Quantifying the agricultural footprint on the silicon cycle: insights from silicon isotopes and Ge∕Si ratios

Abstract. Silicon (Si) is essential for ecosystem function, supports primary productivity, and is intricately linked to the carbon cycle, which regulates Earth's climate. However, anthropogenic activities, such as agriculture, deforestation, and river damming, have disrupted the natural Si cycle, altering biogenic and dissolved Si fluxes in soils and rivers. Despite the importance of understanding and quantifying human impacts on Si cycling at local and global scales, few studies address these disruptions, leaving a critical knowledge gap. Here, we analyzed the Si isotope composition (δ30Si) and germanium-silicon (Ge/Si) ratio dynamics across various Critical Zone compartments – soil, bedrock, water and plants – within the Kervidy-Naizin agricultural catchment observatory, France. Our findings reveal a vertical gradient in δ30Si across the water pool in the Critical Zone, from lighter groundwater (δ30Si = 0.56±0.25 ‰) to heavier soil solutions (δ30Si = 1.50±0.22 ‰). This gradient reflects distinct processes: in deep groundwater, weathering and clay precipitation control δ30Si signatures, while at shallower depths, progressive plant uptake and crop removal further enrich δ30Si in soil solutions. Using a mass balance combining δ30Si and Ge/Si ratios, we quantified Si export from the catchment as plant material, both natural and harvested. Additionally, we assessed Si export from agricultural harvesting using two independent approaches: an elemental mass balance based on riverine chemistry and suspended sediments, and a method incorporating Si isotopic fractionation and soil Si loss indices. The export of plant material, including natural and harvested material, emerged as the largest Si export flux from the catchment, accounting for ∼56 % of the Si solubilized from rock and exceeding dissolved Si export by 3.5 to 5.4 times. Through two independent approaches, we estimated that 30±18 % to 50±19 % of total Si export occurs through harvesting, depending on crop species, with the harvesting flux being 1 to 4 times greater than the dissolved Si flux. Reduction in dissolved Si exports because of agriculture may have significantly impacted downstream ecosystems, where Si availability directly influences primary productivity. Our study highlights how human activities have reshaped the Si cycle in agricultural landscapes.