Effects of pyroxsulam on soil enzyme activity, nitrogen and carbon cycle-related gene expression, and bacterial community structure

Pyroxsulam is a new herbicide which is being widely used to prevent the growth of broad-leaved weeds in wheat fields because of its low quantity of application and high efficiency. However, its ecotoxic effects on soil microorganisms have yet to be clarified. In the present study, the effects of pyroxsulam on soil enzymes, nitrogen and carbon cycle functional genes, and bacterial community structure in the soil were investigated. The effect of pyroxsulam on urease and dehydrogenase activities was not obvious at the end of the experiment (56 d), but β-glucoside activity was consistently promoted. At the enzyme level, the effect of different concentrations of pyroxsulam on soil enzymes was within an acceptable range. Functional gene expression analysis indicated that the expression of the nitrogen-fixing gene (nifH), nitrifying gene (amoA), and carbon fixation gene (cbbL) was inhibited by three different doses of pyroxsulam, and the inhibition was most effective at 2.5 mg kg−1. Therefore, the 2.5 mg kg−1 pyroxsulam dose had adverse effects on functional genes, and application should be controlled. Combined with the results of soil bacterial community structure analysis, we concluded that pyroxsulam inhibited soil nitrogen fixation, nitrification, and carbon dioxide fixation and promoted soil denitrification. In addition, pyroxsulam increased the abundance of beneficial bacteria with the ability to inhibit the spread of soil diseases and degrade organic pollutants. According to the integrated biomarker response calculations, the green-like cbbL gene was the most sensitive indicator of pyroxsulam toxicity. A high concentration of pyroxsulam may have a negative impact on soil microorganisms in different aspects and dosage should therefore be carefully considered.