Objective The study was conducted to monitor the effects of imazapic on the number of culturable microorganisms, the number of herbicide-resistant microorganisms and soil enzyme activities of rice fields in Guangdong area, aiming to provide a reference basis for the effects of herbicide application on soil ecology in agricultural fields.

Method The number of culturable microorganisms and herbicide-resistant bacteria in paddy field soil under water direct-seeding and dry direct-seeding modes were monitored by plate counting method. Activities of soil urease, phosphatase, peroxidase and peroxidase were determined by sodium phenol-sodium hypochlorite colorimetric method, sodium phosphate colorimetric method, o-phenylene triol-ether extraction method and potassium permanganate titration method.

Result After planting one crop of rice, the application of imazapic enhanced soil peroxidase activity significantly by 30% under water direct-seeding mode, and increasing soil acid phosphatase and urease enzyme activities significantly by 136.2% and 9.6% respectively under dry direct-seeding mode. Under both direct-seeding modes, the imazapic had no significant influence on the number of culturable fungi. However, under water direct-seeding mode, the number of bacteria and imazapic-tolerant bacteria decreased by 28.7% and 29.5%, respectively. The total number of actinomycetes and imazapic-resistant actinomycetes decreased by 24.2% and 40.7%, respectively. Under the dry direct-seeding mode, imazapic treatment had little effect on bacteria. The total number of actinomycetes decreased by 26.7%, and the number of herbicide-tolerant actinomycetes decreased by 45.9%.

Conclusion The application of imazapic was effective in enhancing soil peroxidase activity under water direct-seeding mode, and enhancing the enzyme activities of soil acid phosphatase and urease under dry direct-seeding mode. Compared with manual weeding treatments, application of imazapic under water direct-seeding mode inhibited the growth of soil bacteria, and the application of herbicide inhibited the growth of soil actinomycetes under both water- and dry direct-seeding modes.