Effects of biochar, black soldier fly larvae frass and their combinations on yields, availability and uptake of copper by finger millet

Abstract

Heavy metal contamination, particularly copper (Cu), poses serious risks to agricultural productivity and food safety in sub-Saharan Africa (Shabbir et al., 2020; Zhao et al., 2022). Excessive Cu in soils disrupts nutrient uptake, reduces biomass, and increases oxidative stress in crops, leading to yield losses and food safety concerns (Wuana & Okieimen, 2011; Sheng et al., 2024). The prevalence of Cu contamination in arable land is exacerbated by intensive agriculture, the use of Cu-based fungicides, and livestock manure derived from Cu-supplemented feed (Oorts, 2013; Guemiza et al., 2017). Sustainable remediation strategies, including organic amendments such as biochar and insect frass, are therefore urgently needed to immobilize Cu, improve soil fertility, and safeguard food production (Beesigamukama et al., 2020; Palansooriya et al., 2022). In this study, a controlled pot experiment was conducted using a completely randomized design with factorial treatments comprising biochar (1% and 3%), black soldier fly (BSF) frass (1% and 3%), and their mixtures in varying ratios (75% and 25% to make the1 % and 3% of biochar and BSF respectively), alongside a control(NPK application). Finger millet (Eleusine coracana) was grown in Cu-contaminated soil (200 mg/kg), and parameters assessed included yield, Cu uptake in plant tissues, and post-harvest soil physicochemical properties. Copper accumulation in millet tissues followed the order root > grain > straw, with the control recording the highest levels. Application of 3% biochar reduced Cu uptake in grain and roots by 53% and 42%, respectively, relative to the control. BSF significantly enhanced productivity, with the highest grain yield (28.2 g pot⁻¹) obtained at 3%. Combined applications provided synergistic effects: biochar-dominant mixtures (3BC:1BSF) maximized Cu immobilization, while frass-dominant mixtures (1BC:3BSF) improved yield and nutrient enrichment, particularly phosphorus, calcium, and magnesium. Both amendments improved soil physicochemical properties, including pH, organic matter, and nutrient availability.