SIDE-EFFECTS OF HERBICIDES ON THE SIZE AND ACTIVITY OF THE SOIL MICROFLORA - DNOC AS A TEST-CASE

Recent advances in measuring soil microbial biomass by chloroform fumi gation-extraction (CFE) and microbial heterotrophic activity interpret ed by quantitative concentration-activity relations (QCAR) have renewe d interest in assessing side-effects of agricultural chemicals on soil microorganisms. We have studied the effects of a herbicide, 4,6-dinit roorthocresol (DNOC), taken as a test chemical, on the rate of microbi al carbon turnover and the size of the soil microbial biomass, We used the CFE technique in combination with in-situ labelling of the soil b iomass. Exposure of prelabelled soil samples to the herbicide resulted in a significant increase in (CO2)-C-14 production during which the r adioactive carbon content of the microflora decreased exponentially wi thout apparent reduction in the size of the biomass. The extra product ion of (CO)-C-14 by DNOC-treated soil over control, or carbon-enriched soil, is the expression of an increased rate of endogenous metabolism to compensate for shortage in energy caused by a decoupling of ATP ge neration at the oxidative phosphorylations level by the DNOC. To asses s the influence of DNOC on soil microbial communities we also compared the advantages of short-term respirometric tests with those resulting from application of heterotrophic activity measurements in connection with QCAR. Both procedures detected modifications in the metabolic be haviour of soil microorganisms when faced with chemical stress, Short- term respirometric tests showed that DNOC causes a decrease in the res pirometric activity of the soil microflora. Measuring heterotrophic ac tivity also makes it possible to interpret microbial responses in term s of changes in the physiological traits of the microbial communities, DNOC provokes an apparent enrichment in microorganisms with a smaller saturation constant, K-m and, as a consequence, a greater affinity fo r carbon substrates.