Soil organic matter composition controls neonicotinoid transport and accumulation to benthic macroinvertebrate communities in agricultural drains.

Abstract

Neonicotinoids are widely used in agriculture to manage insect pests; however, their application can harm the well-being of aquatic animals and non-target insects in surrounding ecosystems. While previous studies have identified relationships between neonicotinoid transport and soil chemistry in agricultural ecosystems, there is a lack of research exploring how the molecular properties of organic carbon in soils affect neonicotinoid mobility. In addition, there is even less research exploring the interconnected dynamics between neonicotinoid transport and nontarget biological endpoints, such as benthic macroinvertebrates. In this study, soil, stream sediment, and benthic macroinvertebrates were collected monthly across agricultural and naturalized land cover in Southern Ontario from April to October 2023 to quantify neonicotinoid levels in agricultural watersheds. Fourier transform ion cyclotron resonance mass spectrometry was used to characterize the molecular composition of water extractable organic matter in soil and sediment samples, and three neonicotinoids were detected using a triple quadrupole mass spectrometer. Thiamethoxam concentrations peaked early in the sampling period, reaching 6.27 ± 0.31 ng g^-1 in April before steadily declining, coinciding with reductions in scrapers-grazer functional feeding groups. Conversely, thiamethoxam levels in stream sediments exhibited a delayed peak, rising to 6.82 ± 0.94 ng g^-1 in later summer months, suggesting mobilization from soils in as little as three months after application. Overall, thiamethoxam emerged as the most frequently detected neonicotinoids in both soil and sediment samples and a significant positive correlation was observed between WEOM and thiamethoxam (Spearman ρ = 0.803, p = 0.022). Accumulated thiamethoxam is also likely transported by phoshrpus and sulphur-containing WEOM from soils to sediments, where thiamethoxam correlated with significant decreases in scraper-grazers population in stream sediments. Together, our results identify molecular signatures within the WEOM that can serve as indicators of enhanced thiamethoxam transport from soils to sediments and highlight the rapid effects of neonicotinoids on benthic macroinvertebrate communities.