Stimulation of Microbial Degradation of Methyl Bromide in Soil during Oxidation of an Ammonia Fertilizer by Nitrifiers

To reduce volatilization of the fumigant methyl bromide (MeBr) from soil into the atmosphere, attempts were made to enhance microbial degradation of MeBr in soil by stimulating the activity of soil nitrifiers. Disappearance of MeBr in limed Arredondo soil (pH 7.70) treated with an ammonia-based nitrogen fertilizer, (NH4)2SO4, was initially more rapid than in unlimed Arredondo soil (pH 5.5−5.7). Disappearance of MeBr in limed soil with or without treatment of (NH4)2SO4 that received 20 μg/g MeBr was more rapid than in the corresponding soil samples that received a higher rate of MeBr (50 μg/g). Due to higher nitrification activity in limed surface soil (0−15 cm depth) than in limed subsurface soil (15−30 cm depth), disappearance of MeBr in the surface soil with or without (NH4)2SO4 treatment was also more rapid than in the corresponding subsurface soil. Both microbial and chemical degradation were involved in the MeBr degradation in soil, with chemical degradation possibly being the major factor. Contribution from microbial degradation was greater in soil treated with (NH4)2SO4 than in untreated soil, and up to 57% of MeBr degradation was attributed to microbial degradation, mainly by the activity of nitrifiers during the oxidation of ammonia. Inoculation of an ammonia oxidation bacterium, Nitrosomonas europaea ATCC 19718, to soil greatly stimulated the initial degradation of MeBr. In conclusion, stimulation of MeBr degradation in soil can be achieved through liming and application of an ammonia fertilizer as well as through inoculation of an ammonia oxidation bacterium. Consequently, the rate of MeBr flux into the atmosphere may be reduced after fumigation is completed.