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

The intimate association of soil microorganisms with the soil matrix compli cates analysis of their metabolism, since thorough separation of intact cel ls 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' C-13 NMR, which can be used in a noninvasive manner. [U-C-13] glucos e was added at a concentration of 1.73 mmol C g(-1) dry organic matter, whi ch is known to allow maximal substrate induced respiration (SIR), and the m icrobial metabolism of the added C was followed over a period of 28 days. T he data showed that similar to 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 triacylglyce rols were being formed, probably located in oil droplets. During days two t o three, around 40% of the consumed glucose C was allocated into solid stat e components, about 40% was respired and about 20% was found as triglycerol s. The triacylglycerol signal reached a maximum after 13 days, but subseque ntly declined by 60%, as the triacylglycerols were apparently consumed, by day 28 of the incubation. Our results indicate there was an initial formati on 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 mos t active organisms in the glucose degradation. (C) 2001 Elsevier Science Lt d. All rights reserved.