Plant species produce a wide range of toxic secondary metabolites (phytotoxins). In the last decade, there has been an increased interest in the fate of natural toxins in environment, thus several of the toxins have been detected in the soil, soil water, drainage and surface water. Among the secondary metabolites, alkaloids are the most abundant metabolites. However, the environmental distribution and fate of phytotoxins and natural alkaloids are largely unknown. The presence of alkaloids need a concern, as they may have unintended effects on various organisms and contaminate drinking water resources.
The aim of the PhD project was to study the environmental fate of natural alkaloids - phytotoxins, with a focus on locate alkaloids producing vegetation areas, explain the occurrence of the toxins in the plants, soil and surface water, in relation to seasonal and microclimate variation. For that, alkaloids from ragwort (Jacobaea vulgaris), butterbur (Petasites sp.), lupin (Lupinus spp.) and soybean [Glycine max (Merr.) L.] were targeted and their environmental fate investigated.
In Denmark, impacted plant sites were selected to investigate the environmental transport of pyrrolizidine alkaloids (PA)s from the plants (ragwort and butterburs) to soil and proximal water. PAs were continuously detected and quantified, where ragwort was the source and expected to contain average 506 kg PAs/ha, their load in soil and pond - surface water were 1.7 kg/ha and 5 g/10000 L, respectively, Manuscript I. In addition, in surface water and shallow groundwater wells (2.2–3.0 m depth), adjacent to butterbur plants PAs were detected, the total concentrations were 0.53 and 0.23 μg/L, respectively. However, in deep groundwater (~60 m depth) no PAs were detected, Manuscript II.
Agricultural field experiments on lupin conducted in Denmark and Switzerland, where the quinolizidine (QA)s and indole alkaloids (IA)s contents were investigated in the plant, soil, soil, pore water, and drainage water, Manuscript III and VII. Overall, during the field experiments, lupin biomass and alkaloid contents increased toward the harvest season. In lupin field-Switzerland, lupin plant biomass and density were 22.9 g (dry weight (dw) per plant) and 30±4 per m2, respectively, this lead to have 6.9x103 kg dw/ha of lupin production and 4.0 to 11.1 kg/ha of total alkaloids in lupin, per season. During the field experiment 0.55 kg/ha of total alkaloids transported to topsoil (10 cm), also cumulative loads emitted via drainage water were around 0.2-9 mg/ha for individual alkaloids, and 13 mg/ha for total alkaloids.
To understand the distribution of phytotoxins in soybean-impacted rivers and streams, a network of six sampling sites were studied in Iowa. Chosen streams covered a range of basin size of 12–32,400 km2. In the water samples collected from September 2019 to March 2020, alkaloids and phytoestrogens were detected with concentrations ranged from no detection to 37 and 40 ng/L, respectively; alkaloids detected more frequently. They were transported from soybeans field to soil and to proximal surface water, Manuscript IV.
Laboratory experiments showed PAs distribution coefficient for Freundlich (KF) were 0.4-2.3 L/kg for sandy soil, and 0.5-3.4 L/kg for sandy loam soil. Their sorption capacities increased with decreasing soil pH and increasing soil organic matter content, Manuscript V. In addition, during the degradation experiments their half-lives at 5 °C were ranged from 5.1 to 9.2 days for sandy and sandy loam soil, respectively. The results indicate PAs are stable and sorbed weakly to soils, likely to be transported to field drains and ultimately enter surface water.
As a part of the project, an analytical method was developed, to quantify alkaloids in environmental samples. The method is based on sample preparation, and chromatographic separation which coupled to high-resolution mass spectrometry. The liquid chromatography tandem mass spectrometry (LC-MS/MS) method allowed direct and simultaneous quantification of alkaloids for the first time in the environmental samples. The method limit of detection (LOD) for alkaloids ranged from 0.001 μg/L to 7 μg/L, Manuscript VI and Paper I.