The dynamics of neutral sugars in the rhizosphere of wheat.: An approach by 13C pulse-labelling and GC/C/IRMS

Rhizodeposition, i.e. the release of carbon into the soil by growing roots, is an important part of the terrestrial cycle. However the in situ nature and dynamics of root-derived carbon in the soil are still poorly understood. Here we made an investigation of the latter in laboratory experiments using (CO2)-C-13 pulse chase labelling of wheat of (Triticum aestivum L.). We analyzed the kinetics C-13-labelled carbon and more specially C-13 carbohydrates in the rhizosphere. Wheat seedlings-soil mesocosms were exposed to (CO2)-C-13 for 5 hours in controlled chambers and sampled repeatedly during two weeks for C-13/C analysis of organic carbon. After a two-step separation of the soil from the roots, the amount of total orgranic C-13 was determined by isotope ratio mass spectrometry as well as the amounts of C-13 in arabinose. fructose, fucose, glucose, galactose. mannose. rhamnose and xylose. The amount and isotopic ratio of monosaccharides were obtained by capillary gas chromatography coupled with isotope ratio mass spectrometry (GC/C/IRMS) after trimethyl-silyl derivatization. Two fractions were analyzed: total (hydrolysable) and soluble monomeric (water extractable) soil sugars. The amount of organic C-13 found in the soil, expressed as a percentage of the total photosynthetically fixed C-13 at the end of the labelling period. reached 16% in the day following labelling and stabilised at 9% after one week. We concluded that glucose under the form of polymers was the dominant moietie of rhizodeposits. Soluble glucose and fructose were also present. But after 2 days, these soluble sugars had disappeared. Forty percent of the root-derived carbon was in the form of neutral sugars. and exhibited a time-increasing signature of microbial sugars. The Composition of rhizospheric sugars rapidly tended towards that of bulk soil organic matter.