16. Black soldier fly frass (BSFF) was richer in extractable ammonium, phosphorus, potassium and magnesium than mealworm frass (MWF) and buffalo worm frass (BWF) but poorer in extractable calcium. All frass types stimulated carbon mineralisation, nitrification, bacterial and archaeal 16S rRNA gene copy numbers, and fungal biomass as determined by ergosterol concentrations. Bacterial and particularly fungal abundances were stimulated by the 5% frass application rate whereas archaeal abundances were greater in the 2.5% application rate regimes. The 2.5% application rate of MWF and BWF led to a profound build-up of soil extractable nitrite. Correspondingly, these treatments featured the highest 16S rRNA gene copy numbers of archaea, a domain encompassing organisms which oxidise ammonium to nitrite. No nitrite was detectable in soil amended with BSFF. The 5% application rates induced microbial biomass growth (as determined by extractable DNA concentrations) only when BSFF was applied. This was possibly due to differences in the frass types' extractable nutrient or labile carbon contents. BSFF and BWF amendment led to significantly higher microbial biomass in a metal-contaminated substrate. This was likely due to frass providing nutrients, energy and reduced metal bioavailability: extractable zinc, copper, cadmium and nickel concentrations fell due to increased metal sorption and complexation. All frass types could be used as ameliorants in metal-contaminated soils, while BSFF shows most promise as an organic fertiliser as its use did not cause soil nitrite build-up.