Modeling the environmental fate of the natural toxin ptaquiloside: production, release and leaching to groundwater

Plants produce a diverse array of toxic compounds which may be released by precipitation, that explains their wide occurrence in surrounding soil and water. This study presents the first mechanistic model for describing the generation and environmental fate of a natural toxin, viz.  ptaquiloside (PTA), a carcinogenic phytotoxin produced by bracken fern (Pteridium aquilinum L. Kuhn). The newly adapted DAISY model was calibrated based on two-year monitoring performed in the period 2018-2019 in a Danish bracken population located in a forest glade. Several functions related to the fate of PTA were calibrated, covering processes from toxin generation in the canopy, wash off by precipitation and degradation in the soil. Model results (2018-2019) show a good description of observed bracken biomass and PTA contents, indicating that toxin production can be explained by biomass and bracken development stage. The wash off is maximum in the middle of summer, coinciding with the moment of maximum biomass, fully developed canopy and highest PTA content. Model results show that only 4.4 % of the PTA produced in bracken is washed off by precipitation, from both canopy and litter. Once in the soil, PTA degrades rapidly, especially during summer due to the high soil temperatures. Leaching takes place in form of pulses directly connected to precipitation events, with maximum simulated concentrations up to 4.39 µg L^-1 at 50 cm depth. Macropore transport is responsible for the events with highest PTA concentrations, contributing to 72% of the total mass of PTA leached. Based on the results, we identify areas with high precipitation and soils characterized by fast transport, as the most vulnerable to surface and groundwater pollution by phytotoxins