13C NMR spectroscopy studies of forest soil microbial activity: glucose uptake and fatty acid biosynthesis

The intimate association of soil microorganisms with the soil matrix complicates analysis of their metabolism, since thorough separation of intact cells from the matrix is very difficult using standard protocols. Thus, in the study reported here, in situ glucose decomposition and metabolism in humus from a coniferous forest soil was monitored and evaluated using ‘solution state’ 13C NMR, which can be used in a non-invasive manner. [U-13C] glucose was added at a concentration of 1.73 mmol C g−1 dry organic matter, which is known to allow maximal substrate induced respiration (SIR), and the microbial metabolism of the added C was followed over a period of 28 days. The data showed that ∼50% of the added glucose was consumed within three days, coinciding with the appearance of label in CH3, –CH2– and –CH=CH– groups, and in glycerol-carbons, suggesting that olefinic triacylglycerols were being formed, probably located in oil droplets. During days two to three, around 40% of the consumed glucose C was allocated into solid state components, about 40% was respired and about 20% was found as triglycerols. The triacylglycerol signal reached a maximum after 13 days, but subsequently declined by 60%, as the triacylglycerols were apparently consumed, by day 28 of the incubation. Our results indicate there was an initial formation of structural microbial C (solid state carbon) followed by formation of storage lipid C, which subsequently decreased, probably because it was used to provide the organisms with energy when the external energy source (i.e. the glucose) was depleted. The formation of unsaturated triacylglycerols, typical storage metabolites of eucaryotes, suggests that fungi were the most active organisms in the glucose degradation.