Soil microbial biomass and mineralisation of soil organic matter after 19 years of cumulative field applications of pesticides

The effects of 19 years of cumulative annual field application of five pesticides (benomyl, chlorfenvinphos, aldicarb, triadimefon and glyphosate), applied at, or slightly above, the recommended rates in 25 combinations, on soil microbial biomass and the mineralization of soil organic matter were investigated. Soil samples were taken 1 month after the application of benomyl, chlorfenvinphos and aldicarb in April 1992, and again in October 1992, 1 month after the application of triadimefon and glyphosate. The addition of aldicarb caused a significant increase of 7–16% in soil microbial biomass carbon (biomass C), an effect which appeared to be persistent. This effect of aldicarb was not reflected in the mineralization rate of soil organic C, possibly because the measurements of CO2 evolution showed a greater variation than those of biomass C. Measurement of microbial biomass activyty by the substrate-induced respiration method also gave much less precise results than measurements of biomass C by fumigation-extraction. The mineralization of soil organic N to ammonium and then nitrate was mostly unaffected by the pesticide treatments. In the autumn-sampled soil, there was significantly less NH4-N in the aldicarb-treated soil. It is possible that this was due to immobilization by the increased microbial biomass in these treatments, and did not represent a loss to the soil system. The continuous use of these pesticides, either singly or in combination, therefore had no measurable long-term harmful effects on the soil microbial biomass or its activity, as assessed by C or N mineralization.