Contrasting effect of coniferous and broadleaf trees on soil carbon storage during reforestation of forest soils and afforestation of agricultural and post-mining soils

Soils and forest soil in particular represent important pools of carbon (C). The amount of C stored in soil depends on the input of organic matter into the soil, but also on quality of the organic matter, which determines the proportion of organic matter that remains in the soil or that is released from the soil as CO2. Here, we present a quantitative review of common garden experiments in which various tree species were planted alongside each other. The main goals of the study were to determine whether: 1) the amount of sequestered C under broadleaf and coniferous trees could be affected by soil age and previous land use; 2) the C:N ratio of leaf litter is correlated with the amount of sequestered C; 3) the amount of sequestered C under broadleaf and coniferous trees could be affected by pH and clay content. We found that the effects of broadleaf and coniferous trees on soil organic carbon (SOC) sequestration differed with the stage of soil development. We used soils with different previous land uses as a representative of different stages of soil development. Forest soils and agricultural soils represent soils in later stages of soil development and post-mining soils represent soils in early stages of development. In forest soils, more SOC was stored under coniferous trees than under broadleaf trees. In post-mining soils the opposite trend was found, i.e., more SOC was stored under broadleaf than coniferous trees. In afforested agricultural soils, SOC sequestration did not differ between broadleaf and coniferous trees. SOC sequestration under broadleaf trees was highest in soils with high pH. SOC sequestration was negatively correlated with the litter C:N ratio in post-mining soils but not in other more mature soils. Similarly, SOC sequestration was negatively correlated with the litter C:N in alkaline soils and in soils with high clay content. These results suggest that dominant SOC sequestration mechanisms change with stage of soil development such that SOC storage is greater under broadleaf trees in immature soils but is greater under coniferous trees in mature soils.