Carbon Fractions Associated with Silt‐Size Particles in Surface and Subsurface Soil Horizons

Abstract

Information on the distribution and behavior of C fractions in soil particle sizes is crucial for understanding C dynamics in soil. At present little is known about the behavior of the C associated with silt‐size particles. We quantified the concentrations, distribution, and enrichment of total C (TC), readily oxidizable C (ROC), hot‐water‐extractable C (HWC), and cold‐water‐extractable C (CWC) fractions in coarse (63–20‐μm), medium (20–6.3‐μm), and fine (6.3–2‐μm) silt‐size subfractions and in coarse (2000–250 μm) and fine (250–63 μm) sand and clay (<2‐μm) soil fractions isolated from bulk soil (<2 mm), and 2‐ to 4‐mm aggregate‐size fraction of surface (0–25 cm) and subsurface (25–55 cm) soils under different land uses. All measured C fractions varied significantly across all soil particle‐size fractions. The highest C concentrations were associated with the <20‐μm soil fractions and peaked in the medium (20–6.3‐μm) and fine (6.3–2‐μm) silt subfractions in most treatments. Carbon enrichment ratios (ERC) revealed the dual behavior of the C fractions associated with the medium silt‐size fraction, demonstrating the simultaneous enrichment of TC and ROC, and the depletion of HWC and CWC fractions. The medium silt (20–6.3‐μm) subfraction was identified in this study as a zone where the associated C fractions exhibit transitory qualities. Our results show that investigating subfractions within the silt‐size particle fraction provides better understanding of the behavior of C fractions in this soil fraction.