SOIL MICROBIAL BIOMASS ESTIMATED BY FUMIGATION EXTRACTION AND SUBSTRATE-INDUCED RESPIRATION IN 2 PESTICIDE-TREATED SOILS

An incubation experiment was performed with two soils and five pesticide treatments (benomyl, isoproturon, simazine, dinoterb and chloroform). Microbial biomass C and N was estimated by fumigation-extraction (FE). Two modifications of the substrate-induced respiration (SIR) method were used: (1) a continuous flushing system and (2) a static system with soil slurry. In addition, CO2 production was measured. Estimates of biomass C by the two SIR methods were generally higher than those obtained by the FE method. The two SIR measurements and the two biomass estimates were closely correlated, indicating a similar response to pesticide treatments. Pesticide application almost always reduced the size of the microbial biomass. Of the different ways of measuring biomass, the FE method revealed the largest number of significant differences from the nil treatment. The biomass C-to-N ratio was only slightly influenced by soil or pesticide treatments. Apart from the benomyl treatment, pesticide application always increased soil respiration during the 0-10 day incubation.