Dynamics of Polysaccharides in Soils and Its Modelling.
Delphine Derrien, Laboratoire Sols et Environnement, ENSAIA - BP 12, 2, avenue Foret de Haye, Vandoeuvre les Nancy, 54505, France, Christine Marol, Laboratoire d'Ecologie Microbienne de la Rhizosphère, CEA Cadarache, St Paul lez Durance, 13108, France, and Jérôme Balesdent, INRA / Laboratoire d'Ecologie Microbienne de la Rhizosphère, CEA Cadarache, St Paul lez Durance, 13108, France.
Up to now, the simulation models of soil organic carbon dynamics have been poorly related to the chemical composition of soil carbon. There is a clear need to conciliate both representations for at least two reasons. On one hand, the residence times of carbon in soils are suspected to be intimately related to its chemistry, through intrinsic recalcitrance of molecules or organo-mineral interactions. On the other hand, the chemical nature of bulk soil organic matter is determined by dynamic parameters such as flows of biosyntheses and compound specific reaction rates, including humification, biodegradation etc. We built a compound-specific model of the dynamics soil carbon and parametrized it for the carbohydrate component. The turnover of carbon in neutral carbohydrates was measured using the natural 13C labelling in an experimental wheat/maize chronosequence extending from 1 through 23 years. The isotopic composition of individual neutral monosaccharides was determined by GC-IRMS, in hydrolysed particle-size fractions, after trimethylsilylation. In each particle-size fraction, the age distribution of neutral sugar carbon was very similar to that of total soil carbon: few years in particulate organic matter and about one century in <50µm particle size fraction. The rate of biosynthesis of microbial sugars was also determined experimentally using various 13C-labelled substrates and medium term incubations. Based on these experiments we proposed a model of carbohydrate dynamics, respecting the frame of a validated model of soil organic carbon dynamics, the RothC model. It introduces specific parameters concerning individual plant carbohydrates degradability, microbial carbohydrates biosyntheses and carbohydrates preservation in soils. It reproduces nicely the great features of carbohydrates fate in soil: their contribution to soil organic matter decreases during the decomposition process, whereas the microbial sugars signature increases. Such modelling offers new perspectives in the mechanistic approach of the carbon sequestration in soils.