Abstract: Extractable carbon and nitrogen fractions are key components in soils and organic materials, directly influencing soil quality and biogeochemical cycling through their contents and variability. We investigated nine soils and six organic materials that were extracted using two extractants, 0.01 M CaCl₂ (weak extraction) and 0.5 M K₂SO₄ (strong extraction), to compare the variations in extractable organic carbon (EOC) and organic nitrogen (EON). The results showed that: (1) EOC and EON concentrations in soils ranged from 13.1 to 390.8 mg·kg^-1 and 3.3 to 30.3 mg·kg^-1, respectively. On average, CaCl₂ and K₂SO₄ extracted EOC accounted for 79%±22% and 88%±16% of total extractable carbon (TEC), while EON represented 10%±10% and 20%±14% of total extractable nitrogen (TEN), respectively. (2) Organic materials exhibited significantly higher EOC (1.2-77.1 g·kg^-1) and EON (0.4-8.5 g·kg^-1) levels, with EOC/TEC (96%-97%) and EON/TEN (71%-85%) markedly exceeding those in soils. (3) There was a significant negative correlation between SUVA₂₅₄ (specific ultraviolet absorbance at 254 nm) and EOC/EON ratios, indicating that low C/N extractable organic components were enriched in aromatic structures. (4) In soils, CaCl₂ extracted EOC and EON accounted for 19%±8% and 44%±20% of K₂SO₄ extracted amounts, respectively, suggesting that weak extraction preferentially mobilized low C/N fractions and that strong extraction released more high C/N fractions. For organic materials, the absence of stabilization mechanisms resulted in negligible differences in extraction efficiency between the two extractants. This study elucidates variations in extractable C/N fractions based on extractant strength, providing methodological insights for the precise assessment of soil C and N turnover and its associated eco-environmental effects.

Key words: soil, organic material, extractable organic carbon, extractable organic nitrogen, extractant type