Impact of forest conversion to cropland on soil weathering : a geochemical study at the pedon, slope and catchment scales

Forests are under pressure as exploitable land for food production. Forest conversion to cropland often enhances erosion, with consequences for the soil system. This PhD study aims to evaluate the impact of forest conversion on soil weathering, considering the pedon, slope and catchment scales. First, the influence of erosion on soil weathering in an uncultivated setting was investigated (SE Spain). A multiproxy approach combining different weathering indices demonstrated that even with low physical erosion, the soil weathering degree decreases with an increase in denudation rate. Then, the influence of erosion on soil weathering after forest conversion to cropland was investigated in a subtropical setting (Brazil, Rio Grande do Sul). At the slope scale, the key role of slope intensity was highlighted: only in steep terrain it was observed that (i) a redistribution of pedogenic material occurred along the slope, (ii) forest conversion resulted in soil truncation by erosion and a decrease in soil weathering degree. At the pedon scale, investigation of the soil pore water chemistry supported that mineral weathering in soil is not favoured by forest conversion. Interestingly, at the catchment scale, the stream water chemistry (Ge/Si ratio) indicates a contribution from a deeper weathering signal in cropland than in forest, attributed to a higher amount of water and deeper water percolation under cropland induced by forest conversion. This case study reinforces the importance of a multiscale approach to assess the impact of forest conversion to cropland on the soil system.