KINETICS OF SUBSTRATE-INDUCED RESPIRATION (SIR) - THEORY

The substrate-induced respiration (SIR) is a widely used physiological method for measurement of the soil microbial biomass. The equation dp /dt = re(mu t) + K respiration rate dp/dt after substrate addition in the SIR method represents the sum of the respiration rates of growing (re(mu t)) and nongrowing microorganisms (K). According to the equatio n SIR = r + K at the time of substrate addition, where r is the initia l respiration rate of growing microorganisms. The equation is applied to examine the effects of soil moisture content and the antibiotic cyc loheximide on SIR and its components. The SIR, K and mu increased with increasing soil moisture content while r decreased. The SIR was unaff ected while K increased as much as r decreased, i.e. a shift from a gr owing to a nongrowing state occurred at the concentration of cyclohexi mide used and at two moisture levels. It is proposed that the qualitat ive division of the soil microflora into autochthonous and zymogenous populations is quantified by the equation. The equation can be used in terrestrial test systems of natural and anthropogenic effects and mec hanisms on the microorganisms. It gives information on treatment effec ts on existing enzymes (r and K) and on enzyme synthesis as reflected in growth (mu). It will also be possible to form new ecophysiological constants in addition to the ones used today for assessing effects on the soil microflora and functional roles of its different parts.