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The interaction effect of two sulfonylurea herbicides, bensulfuron-methyl (B) and metsulfuron-methyl(M), were tested on microbial biomass C, N, N-mineralization and C/N ratio in a loamy sand soil. The herbicides were applied at various levels of: control (B0M0), 0.01and 0.01 (B1M1), 0.01 and 0.1 (B1M2), and 0.01 and 1.0 (B1M3) μg/g soil. Determinations of soil microbial biomass-C, N and Nmineralization contents were carried out at 1, 3, 5, 7, 10, 15, 25 and 45 days after herbicides application. The results showed that the soil microbial biomass-C (Cmic) and microbial biomass-N (Nmic) decreased consistently with the increasing rates of herbicides. The results further indicated that B1M1 and B1M2 caused a significant reduction in Cmic and Nmic within first 10 and 7 days of incubation, respectively, as compared with the control. These reductions in Cmic and Nmic were also significant ( P = 0.05) with B1M3 application especially within first 15days of incubation. A significant reduction in N-mineralization (N-min) was observed with high doses (B1M2, B1M3 ) of herbicides within first 5 days of incubation, while low rate (B1M1) failed to produce any significant effect. An increase in the soil microbial biomass C:N ratio was also noted.
