Terrigenous clay-fixed inorganic nitrogen (IN) constitutes a massive marine sedimentary pool, yet the lack of mechanistic constraints on its isotopic signature (δ¹⁵N_IN) causes persistent interpretative ambiguities in paleoceanography. To resolve this, we present a systematic source-to-sink investigation into the environmental controls and isotopic fractionation of the IN system. Here, we analyzed 26 modern soil profiles across a steep environmental gradient in Taiwan, validating these terrestrial signals against a 29-kyr sedimentary record (Core MD012403) from the southern Okinawa Trough. Our results demonstrate that IN in soils is primarily sequestered within K-bearing 2:1 phyllosilicates, geochemically inferred to be dominated by illite. Crucially, rather than directly responding to macroscopic climatic parameters (temperature and precipitation), both IN and δ¹⁵N_IN (ranging from 2.6‰ to 6.1‰) are simultaneously governed by coupled weathering and oxidation processes. Intense chemical weathering triggers the structural degradation of mineral lattices, which diminishes their overall ammonium retention capacity (lowering IN content). Concurrently, this degradation drives an isotopic fractionation process: the preferential desorption and subsequent biogeochemical consumption of ¹⁴N leaves the shrinking residual fixed-nitrogen pool progressively enriched in¹⁵N. This modern pedogenic mechanism is robustly validated by the marine sedimentary record, which demonstrates a high source-to-sink fidelity, where the temporal evolution of δ¹⁵N_IN has persistently mirrored independent chemical weathering indices despite major Late Quaternary provenance shifts. By validating this isotopic distillation process, this study establishes δ¹⁵N_IN as an independent geochemical proxy for reconstructing continental weathering history from marginal sea sedimentary archives.