Fertile soil is the basis for our diet. However, agricultural use can accelerate the erosion of the soils. In the European Alps, this has left particularly intensive traces in the past 3800 years, as rock samples from a French mountain lake show. According to this, mountain farmers destroyed the soil through agriculture and pasture keeping, which had previously formed through natural processes for millennium.
On our planet, erosion and soil formation are two complementary natural processes that are strongly controlled by the climate and ideally balancing in the long term. In some areas, especially in the mountains, the surface is removed faster than new soil can form. It is known that in addition to the climate, agriculture also contributes strongly to the erosion of the soils. But how big is your influence in the mountains?
Sediments reveal erosion phases in the Alps
Researchers around William Rapuc from the University of Savoie Mont-Blanc in Chambéry have now examined this based on the European Alps. To do this, they took sediment samples from the bottom of the Lac de Bourget – a large mountain lake in the French Alps, which formed after the last ice age. In it, eroded soil gathered from catchment areas around the Mont Blanc massif. In addition, the Rhône sediments flooded back into the higher lake during flood. In the rock samples up to 9,500 years old, the geologists searched for lithium isotopes and compared them in the individual sediment layers of the lake and with soil samples from the traces of the lake, which are more recent. The lithium isotopes provide information about the phases in which weathering and erosion mainly took place and when new soil formed.
The rock analyzes showed: “During the early phase of the records, from 9500 to 6200 years ago, physical erosion remained low and relatively stable,” said the team. At that time, the climate in the Alps was warm and dry, soil formation and erosion kept the scales. Then the climate became colder and wet, the erosion increased slightly. With the end of the ice age, however, this trend turned back, new soil was created. In the last 3800 years there have been three phases of strong erosion, which almost seamlessly merged into one another and in which the erosion continued to increase. Overall, the erosion has been four times as quickly since then as the simultaneous new ground formation. The soil was removed so much that the rock is now almost again in its “initial state” of the series of measurements around 10,000 years ago. The natural balance from the assembly and dismantling of the soil was nailed.
Fear keeping and agriculture as the main driver
The erosion of the past 3800 years is clearly due to the agricultural activities of the mountain residents, as the team explains. During this time, human action became the main driver and accelerator of erosion, no longer the climate. In the Iron Age, these were initially the pasture economy and forest drainage in the higher altitudes over 2000 meters in order to facilitate the movement in the terrain. Since the Roman period around 2200 years ago, agriculture has been added in medium and lower locations, with increasingly modern devices such as plowing. This additionally accelerated the erosion in the Alps. The landscape changes through these processes were so great that the researchers describe the three phases together as the epoch of the “pedological anthropocene”.
The findings show that humans can have a greater influence on erosion in the mountains than the climate. “By separating the respective contributions of bioclim development and human land use from each other, we provide a clearer understanding of soil courses in mountain environments – a pattern that probably occurs in many mountain regions worldwide,” the researchers write. Comparisons with further samples confirm, for example, that the same man -made erosion process also appeared in the Andes when agriculture increased there about 2400 years ago. This connection also has an impact on global soil fertility, biodiversity and the water and carbon cycle. With the knowledge, however, new agricultural conventions could now be developed, with which the mountains could better protect worldwide and the mountainous arable and meadow country could be used more sustainably in the future.
Source: William Rapuc (University of Savoie Mont-Blanc) et al.; Proceedings of the National Academy of Sciences, DOI: 10.1073/PNAS.2506030122
