Response of Enzyme Activities to Agricultural Soil Fumigation with Methyl Bromide and Alternative Biocides and Implications for Agricultural Sustainability.

Preplant soil fumigation with methyl bromide (MeBr) has been used widely around the world to control insects, nematodes, weeds, and pathogens in many vegetable, fruit, nut, ornamental and nursery crops. The stringent regulations limiting the use of MeBr in recent years prior to its complete phase-out stimulated the use of alternative biocides because soil fumigation remains a central tool in these production systems. Little is known about the impacts of MeBr and alternative biocides on soil microbial processes. A sandy loam soil was fumigated in microcosms for 24 h with methyl bromide and chloropicrin (MeBr + CP), propargyl bromide (PrBr), combinations of 1,3-dichloropropene and CP (InLine), iodomethane and CP (Midas), an emulsifiable concentrate of CP (CP-EC), or methyl isothiocyanate (MITC). The effects of these pesticides on selected enzymatic activities were evaluated in fumigated soils and a nonfumigated control at 1, 3, 7, 14, 21, 28, and 90 d post-fumigation. Soil enzyme activities in fumigated microcosms were significantly (P≤ 0.037) different from the nonfumigated soil, with the exception of β-glucosidase in soils treated with PrBr and MITC, and dehydrogenase in MeBr + CP-fumigated soils. Over the 90-d study, soil fumigation (average of all fumigants and sampling dates) reduced the activities of arylsulfatase (62%), dehydrogenase (35%), acid phosphatase (22%), and β-glucosidase (6%), suggesting that S mineralization in soils and the total oxidative potential of microorganisms were more affected by fumigation than P and C mineralization. This study also indicates that soil fumigation with MeBr + CP alternative biocides has the potential to alter important key reactions involved in nutrient transformation, and thus, the long-term productivity of agricultural soils.