Saturday, 15 July 2006

Physical and Physico-Chimical Stabilization of Organic Matter in the Subsoil.

Christophe Moni, Abad Chabbi, Cornelia Rumpel, and Claire Chenu. INRA, UMR Bioemco, bātiment EGER, 78850 Thiverval-Grignon, France

Very few studies deal with organic matter stabilisation in subsoil, a compartment that represents 2/3 of the soil C stocks and displays a very high potential for organic matter storage. Compared to the topsoils, subsoils are depleted in organic matter but show high Mean Residence Time (MRT). The goals of the current study are (1) to quantify the stocks and the mean residence time of organic carbon in soil down to two meters depth, (2) to monitor the influence of different vegetal cover on the OM distribution in subsoil and, (3) to evaluate the relative importance of physical and physico-chemical stabilisation processes in two loamy soils. Some pedologicals parameters that may exert a control on the carbon stabilisation (e.g. texture, pH, CEC, exchangeable cations, mineralogical composition of clay, and the content in Fe, Al, Si amorphous and crystallised) were measured. The soils had developed from loess and are located at the Institut National de la Recherche Agronomique (INRA) research stations of Versailles (north-western France) and Lusignan (south-western France). The Versailles station displays a 12 years old chronosequence of C3-C4 plants that allowed us to follow the OM dynamics in soil through the stable isotopic signature of carbon. The soils of Lusignan that have been developed on a tropical paleosoil, allowed us to follow both the influence of a strong vertical structuration of the profiles, and different vegetal cover (culture, grassland, fallow), on the organic matter stabilisation in deep horizons. Soils were sampled from surface up to two meters depth. The carbon stocks estimated for all profiles ranged from 118 t C.ha-1 to 173 t C.ha-1. The average C stock of cultivated soils is 124±6 t C ha-1. These soils contain less carbon than the uncultivated soils that store in average 139±37 t C ha-1. However carbon stocks under grassland and fallow display a much more important variability, probably due to the heterogeneous vertical structuration of the soil. For each profile, more than half of the organic carbon was stored in the subsoil (i.e. under 30 cm depth). At Versailles in the soil under corn the OM isotopic signature decreased with depth and became more negative below 1m depth than that of the soil under wheat. This suggests that the OM isotopic signature is controlled by a process different from the simple succession (C3-C4) of the vegetal cover. The fluctuation of a water table around 80 cm depth might induce that process. At Versailles, after 12 years of corn cultivation, 11% of corn carbon have been incorporated within soil organic matter in the plough horizon. That value decreases quickly with increasing depth. Good correlations were found between the soil carbon content and the vertical distribution of Mn3+(R2=0,98), Na+(R2=-0,83), K+(R2=0,82), and silt (R2= 0,70). Whereas no significant correlation had been observed with the soil Al and Fe oxide/hydroxide content, neither with clay.

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