Morphology of soil Zn and Mn in main forest types and potential ecological risk in the Great Xing'an Mountains

This study was designed to analyze morphological characteristics of heavy metals in soil such as Zn and Mn and to evaluate their potential ecological risk by methods of BCR and RSP (rations secondary phase and primary phase) in three typical forest types, namely, Larix gmelinii, Betula platyphylla and Pinus sylvestris var. mongolica in the Great Xing'an Mountains, northeastern China. Heavy metals were divided into the following four morphologies by BCR, extractable, bound to carbonates, bound to Fe-Mn oxides and residual morphology. RSP which indicates the degree of heavy metals pollution was divided into four levels: no pollution, light pollution, moderate pollution, and severe pollution. The value of RSP is expressed by the ratio of the soil heavy metals content (Msec) at secondary phase to that (Mprim) at the primary phase. The results showed that: soil Mn content was significantly higher than soil Zn content. Soil Mn content in L. gmelinii forest was the highest (1169.01±0.74) mg/kg, while it was the lowest in B. platyphylla forest (470.62±0.65) mg/kg, and their difference was highly significant (P<0.01); Soil Zn content was ranked in the decreasing order of P. sylvestris var. mongolica forest (103.09±0.40mg/kg), L. gmelinii (61.02±0.14) mg/kg, B. platyphylla (46.15±0.15) mg/kg. The main existing morphology of soil Mn was RED-Mn, and those of soil Zn were RES-Zn and OXI-Zn. Their proportion of extractable form was both over 40%, with high potential of twice releasing, and was distributed mainly in topsoil. RSP index of Mn (18.24) was higher than that of Zn (1.32), and decreased in the order of B. platyphylla, P. sylvestris var. mongolica forest, L. gmelinii, with apparent characteristics in spatial distribution.