Soil organic carbon drives the spatiotemporal variations of heavy metal contents in burned taiga forest soils

Forest fires profoundly alter soil heavy metal cycling, yet their long-term spatiotemporal evolution and coupling mechanisms with soil organic carbon (SOC) remain unclear. To address this gap, we established a chronosequence in a Greater Khingan Mountains taiga forest, encompassing four fire severities across four recovery periods. We analyzed total and available contents of nine heavy metals using multivariate models and spectroscopy. Results revealed significant spatiotemporal asynchrony between total and available metals. Total contents peaked during middle-term recovery, with Mn and As reaching 2.84 and 4.21 times the unburned control, inducing moderate ecological risks. Conversely, available fractions exhibited heterogeneous trajectories: Cu and Zn peaked immediately post-fire, whereas Hg surged during long-term recovery. Variance partitioning demonstrated that recovery time (39.5%) and fire severity (20.2%) co-dominated these variations, shifting from early-stage pH-activation to subsequent organic-biological controls. Crucially, middle-term severe fires triggered massive SOC accumulation (109.47 g/kg), characterized by significantly enriched oxygen-containing functional groups and heightened humification. Consequently, the availability of strong-complexing metals (Cu, Pb, Hg) correlated negatively with macromolecular aromaticity indicators (P < 0.01), confirming that metal speciation is governed by organic matter molecular quality rather than mere quantity. Ultimately, carbon quality evolution drives strong complexation, fundamentally decoupling high total metal loads from low ecotoxicological availability. Post-fire environmental assessments must therefore transcend traditional total-content paradigms, shifting towards comprehensive frameworks integrating speciation availability and carbon pool dynamics for precise risk management.