Microbial biosyntheses of individual neutral sugars among sets of substrates and soils

The presence of neutral carbohydrates in soil organic matter (SOM) in significant amount is apparently in contradiction with their lability. It may result either from stabilization or from continuous recycling of SOM by micro-organisms and biosynthesis of microbial sugars. To address this question, it was necessary to precisely determine the average yield of sugar biosyntheses and the distribution of individual sugars. We performed incubations of 13C labelled substrates and quantified biosyntheses of neutral carbohydrates using compound-specific isotope ratio mass spectrometry. To investigate the natural variation of the microbial biotransformations, we varied the soil characteristics on the one hand and the substrate on the other. Incubation kinetics reflected the existence of two pools of carbohydrate metabolites. One pool was characterized by its lability with a mean residence time of about two weeks, whereas the second was preserved from biodegradation. The yield of sugar production by soil micro-organisms was poorly affected by the quality of the substrates glucose, cellulose and glycine, and very similar distributions of individual sugars were observed (Glc > Gal = Man = Rham > Xyl = Ara = Fuc). Soil type affected sugar biosyntheses slightly more, reflecting some diversity of either microbial communities or ecophysiology among soils. These results provided important indications to further calibrate a comprehensive model of carbohydrate dynamics in soils.