A modified static respiration assay and its relationship with an enzymatic test to assess compost stability and maturity

Despite the numerous compost stability and maturity tests, no universally accepted compost stability or maturity index exists. The fluorescein di-acetate (FDA) enzymatic assay, originating from soil studies, is examined here as a potential new compost stability test, and is compared to microbial respiration and phytotoxicity indices. Thirteen composts were used in the study from different source materials. Static microbial respiration activity indices calculated were the cumulative O₂ consumptions, O₂ consumption rates, total C-CO₂ production, the respiratory quotient and the bio C/N ratio. Compost phytotoxicity was quantified via a 7-day tomato seed germination assay. Results showed that the net fluorescein release rates correlated with all stability indices. The germination index marginally correlated with the fluorescein release rates, but not with any of the other stability indices. New limits to classify composts regarding their stability were proposed.