Multi-source apportionment and speciation drivers of heavy metals in the desert-oasis ecotone

We systematically evaluated soil heavy-metal contamination, speciation, ecological risks, and sources across the arid desert-oasis ecotone along the southern Tarim Basin margin (Northwest China). Total concentrations and sequential fractions of nine metals (As, Cr, Cu, Hg, Mn, Ni, Pb, V, Zn) were determined. An integrated framework combined probabilistic risk indices (Nemerow integrated pollution index, NIPI; potential ecological risk index; modified Nemerow integrated ecological risk index, mNIER; Monte Carlo), fraction-based risk (risk assessment code, RAC; ratio of secondary phase to primary phase, RSP), speciation drivers (partial least squares path modeling, PLS-PM; redundancy analysis, RDA), and source apportionment (absolute principal component score-multiple linear regression, APCS-MLR). Regionally, only Hg and Zn exceeded standards, and metals exhibited strong spatial heterogeneity. As, Cr, Cu, Mn, Ni, Pb, V and Zn were dominated by the table residual state fraction (F4) with a conserved hierarchy of F4 > the reducible fraction (F2) > the weak acid extractable fraction (F1) > the oxidizable fraction (F3), whereas Hg showed more labile partitioning and the lowest residual share. Soil physical properties, especially specific surface area and clay-silt fraction, primarily controlled heavy metal retention and fractionation, whereas sand played an inhibitory role. Multi-model assessments indicated generally low ecological risk, but Hg reached moderate risk in parts of the ecotone. Source apportionment attributed metals mainly to natural weathering (64.80 %), plus contributions from coal combustion (7.08 %) and traffic activities (8.25 %). Coal-emission control, low-disturbance farming, and roadside barriers are recommended to mitigate Hg input, metal activation, and traffic deposition, respectively.