Quantifying the impact of agriculture on silicon riverine exports in the Critical Zone

Water plays a fundamental role in the Critical Zone driving chemical weathering, releasing nutrients, and sustaining ecosystem productivity. In natural systems, river chemistry reflects the balance between solutes dissolved from bedrock and those sequestered by mineral precipitation and/or ecosystem uptake. However, human activities like agriculture have disrupted this balance, altering solute cycling. Silicon (Si), a crucial nutrient that supports primary productivity in terrestrial and aquatic ecosystems, is particularly impacted by agriculture. Crop harvesting alters the fluxes of dissolved and biogenic Si in soils, rivers, and ecosystems. Despite its importance, the effects of these changes on riverine solute exports remain largely unexplored. In this study, we investigated the Si isotope composition (δ³⁰Si) and germanium-silicon (Ge/Si) ratio dynamics across Critical Zone compartments—soil, bedrock, water, and vegetation— within the Kervidy-Naizin agricultural catchment, France. We observed a vertical gradient in δ 30 Si across the water pool in the Critical Zone, from lighter groundwater (δ 30 Si = 0.56 ± 0.25 ‰) to heavier soil solutions (δ 30 Si = 1.50 ± 0.22 ‰). This gradient reflects depth-dependent processes: mineral weathering and clay precipitation control δ 30 Si signatures in groundwater, while plant uptake and crop removal significantly enrich δ³⁰Si in soil solutions. Using a mass balance that combines δ 30 Si and Ge/Si ratios, we quantified Si export from the catchment as plant material, both natural and harvested. Additionally, we employed two independent methods to assess Si export through agricultural harvesting: an elemental mass balance based on riverine chemistry and suspended sediments, and a method incorporating isotope fractionation factors and soil Si loss indices. an elemental mass balance based on riverine chemistry and suspended sediments, and a method incorporating isotope fractionation factors and soil Si loss indices. Our results show that plant-mediated Si export—both natural and harvested—is the largest Si flux from the catchment, accounting for ~74 % of the Si solubilized from rock and exceeding dissolved Si export by 3.2 to 5.4 times. Through two independent methods, we estimated that harvesting alone accounts for 37 ± 10 % to 50 ± 19 % of total Si export, depending on crop species, with the harvesting flux being 1 to 4 times greater than the dissolved Si flux. These findings suggest that agricultural activities significantly reduce riverine Si exports, potentially altering downstream ecosystems where Si availability regulates primary productivity. By quantifying Si fluxes and exploring their drivers, this study provides critical insights into the interplay between water partitioning, nutrient cycling, and solute transport in the Critical Zone, highlighting the transformative effects of agriculture.