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.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 201

Imprint of chemical weathering and hydrothermalism on the Ge/Si ratio and Si isotope composition of rivers in a volcanic tropical island, Basse-Terre, Guadeloupe (French West Indies)

A significant portion of the disproportionally high chemical weathering flux in volcanic island arcs may originate from hydrothermal fluid-rock interaction, thereby compromising the accurate estimate of atmospheric CO2 consumption rates. The objective of this study is to evaluate how the riverine Ge/Si ratio and Si isotopes, two well-established tracers of weathering, respond to hydrothermal inputs. The work took place in Basse-Terre, Guadeloupe, a tropical volcanic island with a dense river network, high chemical weathering fluxes and various hydrothermal surface manifestations. We characterized the Ge/Si ratio and δ30Si of 15 thermal springs, nine non-impacted (NI) rivers and 13 hydrothermally-impacted (HI) rivers. The soil solution from a highly weathered soil profile (Ferralsol) and a clayey-rock corresponding to the material exposed in an extinct hydrothermal system were also measured. A new purification method was successfully developed in order to allow the reliable measurement of Si isotopes in SO42−- and Cl−-rich thermal spring and HI river waters by mass spectrometry. Basse-Terre's thermal springs have variable Ge/Si ratios (0.05–21.03 μmol.mol−1) and δ30Si (0.71–1.50‰), but with no apparent relationship to the water compositional type. The Ge/Si ratio (0.15–2.57 μmol.mol−1) and Si isotope composition (0.26–1.21‰) values of the NI rivers reveal differences in the watersheds' weathering degree. Dissolution of Ge- and 28Si-rich secondary minerals explains the high Ge/Si and isotopically light composition of the northern NI rivers draining strongly weathered terranes. The Ge/Si ratio and δ30Si values measured for the NI and HI rivers overlap, implying that they cannot be used to diagnose hydrothermal contributions to river basins unambiguously. However, when combined with the Cl− and SO42− concentrations, the analysis of Ge and Si in the HI rivers suggests that water seeping through an extinct hydrothermal system produces SO4-rich drainages with distinctively lower Ge/Si ratios than those inferred for watersheds receiving thermal spring discharges associated with an active hydrothermal system. Overall, our results provide new constraints for applying and interpreting Ge/Si and Si isotope measurements to study weathering in volcanic environments

The impact of land use changes on water pathways, soil formation and soil functioning

status: publishe

Kinetically limited weathering at low denudation rates in semi-arid climates

On Earth, the Critical Zone supports terrestrial life, being the near-surface environment where interactions between the atmosphere, lithosphere, hydrosphere, and biosphere take place Quantitative understanding of the interaction between mechanical rock breakdown, chemical weathering, and physical erosion is essential for unraveling Earth’s biogeochemical cycles. In this study, we explore the role of soil water balance on regulating soil chemical weathering under water deficit regimes. Weathering rates and intensities were evaluated for nine soil profiles located on convex ridge crests of three mountain ranges in the Spanish Betic Cordillera. We present and compare quantitative information on soil weathering, chemical depletion and total denudation that were derived based on geochemical mass balance, 10Be cosmogenic nuclides and U-series disequilibria. Soil production rates determined based on U-series isotopes (238U, 234U, 230Th and 226Ra) are of the same order of magnitude as 10Be-derived denudation rates, suggesting steady state soil thickness, in two out of three sampling sites. The chemical weathering intensities are relatively low (∼5 to 30% of the total denudation of the soil) and negatively correlated with the magnitude of the water deficit in soils. . Soil weathering extents increase (nonlinearly) with soil thickness and decrease with increasing surface denudation rates, consistent with kinetically limited or controlled weathering. Our study suggests that soil residence time and water availability limit weathering processes in semi-arid climates, which has not been validated previously with field data. An important implication of this finding is that climatic regimes may strongly regulate soil weathering by modulating soil solute fluxes

Has Land Use/Cover Change affected the soil weathering degree in Southern Brazil?

status: publishe