Effect of Soil Properties on Degradation and Sorption of Methyl Bromide in Soil

Methyl bromide (CH,Br) is currently the most widely used soil fumigant, and its emission into the atmosphere after application reportedly contributes to ozone depletion in the stratosphere. Irreversible degradation and partially reversible sorption reactions affect the quantity of this fumigant reaching the soil surface and escaping into the atmosphere. Incubation studies in closed headspace vials under controlled conditions showed that degradation of CH,Br was highly dependent on soil organic matter content, and to a lesser extent, on the moisture level in the soil. Methylation of CH,Br on organic matter was suggested to be the major reaction that CH,Br undergoes in the soil environment. Other soil constituents such as clay did not contribute to the degradation under moist or air-dried conditions, though enhanced degradation was observed on oven-dried montmorillonite and kaolinite clays. Within soil profiles, degradation of CH,Br decreased with soil depth mainly due to the reduction of soil organic matter content with depth. In both Greenfield and Wasco sandy loams, the degradation rate of CH,Br in soil layers from 0 to 270 cm could be estimated from soil organic matter content. Sorption of CH,Br on moist soils was generally limited, and varied with soil depth. The degree of sorption could be predicted from soil moisture alone or soil moisture and organic matter content.