Harmonized Sequential Extraction Parameters for Soil Organic Matter Fractions Across Contrasting Land-Use Soils: Optimization and Mass-Balance Assessment

Fine fractionation of soil organic matter (SOM) underpins the interpretation of element speciation in different SOM pools, yet sequential extraction methods are mostly optimized for single soil types, limiting inter-study comparability. We optimized extraction time and concentration for six SOM fractions—exchangeable SOM (Ex), humic acid (HA), fulvic acid (FA), iron-bound humin (HMi), clay-bound humin (HMc), and insoluble humin (HMr)—using forest, vegetable, and farmland soils from Lincang, Yunnan, and characterized the extraction residues by FTIR, SEM, and EEM analyses. Optimal parameters fell within 8–12 h and 0.5–1.0 mol L−1: Ex, 1 mol L−1 NH4Ac (pH 5.0, 12 h); HA/FA, 1 mol L−1 NaOH + 0.05 mol L−1 Na4P2O7 (12 h); HMi, 0.5 mol L−1 H2SO4 (8 h), then 0.5 mol L−1 NaOH + saturated Na2SO4 (12 h); HMc, 1 mol L−1 H2SO4–HF (1:1, v/v, 12 h), then the same alkali extraction. Total SOM extraction rates reached 90.05% ± 1.12% to 96.97% ± 1.27% (n = 3); residues showed progressive stripping from weakly to strongly bound fractions, and fraction distributions differed consistently among land-use types. This framework provides a methodological basis for cross-soil SOM fractionation.