Sei. Géol., Bull., 50, 1-4, p. 123 -129, Strasbourg, 1997

VALIDATION OF SOIL ORGANIC CARBON IN THE HIGH RESOLUTION BIOSPHERE MODEL

Johannes HOFFSTADT1, D.M. HOWARD2 and Peter J. A. HOWARD2

Soil organic carbon in the model

The soil organic carbon (SOC) pool is connected to the biospheric carbon cycle through input and output fluxes (fig. 1). Part of the carbon stored in litter is turned into atmospheric C02 by decomposition processes. The rest reaches the soil, filling the SOC pool, where it is more or less slowly converted into C02. The conversion rate depends on temperature and humidity similar to the litter decomposition, but also on how much oxygen is available in the soil. When peats are saturated with water, which may vary during the seasons, there is a lower, anoxic layer, where no decomposition to C02 is possible. This layer, fed from the oxic layer above, accumulates carbon up to a saturation level, a process which may take several thousand years (IMMIRZI et ai, 1992). Under anaerobic conditions, methanogenesis takes place instead.

In the ESCOBA project, the HRBM is concerned with events on a smaller time scale (decades). The model does not account for two distinct peat layers. Instead, the decomposition of the single SOC pool is drastically reduced within appropriate soils: by 80% on Histosols and by 50% on Gelic Gleysols (Esser et ai, 1994).

These percentages may seem arbitrary -they are indeed a rather old component in the model and have been used already in its predecessor, but they also have been well established by some previous tests of SOC. In relation to the Zinke database (ZlNKE et al., 1984), the agreement can be said to be good enough globally, even though there is a little shortcoming in peats of higher latitudes.

In any case, it is important to understand that wherever the HRBM lists Histosols, there will be a much higher amount of SOC due to accumulation than elsewhere.