The effect of tree species on soil organic carbon recovery in a restoration project is associated with vegetation biomass: Evidence from the Pingshuo Mine reclaimed ecosystem, North China

Vegetation and soil surveys were made within three mixed forests and four monoculture forests on opencast coal mine spoils in the Loess Plateau (China) to reveal the tree species effect on reclaimed mine soil (RMS) and carbon (C) sequestration. Results showed that tree biomass, annual litter biomass, and reserved litter biomass varied among forests, with Robinia pseudoacacia L.-Pinus tabuliformis Carr. recording the highest tree biomass, leaf C, and leaf nitrogen (N) values. The greatest C and N contents in RMS were recorded in the R. pseudoacacia - P. tabuliformis forest in both small macroaggregate (250-2000 mu m) and large macroaggregate (>2000 mu m) fractions. However, no significant differences among forests were observed in microaggregate (53-250 mu m) and silt+clay (<53 mu m) associate C and N. Significant positive correlations were observed among soil C and vegetation biomass and vegetation biomass associated C. It was in line with the C isotope analysis, which indicated that C flowed in the order: leaf -> litter/root -> soil macroaggregate -> soil microaggregate/silt+clay. Moreover, negative correlations between soil C and soil delta N-15 in large and small macroaggregate fraction were recorded, indicating C sequestration in macroaggregate was potentially associated with soil N cycling. Overall, Tree species effect on soil C sequestration was derived from the amount and quality of the vegetation biomass and it was mediated by soil N cycling to some extent. R. pseudoacacia - and P. tabuliformis mixed forest could be proposed as a favourable reclamation choice, due to these species prominent C sequestration ability.