ORIGINAL ABSTRACT

Glyphosate is widely used in agriculture and often co-applied with urea. However, the impact of urea on glyphosate degradation and its potential soil toxicity remains unclear. This study investigated the effects of urea co-application (240 kg pure nitrogen per hectare) on glyphosate degradation and related ecological risks through laboratory soil experiments. Results showed that urea extended the half-life of glyphosate by 1.99 times. Co-application altered soil available nitrogen levels, reduced the abundance of microbes including Pseudomonas and Bacillus, and significantly decreased the relative abundance of glyphosate degradation genes such as goxB. Partial least squares structural equation modeling (PLS-SEM) confirmed that soil available nitrogen content was the main factor affecting glyphosate residues. Co-application also reduced β-D-cellobiosidase activity, hindered organic carbon decomposition, and disrupted the coupling of soil carbon and nitrogen cycles. Urease and leucine aminopeptidase activities were inhibited, nitrogen-cycling microbes such as Methylomirabilota declined, and nitrogen availability decreased. Microbial network complexity increased under co-application, but resilience declined, indicating reduced soil ecological stability. This study reveals the mechanisms by which urea affects glyphosate degradation and provides theoretical support for the rational application of urea and glyphosate.