Side‐effects of herbicides on the size and activity of the soil microflora: DNOC as a test case

Abstract

SummaryRecent advances in measuring soil microbial biomass by chloroform fumigation–extraction (CFE) and microbial heterotrophic activity interpreted by quantitative concentration–activity relations (QCAR) have renewed interest in assessing side–effects of agricultural chemicals on soil microorganisms. We have studied the effects of a herbicide, 4,6‐dinitroorthocresol (DNOC), taken as a test chemical, on the rate of microbial carbon turnover and the size of the soil microbial biomass. We used the CFE technique in combination with in‐situ labelling of the soil biomass. Exposure of prelabelled soil samples to the herbicide resulted in a significant increase in 14CO2 production during which the radioactive carbon content of the microflora decreased exponentially without apparent reduction in the size of the biomass. The extra production of 14CO2 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 generation at the oxidative phosphorylations level by the DNOC. To assess 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 behaviour of soil microorganisms when faced with chemical stress. Short‐term respirometric tests showed that DNOC causes a decrease in the respirometric activity of the soil microflora. Measuring heterotrophic activity also makes it possible to interpret microbial responses in terms of changes in the physiological traits of the microbial communities. DNOC provokes an apparent enrichment in microorganisms with a smaller saturation constant, Km and, as a consequence, a greater affinity for carbon substrates.