Enhanced degradation of the volatile fumigant-nematicides 1,3-D and methylbromide in soil

The use of the gaseous fumigant-nematicide methyl bromide in agriculture is scheduled to be phased out in the year 2001. 1,3-Dichloropropene- (1,3-D) in combination with chloropicrin and an herbicide is considered to be a via ble alternative to methyl bromide for some crops. 1,3-Dichloropropene consi sts of two isomers, cis- and trans-1,3-D. A number of soil bacteria have be en shown to initially degrade 1,3-D or one of its isomers, cis-1,3-D, via h ydrolysis. Until recently, the degradation of cis- and trans-1,3-D in soils was considered to exhibit similar kinetics, with their degradation rates i ncreasing with increases in soil temperature. Enhanced degradation of 1,3-D in soil from a site in Florida with a history of repeated annual applicati ons of 1,3-D was observed in 1994. Biological hydrolysis was involved in th e initial degradation of cis- and trans-1,3-D. The two isomers were degrade d at different rates, with the trans isomer being degraded more rapidly tha n the cis isomer. Cis and trans-1,3-D in soil from the control site were de graded at a similar rate but more slowly than in the enhanced soil. Methyl bromide in soils can be degraded through chemical hydrolysis and methylatio n to soil organic matter. Some methanotrophic bacteria and ammonia-oxidatio n bacteria during the oxidation of their primary substrates (methane and am monia) also have the capacity to cooxidize methyl bromide to formaldehyde a nd bromide ion. It was recently observed that degradation of methyl bromide was stimulated in methanotrophic soils and in soils treated with ammonium sulfate. Soil methanotrophic bacteria and soil nitrifiers are apparently re sponsible for cooxidation of methyl bromide in methanotrophic and ammonia t reated soils, respectively.