Controls of the spatial variability of Cr concentration in topsoils of a central French landscape

The aim of this study is to determine the factors that control the spatial variability of chromium in the topsoil at the landscape scale by using a combination of both individual soil pit informations and systematic soil sampling analyses. The 11 ha study area, located in Massif Central (France), includes contrasted bedrock and topography.In a first approach we determined the relation between the concentrations in chromium and other major elements reflecting the soil pedogenic processes and some particular minerals indicating the original materials the soils are derived from. For this purpose, we used data collected at the catena scale from 10 soil profiles in two toposequences matching all geological and topographic situations. In a second approach, we studied the spatial variability of Cr in both topsoil and alterite through a systematic sampling study.Chromium concentrations range between 16 and 355 mg kg− 1 in the alterite, and 31 to 129 mg kg− 1 in the topsoil. Chromium concentration in alterite presents a complex spatial distribution governed principally by the geological origin of the alterite. Two major differences between chromium concentration in the alterite and in the topsoil may be deduced. First, topsoil chromium distribution depends on topography. Increased weathering together with selective erosion of the topsoil resulted in smaller chromium concentration in the concave compared to the convex area. Secondly, chromium concentration in the topsoil varies spatially much less than in the alterite. Such differences can probably be observed in similar geomorphic conditions and for other chemical elements that can be considered as geochemically immobile through weathering.