Biogeochemical cycling and mineral fertility dudgets in a beech forest

In the context of climatic change and wood for energy production, there seems to be a need for an improved understanding of the way lowland beech forests function in their present distribution areas. Research in this sector is primordial, especially for beech forests with low mineral fertility, as these are, by definition, more sensitive to external disturbance. Fougères beech forest in Brittany, France, characterised by poor mineral fertility, was studied during this work The overall aim of this work was to produce an inventory of mineral cycling in Fougères beech forest, for a complete forest rotation. A chronosequence approach was used, and the stands studied were 8, 25, 80 and 145years old, thus covering the whole forest rotation. A fertility budget approach was chosen as this is a useful tool to diagnose the overall evolution of a system. The calculation of element fertility budgets and the study of nutrient cycles within the ecosystem were carried out in each stand over a period of 7 years, from 1998 to 2004. The parent materials in which the soils are developed at Fougères are poor and produce limited quantities of nutrient elements as a result of weathering. However, K and Mg fluxes released by weathering are relatively high throughout the profile, in particular the Mg flux in the zone where there is mixing of the silty loess and weathered granite saprolite. Estimated quantities of Ca and P released by mineral weathering in the 0-120 cm zone of the soils are very low. Atmospheric inputs play a very important role in the sustainability of this ecosystem with poor mineral fertility, especially for a limiting element like Ca. Most of the atmospheric inputs of nutrient elements diminished between 1998 and 2004 and this reduction was also visible in the quantities of elements in circulation in the soils at Fougères. If this tendency continues over time, the fertility of the ecosystem could suffer. Nutrient losses by drainage, at depths of 120 cm, are similar whatever the stand age. Moreover, we have demonstrated that the effect of beech stand age on available stocks present in the humus and the soils is not significant. The low fertility levels in the ecosystem at Fougères may explain these observations. Mineral element fertility budgets are positive for Mg, K and P, whatever the stand age in the chronosequence. For Ca, the fertility budgets are positive in the young, 8- and 25-year-old stands, but are negative in the older, 81- and 140-year-old stands. Losses related to biomass harvest, which increase strongly with stand age, are responsible for the negative Ca budgets in the stands over 50 years old. The calculated mineral element fertility budgets, show that losses linked to biomass harvest explain the majority of the changes with stand age observed in these budgets.