Long-term effects of glyphosate on substrates with varying organic matter content: an ecotoxicological assessment under laboratory conditions

The study aimed to determine the time required for glyphosate and its primary decomposition product, AMPA, to degrade in substrates with varying organic matter content, and to identify when the toxic effects of these substances diminish due to decomposition in the tested substrates. Additionally, the study investigated how nitrogen and phosphorus levels change during the decomposition of glyphosate and AMPA in relation to the organic matter content of the substrates. Ecotoxicological assessments were conducted using biotests (Phytotoxkit, Lemna , and Microtox). Chemical analyses of substrates with different organic matter contents were performed to measure glyphosate and AMPA levels, as well as various forms of nitrogen and phosphorus. The experiments were carried out under laboratory conditions over a time course from 0 to 24 months after herbicide application. It has been shown that sand is the substrate in which glyphosate and AMPA degrade most slowly (up to 24 months). The presence of undegraded glyphosate and AMPA in sand resulted in the longest-lasting toxic effects on test organisms in the bioassays conducted with this substrate. In agricultural soil, however, undecomposed glyphosate and AMPA had a toxic effect on the studied organisms only for a short period after the herbicide was applied, due to the rapid degradation of glyphosate and AMPA (3 months). There was an increase in the levels of phosphorus and nitrogen in the substrates after the herbicide was added, compared to the control. Although the experiments were conducted under laboratory conditions, the observed increase in nitrogen and phosphorus content in the tested substrates following the degradation of glyphosate and AMPA suggests that there may be potential implications for processes in the natural environment. This could be particularly relevant in areas where high doses of glyphosate have been used over many years. Additionally, it appears that in substrates with lower organic matter content, the degradation of glyphosate and AMPA might take longer, potentially leading to a more extended toxic effect on living organisms compared to substrates with higher organic matter. These observations could, in some cases, contribute to the development of increased tolerance to glyphosate in plants from such areas. However, further studies would be necessary to fully understand these effects.