Effects of pesticides on soil microbial community structure and nitrogen transformation in tobacco fields affected by root rot

The results revealed that pesticide application significantly changed the content of nitrogen forms and their transformation rate. T1 and T2 treatments significantly increased the accumulation of ammonium nitrogen (NH₄ ⁺-N), while T2 and T4 markedly promoted the accumulation of nitrate nitrogen (NO₃ ^--N). Microbial community analysis indicated that the T2 and T4 treatments significantly affected the microbial structure. Analysis of xenobiotic degradation pathways showed that multiple pathways were suppressed by the four pesticide treatments, with the T2 treatment exhibiting the broadest suppressive effect. Metagenomic analysis further revealed that the T2 treatment promoted the accumulation of both NH₄ ⁺-N and NO₃ ^--N by up-regulating the mineralization gene (<i>gdh</i>) and nitrification genes (<i>hao</i> and <i>nxrAB</i>), while the T4 treatment facilitated NO₃ ^--N accumulation by up-regulating nitrification genes (<i>hao</i> and <i>nxrAB</i>). Correlation network analysis uncovered relationships between key nitrogen cycle genes and microbial genera, showing that nitrification genes (<i>hao</i> and <i>nxrAB</i>) in the T2 and T4 treatment groups exhibited positive correlations with <i>Nitrobacter</i> and <i>Nitrosovibrio</i>. This research clarifies the pathways through which these four pesticides influence the soil nitrogen cycle, providing an important theoretical basis for their ecological risk assessment and rational application.