Carbon sequestration and nitrogen loss drive the evolution of French forest soils

The temporal change of soil chemistry in the forest floor and mineral soil down to a depth of 40 cm was assessed for the 102 permanent plots of the French Network for the Monitoring of Forest Ecosystems (RENECOFOR), over a 15-year period (from 1993-1995 to 2007-2012). In examining the separate and joint evolutions of a large set of parameters, many significant changes were detected reflecting the fact that French forest soils were not in a steady state. A significant increase in soil organic carbon (SOC) stocks was found, mainly in the surface soil (13.0% increase over the forest floor and the 0-10 cm layer). Conversely, the relative increase of the total nitrogen (N-tot) stocks was lower in the surface soil (4.8% increase), and a general and sharp decline of N-tot was detected between 10 and 40 cm depth (12.0% decrease). These results led to a substantial raise of C/N ratio over the whole soil profile. Another major finding is the difference in soil acidification recovery depending on the initial trophic level. In highly acidified contexts (top soil pH H2O < 4.5), increased soil acidification (pH and base saturation decrease, exchangeable Al increase) over the profile was observed while exchangeable base cation (Ca, Mg, K) pools increased. On the other hand, less acidic soils saw their global buffer capacity enhanced. These observations contrast with what is measured in other European inventories. While a previous study carried out on the same plots and over the same period highlighted SOC as a major driver of soil evolution in the top mineral soil, the possible mechanisms behind the large N decrease in the lower mineral soil remain to be confirmed.