LAUSR

As forest ecosystems generally develop on poor and unfertilized soils, nutrient cycles and input/output budgets, notably in the humus compartment, are key to determining forest sustainability. Previous studies generally focused on the ecosystem scale or only on C, N, P in the humus layer. Our objectives in this study are to establish these budgets for principal elements, considering dust inputs and, for the first time, particulate outputs. To achieve these objectives, the inputs and outputs of the elements in litterfall, particulates (dust inputs and particulates in the outgoing solution) and solution were quantified in mulls in two contrasting soils in beech ecosystems monitored for 7 years. The concentrations of the elements in the particulate outputs were 3 or 16 times higher than those in dust, because some particles were produced in the humus. The input/output budgets were balanced for K, Na and S in both soils and for P, Si and Mg in one soil. These elements were transported in the humus mainly as a solution or as a soluble form in plant tissues. For Ca and Mn in both soils and for P, Si and Mg in one soil, the elements found in biominerals, the inputs were higher than the outputs, indicating that other outputs were not taken into account in this balance, such as biomineral sedimentation or uptake by fungi. These results show the importance of the humus particles in element cycles.