Occurrence and Mass Balance of Isoflavones on an Experimental Grassland Field

Isoflavones and coumestrol (COU) are estrogenic compounds that are naturally produced by plants (e.g., red clover, soybeans). Although these compounds have been extensively studied in food and feed, only little is known about their environmental fate. Therefore, we investigated the occurrence of isoflavones (formononetin, daidzein, equol, biochanin A, and genistein) and COU over 3.5 years in red clover, manure, and soil of a grassland field with and without manure application, as well as their emission via drainage water. Isoflavones were regularly quantified in plant (≤15 × 106 ng/gdry weight (dw)), manure (≤230 × 103 ng/gdw), soil (≤3.4 × 103 ng/gdw), and drainage water samples (≤3.6 × 103 ng/L). In contrast, COU was observed only in manure and soil. Cumulative isoflavone loads emitted via drainage water were around 0.2 × 10–3 kg/ha/y, which is very little compared to the amounts present in red clover (105–220 kg/ha/y), manure (0.5–1.0 kg/ha/y), and soil (0.1–5.1 kg/ha/y). Under good agricultural practice, no additional emission of isoflavones into drainage water was observed after manure application. With calculated 17β-estradiol equivalents up to 0.46 ng/L in drainage water, isoflavones can constitute a dominant and ecotoxicological relevant portion of the total estrogenicity in small rural river catchments.