Degradation Pathways and Environmental Fate of Selected Herbicides in Irrigated Farmland Soils of the Benue Valley, North Central Nigeria

Repeated herbicide applications in the intensively irrigated farmlands of the Benue Valley generate soil residue accumulation and potential leaching to groundwater sources used by farming communities, yet the specific degradation pathways, transformation products, and environmental fate of commonly used herbicides under the tropical soil and climatic conditions of North Central Nigeria remain insufficiently studied. This study investigates the degradation kinetics, transformation products, and environmental fate of atrazine and glyphosate herbicides in irrigated alluvial soils from the Benue Valley, Benue State, North Central Nigeria. Laboratory incubation experiments are conducted at two temperatures (25 and 35 degrees Celsius) and two moisture conditions (field capacity and one-half field capacity) using soil microcosms spiked with individually labelled herbicide concentrations. Herbicide dissipation is monitored by liquid chromatography-tandem mass spectrometry at seven time intervals over ninety days. Transformation products are identified by non-target high-resolution mass spectrometry screening. Soil sorption parameters are determined from batch Freundlich isotherm experiments and expressed as Koc values. Leaching potential is evaluated using the FOCUS groundwater modelling framework parameterised with measured soil properties. Microbial community dynamics in response to herbicide application are monitored by terminal restriction fragment length polymorphism analysis. Findings fill an important knowledge gap on tropical soil-specific herbicide fate and support the development of pesticide risk assessment models applicable to the Nigerian regulatory context. Keywords: herbicide degradation, atrazine, glyphosate, Benue Valley, environmental fate