Plants produce a wide array of toxic secondary metabolites that serve functions for plant protection against different stressors and in the competition with neighboring plants for resources. Experimental evidence shows that phytotoxins are widely occurring in the environment, and thus of concern as emerging aquatic micropollutants. This work aims at adapting the soil-water-plant-atmosphere model DAISY for describing biomass and phytotoxin production of a non-crop plant growing in monoculture, transfer of phytotoxin to the soil and fate and leaching of the toxin in the soil. Throughout the work, bracken fern (Pteridium aquilinum L. Kuhn) and its derived carcinogenic toxin, ptaquiloside (PTA), has been used for model development and optimization.
This study is the first to report a two-year monitoring of bracken biomass growth, PTA production in bracken, release by precipitation and concentrations in soil solution. Results from the monitoring show that PTA is produced by bracken in amounts exceeding 1 g m-2. Moreover, the toxin content present in the biomass is washed off by precipitation events in high amounts, with observed values up to 13 mg released per square meter during a single rain event. Even though PTA degrades rapidly in the soil, observed concentration in soil solution at 50 cm depth show that PTA can leach in form of pulses of high concentrations connected to precipitation events. Concentrations in the soil solution were in the order of microgram per liter, three orders of magnitude above maximum tolerable concentrations determined by previous studies. Hence, the presence of bracken might pose a risk towards water quality, both surface and groundwater.
The calibrated model DAISY shows a good description of bracken biomass and PTA contents observed in the field, indicating that toxin production can be explained by biomass and development stage. Wash off by precipitation was the process with the highest uncertainty, showing discrepancies between simulated and observed values. Differences are explained by the high variability in the wash off process between different events due to the combined effect of environmental factors. Simulations show that preferential transport contributes to 47% of the overall mass of PTA being leached, explaining the pulse of concentrations observed in the field during summer. Results from this study shows that the adapted DAISY model represents a flexible and suitable platform for modelling the fate of phytotoxins