Afforestation effects on soil organic carbon and nitrogen pools modulated by lithology

•Lithology modulates the effect of afforestation on soil C and N pools.•Soil C pool is not altered by afforestation for areas underlain by limestone.•Soil C pool is increased for areas underlain by clasolite.•Soil N pool is decreased by afforestation for areas underlain by limestone.•Soil N pool is not altered for areas underlain by clasolite.

Afforestation is regarded as an effective way to sequester soil organic carbon (C) and hence to attenuate global warming. Nevertheless, so far mechanisms underlying the direction and magnitude of soil C pool variation following afforestation are not well understood. Here soil organic C, total nitrogen (N) and total phosphorus (P) pools in surface mineral soil (0-15 cm) were measured in cropland and plantation forest over two lithology types, i.e., limestone and clasolite, using a paired-site approach in a region of southwest China. The major objective was to test whether the effects of afforestation on soil C, N and P pools were affected by lithology. We found that lithology modulated the effect of afforestation on soil C or N pool, but not on soil P pool. Soil C pool was not altered by afforestation over limestone, but was significantly increased over clasolite (P < 0.001). Instead, soil N pool was significantly decreased by afforestation over limestone (P < 0.001), but was not altered over clasolite. Soil P pool was consistently decreased by afforestation regardless of lithology type (P < 0.05). The molar ratios among soil C, N and P were not affected by lithology, but were significantly elevated by afforestation (P < 0.01), indicating that soil C sequestration would increasingly be limited by soil N or P following afforestation. Our study suggests that afforestation effects on soil C and N pools may be controlled by lithology, and thus provides a new mechanism to explain the divergent responses of soil C and N following afforestation.