Extracellular enzymes as reliable indicators of long-term antimony contamination

Antimony (Sb) contamination in soil has become a growing concern due to its toxic effects on ecological soil functions. Soil enzymes, which are effective biological indicators, play a crucial role in assessing the ecological impact of heavy metals in soil. However, the effects of Sb on soil enzyme activity, particularly during the ageing process, remain poorly understood. This study examines the ageing process of Sb in soil and its biological toxicity on three key soil enzyme activities (arylsulfatase, urease, phosphatase) at different enzyme pool levels (total, intracellular and extracellular). Our findings reveal that the ageing of exogenous Sb in soil follows a heterogeneous dispersion process, with the Sb ageing rate constant (|b|) in acidic soil (S1, red soil, pH 4.90) being 1.21 to 1.90 times higher than in alkaline soil (S2, gray desert soil, pH 8.12). This suggests that Sb stabilizes more rapidly in acidic conditions. Regarding Sb effects on soil enzymes, extracellular urease activity and the total enzyme activity index (TEI) of extracellular enzymes were particularly sensitive to Sb stress. Over the course of the ageing period, extracellular urease activity decreased by 23.46-57.85 % in both soils under Sb stress at 7000 mg·kg^-1. The inhibition of TEI by Sb ranged from 29.08 to 42.47 % in S1 soil, and from 12.47 to 20.65 % in S2 soil. Ecological dose (ED₁₀) values indicated that Sb concentrations of 24.86-184.00 mg·kg^-1 caused a 10 % reduction in extracellular enzyme TEI, whereas higher Sb concentrations (29.63-1791.86 mg·kg^-1) were needed to inhibit 10 % of extracellular urease activity. Overall, TEI of extracellular enzymes emerges as a more sensitive and reliable indicator of long-term Sb contamination. This study provides essential insights for monitoring Sb pollution and provides a basis for establishing a soil Sb pollution assessment and early-warning system.