Long-term changes in alpine pedogenetic processes : effect of millennial agro-pastoralism activities (French-Italian Alps)

Human activities are known to modify soil properties; however, the associated modifications to soil processes are poorly documented, as they must be studied over long time scales. Lake Verney, which is on the Italian side of the Petit Saint-Bernard Pass in the French-Italian Alps (2188 m a.s.l.) provides a sediment record of the last 11,000 cal. yrs BP. Analysis of multiple proxies within this sediment sequence, including sedimentological characteristics, mineral geochemistry (as determined using XRF and extractable Fe fractions), pollen and non pollen palynomorphs (NPPs) and sediment DNA (sedDNA) analysis, is compared with analyses of current soils and paleosols within this mountain ecosystem in order to understand the main drivers of long-term pedogenesis. We performed principle component analysis on both the sediment proxies and the soil geochemical properties to identify different sediment endmembers that reflect different types of soil horizons, mainly stagnic and spodic. These horizons are characteristic of specific soil processes and their associated land uses. During the first part of the Holocene, a decrease in the carbonate fraction in the sediment reflects the loss of carbonate material from soils that occurred as early as postglacial vegetation became established. The migration of Fe-complexes until 6000 cal. yrs BP indicates the development of Podzols in the catchment. The first signs of human land use are detected at 4300 cal. yrs BP according to analyses of sedDNA and NPPs. Increases in the input of terrestrial organic matter, associated with forest clearance suggests degradation of the surface horizons of the Podzols. Erosion increased during the Roman Period due to sheep grazing. Then, while erosion was still increasing, Podzols developed into Stagnosols after the Middle Ages with cow grazing which is consistent with the current functioning of the soils. The history of the paleosols and archaeological stratigraphy within the study area confirm the model of soil evolution inferred from the lake sediments and allow us to characterize the human-induced "metapedogenetic phase" of the evolution of the soil cycle. Anthropo-pedogenesis may define the development of soils during the Anthropocene. The main consequence of this change in the functioning of the soils is a reduced sequestration of soil carbon.