The Top 1.2 m as a δ 56 Fe ‐Sensitive Zone: Synergistic Regulation of Fe Cycling by Soil Texture and Land Use Intensity in Tropical Soils

Soil Fe cycling is co‐regulated by texture and land use intensity (LUI), but their synergistic mechanism and depth‐dependent effects remain unclear. We analyzed 68 samples (0–200 cm) from one cultivated (CL, high LUI, silty loam, clay = 11%–18%) and three non‐cultivated (AC1–AC3, low LUI, sandy loam, clay = 2.65%–9.00%) profiles in Thailand's Mun River Basin, using hierarchical regression, slope difference tests, and CV decomposition. Results showed: (1) Soil texture best explains Fe content variation—silty loam maintains stable high Fe (25.4–39.7 g kg −1 ) via clay adsorption, while sandy loam has variable Fe (1.1–118.5 g kg −1 ) with severe depletion in topsoil. (2) The top 1.2 m is a δ 56 Fe‐sensitive zone (CV = 35.7%), more variable than deep soil (CV = 21.8%). (3) Texture ×LUI synergy is statistically significant (Fe: Δ R 2 = 0.09, p < 0.01; δ 56 Fe: Δ R 2 = 0.13, p < 0.001). (4) Fe content (static pool) and δ 56 Fe (dynamic tracer) are complementary, with LUI acting as a “switch” for texture‐Fe relationships. This study clarifies the vertical differentiation and synergistic controls of Fe biogeochemistry in tropical soils, and strengthens the mechanistic basis for land degradation assessment and restoration in tropical agroecosystems.