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The herbicide chlorimuron-ethyl has been applied widely for weed control in farmland, especially in soybean fields in China over the past decade, but the chronic effects of this herbicide on soil microorganisms, particularly Pseudomonas spp., is not well understood. Taking a continuously cropped soybean field in the town of Fuyuan—a soybean production base of Heilongjiang Province in Northeast China—as a case study, soil samples were collected from plots having received 0-, 5-, and 10-year applications of chlorimuron-ethyl (30 g active component of chlorimuron-ethyl/ha/year) to study the abundance and diversity of Pseudomonas spp. Meanwhile, an in vitro assay was used to examine the antifungal activities of isolated Pseudomonas spp. against soil-borne pathogens (Fusarium graminearum, Fusarium oxysporum, and Rhizoctonia solani) causing soybean root rot disease. The production of siderophore, hydrogen cyanide (HCN), and lytic enzymes (cellulase, pectinase, and chitinase) by Pseudomonas spp. was also investigated. With 5- and 10- year chlorimuron-ethyl application, the numbers of soil Pseudomonas spp. decreased from 121 × 102 CFU/g dry soil in the control to 40 × 102 CFU/g dry soil and 13 × 102 CFU/g dry soil, and the Shannon index values decreased from 6.23 to 3.71 and 1.73, respectively. The numbers of antifungal Pseudomonas spp. also decreased, and the proportions of Pseudomonas spp. with antifungal activities against the different test pathogens altered. All the antifungal Pseudomonas spp. could produce siderophore and HCN but not lytic enzymes. The results suggest that long-term application of chlorimuron-ethyl in continuously cropped soybean field had negative effects on the abundance and diversity of soil Pseudomonas spp., including species with different antifungal activities against pathogens. Siderophore and HCN rather than lytic enzymes formed the antifungal metabolites of Pseudomonas spp., and the number of antifungal Pseudomonas that can produce siderophore and HCN decreased markedly under application of chlorimuron-ethyl, especially after 10-year application.

Chlorimuron-ethyl (CE) has been widely used in modern agriculture, but little is known regarding the influence of CE on ammonia-oxidizing bacteria (AOB) and archaea (AOA) populations in soils. In this study, microcosm incubation of aquic brown soil was conducted for 60 d. Associated changes in the population sizes of AOB and AOA in response to CE application with or without urea fertilization were examined via quantitative real-time PCR (qPCR) assays of the ammonia monooxygenase gene (amoA). The half-life of CE ranged from 11.80 d to 14.54 d in the tested soil. Compared to the untreated control, the application of CE alone had no strong effects on soil pH, and urea fertilization temporarily increased soil pH in the first 7 days. The abundance of the AOA amoA gene was greater than the abundance of the AOB amoA gene in all treatments, but both were significantly suppressed by CE application in a dose-dependent manner. Urea fertilization generally increased AOB and AOA amoA gene abundances, except that the AOA amoA gene level was slightly reduced at the early stage of the incubation period. AOB and AOA preferred different N levels for growth, with AOB only growing significantly at high NH4(+) levels and AOA growing substantially at low NH₄(+) levels. The stimulation effects of urea fertilization on AOA and AOB amoA gene abundances were strongly suppressed by the CE application. This study indicated that the CE application substantially suppressed soil nitrification via inhibiting the AOB and AOA population regardless of urea fertilization, which resulted in significant changes in the soil NH₄(+)-N and NO₃(-)-N levels. Furthermore, AOB and AOA inhabiting separate ecological niches with different NH₄(+) levels played various roles in N cycling.