Black Soldier Fly frass as a soil amendment for improving soil fertility and stabilising cadmium in acidic paddy soils

Acidic paddy soils with low organic matter often retain cadmium (Cd) in relatively mobile forms accessible to plant roots. This study evaluated Black Soldier Fly (BSF) frass as an amendment for improving degraded paddy soil and limiting Cd transfer to aboveground plant tissue. The tested soil was acidic and poor in organic carbon, with a total Cd of 1.58 mg kg-1. A randomised pot experiment comprised four media treatments: untreated soil, soil:frass (50:50), soil:compost (50:50), and soil:frass:compost (50:25:25). Each treatment was replicated three times using chilli (Capsicum annuum L.) as an indicator plant. Soil pH, organic carbon, total N, P, K, total Cd, microbial population, shoot Cd, and root Cd were measured and the bioconcentration factor (BCF) and translocation factor (TF) were calculated. The frass contained 37.51% organic carbon and had a near-neutral pH of 6.60. Frass application improved soil pH, organic carbon, macronutrient status, and microbial population from 1.10 x 107 to 3.75 x 107 g-1. Cd occurred mainly in roots, while TF values remained below one, indicating phytostabilisation rather than phytoextraction. BSF frass may combine fertility restoration with Cd risk reduction; however, frass quality, soil properties, and tissue Cd concentrations should be carefully monitored before field-scale application.