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Chemical weathering of volcanic rocks in warm and humid climates contributes disproportionately to global solute fluxes. Geochemical signatures of solutes and solids formed during this process can help quantify and reconstruct weathering intensity in the past. Here, we measured silicon (Si) and germanium (Ge) isotope ratios of the soils, clays, and fluids from a tropical lowland rainforest in Costa Rica. The bulk topsoil is intensely weathered and isotopically light (mean ± 1σ:δ³⁰Si = −2.1 ± 0.3‰,δ⁷⁴Ge = −0.13 ± 0.12‰) compared to the parent rock (δ³⁰Si = −0.11 ± 0.05‰,δ⁷⁴Ge = 0.59 ± 0.07‰). Neoforming clays have even lower values (δ³⁰Si = −2.5 ± 0.2‰,δ⁷⁴Ge = −0.16 ± 0.09‰), demonstrating a whole‐system isotopic shift in extremely weathered systems. The lowland streams represent mixing of dilute local fluids (δ³⁰Si = 0.2 − 0.6‰,δ⁷⁴Ge = 2.2 − 2.6‰) with solute‐rich interbasin groundwater (δ³⁰Si = 1.0 ± 0.2‰,δ⁷⁴Ge = 4.0‰). Using a Ge‐Si isotope mass balance model, we calculate that91 ± 9% of Ge released via weathering of lowland soils is sequestered by neoforming clays,9 ± 9% by vegetation, and only0.2 ± 0.2% remains dissolved. Vegetation plays an important role in the Si cycle, directly sequestering39 ± 14% of released Si and enhancing clay neoformation in surface soils via the addition of amorphous phytolith silica. Globally, volcanic soilδ⁷⁴Ge closely tracks the depletion of Ge by chemical weathering (τ_Ge), whereasδ³⁰Si and Ge/Si both reflect the loss of Si (τ_Si). Because of the different chemical mobilities of Ge and Si, aδ⁷⁴Ge‐δ³⁰Si multiproxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation.