Nitrous oxide and methane emission from a flooded rice field as influenced by separate and combined application of herbicides bensulfuron methyl and pretilachlor.

Combination of divergent active principles to achieve broad-spectrum control is gaining popularity to manage the weed menace in intensive agriculture. However, such application could have non-target impacts on the soil processes affecting soil ecology and environmental interactions. A field experiment was conducted to investigate the impact of separate and combined applications of herbicides bensulfuron methyl and pretilachlor on the emission of N(2)O and CH(4), and related soil and microbial parameters in a flooded alluvial field planted to rice cv Lalat. Single application of the herbicide bensulfuron methyl or pretilachlor resulted in a significant reduction of N(2)O and CH(4) emissions while the combination of these two herbicides distinctly increased N(2)O and CH(4) emissions. Cumulative N(2)O emissions (kg N(2)O-N) followed the order of bensulfuron methyl (0.35 kg ha(-1))<pretilachlor (0.36 kg ha(-1))<control (0.45 kg ha(-1))<bensulfuron methyl 0.6%+pretilachlor 6.0% single dose (0.49 kg ha(-1))<bensulfuron methyl 0.6%+pretilachlor 6.0% double dose (0.54 kg ha(-1)). Cumulative CH(4) emissions (kg CH(4)), on the other hand, followed the order of bensulfuron methyl (47.89 kg ha(-1))<pretilachlor (73.17 kg ha(-1))<bensulfuron methyl 0.6%+pretilachlor 6.0% single dose (93.50 kg ha(-1))<control (106.54 kg ha(-1))<bensulfuron methyl 0.6%+pretilachlor 6.0% double dose (124.67 kg ha(-1)). The inhibitory effect of separate application of herbicides bensulfuron methyl 0.6% and pretilachlor 6.0% on N(2)O emission was linked to lower mineral N, lower denitrifying and nitrifying activity and low denitrifier and nitrifier populations. Inhibitory effect on CH(4) emission, on the contrary, was linked to prevention in the drop of redox potential, lower readily mineralizable carbon (RMC) and microbial biomass carbon (MBC) contents as well as lower methanogenic and higher methanotrophic bacterial population. Admittedly, stimulatory effect of combined application of herbicides bensulfuron methyl 0.6% and pretilachlor 6.0% at double dose on N(2)O and CH(4) emission was related to reversal of the identified indicators of inhibition. Results indicate that while individual application of herbicides bensulfuron methyl 0.6% or pretilachlor 6.0% can reduce N(2)O and CH(4) emission from flooded soil planted to rice, their combined application at normal dose can keep the emission at a comparatively lower level with significantly higher grain yield as compared to the herbicides applied alone.