STIMULATION OF MICROBIAL-DEGRADATION OF METHYL-BROMIDE IN SOIL DURINGOXIDATION OF AN AMMONIA FERTILIZER BY NITRIFIERS

To reduce volatilization of the fumigant methyl bromide (MeBr) from so il into the atmosphere, attempts were made to enhance microbial degrad ation of MeBr in soil by stimulating the activity of soil nitrifiers. Disappearance of MeBr in limed Arredondo soil (pH 7.70) treated with a n ammonia-based nitrogen fertilizer, (NH4)(2)SO4, was initially more r apid than in unlimed Arredondo soil (pH 5.5-5.7). Disappearance of MeB r in limed soil with or without treatment of (NH4)(2)SO4 that received 20 mu g/g MeBr was more rapid than in the corresponding soil samples that received a higher rate of MeBr (50 mu g/g). Due to higher nitrifi cation activity in limed surface soil (0-15 cm depth) than in limed su bsurface soil (15-30 cm depth), disappearance of MeBr in the surface s oil with or without(NH4)(2)SO4 treatment was also more rapid than in t he corresponding subsurface soil. Both microbial and chemical degradat ion were involved in the MeBr degradation in soil, with chemical degra dation possibly being the major factor. Contribution from microbial de gradation was greater in soil treated with (NH4)(2)SO4 than in untreat ed soil, and up to 57% of MeBr degradation was attributed to microbial degradation, mainly by the activity of nitrifiers during the oxidatio n of ammonia. Inoculation of an ammonia oxidation bacterium, Nitrosomo nas europaea ATCC 19718, to soil greatly stimulated the initial degrad ation of MeBr. In conclusion, stimulation of MeBr degradation in soil can be achieved through timing and application of an ammonia fertilize r as well as through inoculation of an ammonia oxidation bacterium. Co nsequently, the rate of MeBr flux into the atmosphere may be reduced a fter fumigation is completed.