Effect of wildfires on soil organic carbon content and carbon flow pathways: The evidence of BPCAs molecular markers and 13C natural abundance

Frequent wildfires worldwide have severely disturbed soil organic carbon (SOC) pools, yet their effects on organic carbon turnover are still under debate. One possible reason is the insufficient distinction between pyrogenic carbon (PyC) and non-PyC. In this study, soil samples were collected from six areas affected by wildfires. To distinguish PyC from non-PyC, density fractionation, benzene polycarboxylic acids (BPCAs) molecular markers, and ¹³C stable isotope analysis were combined. The results showed that following the wildfires, SOC levels in free light (fLF), occluded light (oLF), and heavy (HF) fractions increased by up to 253 %. PyC accounted for 2.02 % ∼ 13.0 % of the SOC increase. Most of the SOC increase was attributed to plant- and microbial-derived organic carbon accumulated during post-fire recovery. Based on δ¹³C analysis, the direction of organic carbon enrichment was from LF to HF. The wildfires increased the abundance of δ¹³C in bulk soils, with insignificant changes of δ¹³C in fLF and oLF, while δ¹³C in HF was significantly positive, likely because the SOC in HF is mainly the degradation product of that in fLF and oLF. The trend of carbon flow from fLF and/or oLF to HF was reduced, suggesting that the organic carbon in HF probably approached saturation. Thus, wildfires may facilitate the transition of SOC into stable carbon pools by modulating soil carbon flow pathways, which is conducive to soil carbon sequestration.