Glyphosate and urea co-exposure: Impacts on soil nitrogen cycling

Glyphosate, the most widely utilized herbicide, frequently coexists with nitrogen fertilizers such as urea in soil environments. Nitrogen cycling is a key process for maintaining soil ecological functions and nutrient balance. However, the effects of co-exposure to glyphosate and urea on this process have remained unclear. This study investigated the impact of co-exposure to glyphosate (10 mg/kg) and urea (260.87 or 347.83 mg/kg, equivalent to 180 or 240 kg N/ha) on soil nitrogen cycling through a 98-day incubation experiment. Soil nutrients, enzyme activities, bacterial community structure, and functional genes were analyzed. NH₄⁺-N and NO₃^--N contents significantly decreased by 44.70-53.43 % and 36.74-49.12 %, respectively. Co-exposure reduced bacterial diversity and altered nitrogen cycling genes, decreasing nifH while increasing amoA and nosZ, indicating reduced nitrogen input potential and increased inorganic nitrogen loss. Enzyme analysis confirmed excessive activation of nitrification and denitrification, lowering nitrogen availability. Partial least squares structural equation modeling (PLS-SEM) showed co-exposure indirectly decreased NH₄⁺-N and NO₃^--N via enhanced nitrate and nitrite reductase activities. The study highlights the complex interactions between herbicides and fertilizers in soil environments and underscores the need for further research to understand the implications for wider soil health and crop production in agriculture systems.