Enzyme activities in agricultural soils fumigated with methyl bromide alternatives

Pre-plant fumigation of agricultural soils with a combination of methyl bromide (MeBr) and chloropicrin (CP) to control nematodes, soil-borne pathogens and weeds has been a common practice in strawberry (Fragaria X ananassa Duchesne) production since the 1960s. MeBr will be phased out by 2005, but little is known about the impacts of alternative fumigants on soil microbial processes. We investigated the response of microbial biomass and enzyme activities in soils fumigated over two years with MeBr+CP and the alternatives propargyl bromide (PrBr), InLine, Midas and CP. Results were compared to control soils, which were not fumigated for the last 4-5 years for Watsonville and Oxnard, respectively, but had a 10 year history of MeBr+CP fumigation (history soils). Soil samples (0-15 cm) were taken from two sites in the coastal areas of California, USA, in Watsonville and Oxnard, at peak strawberry production after two years of repeated application. In addition to the soil enzymes, the activities of purified reference enzymes of β-glucosidase, acid phosphatase and arylsulfatase were assayed before and after fumigation with MeBr+CP and alternative biocides. At the Oxnard site, microbial respiration significantly decreased in soils fumigated with MeBr+CP (P=0.036), while microbial biomass C and N showed no response to fumigation at both sites. These results may indicate that fumigation promotes the growth of resistant species or that soil microorganisms had recovered at the time of sampling. Repeated soil fumigation with MeBr+CP significantly decreased the activities of β-glucosidase and acid phosphatase at the Watsonville site, and dehydrogenase activity at the Oxnard site. Although, enzyme activities in soils fumigated with PrBr, InLine, Midas and CP were lower compared to the control soil, effects were, in general, not significant. Fumigation with MeBr+CP and alternatives reduced the activities of purified reference enzymes by 13, 76 and 28% for acid phosphatase, β-glucosidase and arylsulfatase, respectively. Mean enzyme protein concentrations in fumigated agricultural soils were 2.93, 0.105, and 2.95 mg protein kg⁻¹ soil for acid phosphatase, β-glucosidase and arylsulfatase, respectively, all lower than in control soils. Organic matter turnover and nutrient cycling, and thus, the long-term productivity of agricultural soils seem unaffected in soils repeatedly fumigated with PrBr, InLine, Midas and CP.