Applying Eco-indicator Sensitivity Distribution (EcoSD) to Evaluate Chlorpyrifos Risk in Grassland Soils with Prescribed Fire and Grazing History

Prescribed fire and grazing can enhance soil ecosystem functions in grasslands but may also induce ecological stress on soil function, making soil more vulnerable to chemical stressors like pesticides, which contaminate grasslands via aerial spray drift. Therefore, this study investigated the effects of prescribed fire-grazing history and chlorpyrifos toxicity on selected soil quality parameters and ecological indicators “eco-indicators” in a fescue prairie grassland soil. Burnt soil from area subjected to prescribed fire in 2017 and annual cattle grazing and unburnt reference soil from adjacent areas subjected to grazing alone were collected in 2021 and exposed to varying doses of chlorpyrifos. Results showed no interactive effect of fire-grazing and chlorpyrifos toxicity on the measured soil quality parameters and eco-indicators. While the fire-grazing effect in the burnt soil significantly enhanced soil quality parameters like base cations and available nitrogen, it exacerbated chlorpyrifos toxicity on key soil eco-indicators like Oribatid mites and soil extracellular enzyme, acid phosphatase. Based on the Eco-indicator Sensitivity Distribution (EcoSD) framework, the burnt soil with history of fire and grazing, was generally susceptible to chlorpyrifos with ecological hazard concentrations at 5% (HC5Eco) to 50% (HC50Eco) ranging from 0.08–1.5 mg/kg compared to 0.5–4.0 mg/kg in the unburnt soil with grazing history alone. Regardless of the fire treatment, arylamidase, an enzyme crucial for nitrogen mineralization, was the most sensitive soil eco-indicator to chlorpyrifos toxicity. These findings suggest that fire, in combination with grazing, may increase the susceptibility of soil eco-indicators to chlorpyrifos toxicity, potentially due to changes in organic matter quality or increased stress from pyric byproducts.