Heating-Induced Redistribution and Isotopic Fractionation of Soil Organic Carbon Among Density Fractions

Fire-induced transformation and isotopic fractionation of soil organic carbon (SOC) among density fractions remain poorly understood when investigating SOC turnover in postfire vegetation recovery. To specifically focus on the heating-induced processes, laboratory-controlled pyrolysis of forest soils was studied in a temperature gradient (simulating fire intensities) by combining density fractionation, molecular biomarker, and δ¹³C analysis. Results showed that increasing heating intensity reduced SOC content, enhanced carbon aromatization, and generated substantial pyrogenic carbon (PyC). The free light fraction (fLF) exhibited higher SOC loss and lower PyC yield compared to the heavy fraction. Preferential loss of light isotopes (¹²C) enriched ¹³C in residual pools, elevating δ¹³C in bulk soil from -26.0‰ to -21.8‰. The most pronounced ¹³C enrichment occurred in fLF due to extensive SOC loss, and this enriched carbon was readily solubilized into dissolved organic matter (DOM). Notably, the isotopic fractionation during heating significantly exceeded typical microbial-induced fractionation of <3‰. DOM extracted from soils heated at 400 °C featured aromatic and phenolic-C structures, indicating PyC origins. In contrast, DOM from the 550 to 700 °C treatments contained mostly carboxyl and carbonyl-C, derived from highly oxidized SOC. These ¹³C-enriched components intensified fractionation between DOM and residual organic carbon. This study clarifies mechanisms of fire-driven SOC redistribution and isotopic fractionation, highlighting the critical role of wildfire in soil carbon cycling.