Responses of soil enzymes, bacterial communities and soil nitrification to the pre-emergence herbicide pyroxasulfone

Pyroxasulfone is a relatively new herbicide that is sprayed on soils to control grassy weeds and some broadleaf weeds during the cultivation of agronomic crops. However, information regarding its environmental risks to soil ecosystems is currently limited. Here, the response of soil characteristics and soil bacterial communities to pyroxasulfone exposure were evaluated. The rate of pyroxasulfone degradation in soil decreased with increasing herbicide concentration, and its half-life at doses of 0.12 (the recommended field rate), 1.2 and 12 mg kg^-1 was estimated to be 15.75 d, 39.46 d and 78.08 d, respectively. Soil pH markedly increased after pyroxasulfone treatment. Pyroxasulfone significantly inhibited urease activity but had a small effect on soil sucrase activity. In the late stages of degradation, the abundance of bacteria clearly decreased in soils treated with pyroxasulfone at doses of 1.2 and 12 mg kg^-1. Compared with the control group, a distinct decrease in bacterial network complexity was observed at a pyroxasulfone dose of 0.12 mg kg^-1, while the opposite phenomenon was observed at a pyroxasulfone dose of 12 mg kg^-1. The copy numbers of the AOA amoA and AOB amoA genes exposed to 10- and 100-fold the recommended rates of pyroxasulfone were significantly lower than those in soils without pyroxasulfone residue at 25 and 60 days after treatment. In summary, pyroxasulfone at the recommended rate had a slight effect on soil enzymes, the bacterial community and soil nitrification; however, the potential adverse impacts of pyroxasulfone at higher concentrations on these soil factors deserve further attention.