The dynamics of neutral sugars in the rhizosphere of wheat. An approach by13C 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 carbon 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 13 CO 2 pulse chase labelling of wheat ( Triticum aestivum L.). We analyzed the kinetics of 13 C-labelled carbon and more specially 13 C carbohydrates in the rhizosphere. Wheat seedlings-soil mesocosms were exposed to 13 CO 2 for 5 hours in controlled chambers and sampled repeatedly during two weeks for 13 C/C analysis of organic carbon. After a two-step separation of the soil from the roots, the amount of total organic 13 C was determined by isotope ratio mass spectrometry as well as the amounts of 13 C 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 13 C found in the soil, expressed as a percentage of the total photosynthetically fixed 13 C 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.