Substrateinduced Respiration

The substrate-induced respiration (SIR) method estimates the amount of C held in living, heterotrophic microorganisms by measuring the initial respiration after the addition of an available substrate (Anderson and Domsch, 1978). In general, soil samples are placed into airtight containers and then amended with glucose. Evolved CO2 is followed for several hours (no proliferation of microorganisms should occur under these conditions). The initial respiratory response is proportional to the amount of microbial C present in the soil sample. Applying the following conversion factor, derived from the calibration of substrate-induced respiration to the chloroform fumigation incubation technique, results can be converted to milligrams of biomass C, y = 40.04x + 0.37, where y is biomass C (mg 100 g "1 dry wt soil) and x is the respiration rate (ml CO2 100 g_1 soil h_1). Respired CO2 can be measured by use of an alkali trap followed by titration or by GC analysis of the headspace gas. The optimum concentration of glucose leading to the maximal initial release of CO2 has to be independently determined for each soil type and should be applied to that soil to standardize the SIR method between different soil types. The SIR method using titrimetric measurement of CO2 is frequently applied because it is simple, fast, and inexpensive. A disadvantage of the static systems with alkaline absorption of evolved CO2 is that the O2 partial pressure may change, causing overestimations in neutral or alkaline soils. Nevertheless, most versions of the three methods for estimating micro-bial biomass (CFI, CFE, and SIR) gave identical ranking from a range of 20 arable and forest sites in an interlaboratory comparison (Beck et al., 1997).

Using selective antibiotics, the SIR approach can also be used to measure the relative biomass of fungi and bacteria in the soil microbial community. Glucose-induced respiration is determined for fungi in the presence of streptomycin, which inhibits prokaryotes, and for bacteria in the presence of cycloheximide (actid-ione), which inhibits eukaryotes. An automated infrared gas analyzer system is used to continuously measure CO2 produced, and a computer program is used to calculate the bacterial/fungal respiration based on the following criteria: (1) proof of no unselective inhibition and (2) proof of no shifts in the biosynthesis rates of bacteria and fungi in favor of one group (Baath and Anderson, 2003).

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