Root rather than leaf litter input drives soil carbon sequestration after afforestation on a marginal cropland

• Leaf litter and fine root added to 13C-enriched soil in incubation experiment.
• Litter input enhanced SOC sequestration although stimulating soil respiration.
• Fine root addition can incorporate more new C into soils.
• Subsurface soil had a greater potential to sequester SOC compared to surface soil.

Afforestation on croplands has been proposed as a means to mitigate the increasing emission of anthropogenic CO2. However, the relative contribution of above- and belowground litter input on soil organic carbon (SOC) sequestration following afforestation is not fully understood. We used a 270-day laboratory incubation experiment to examine the impact of litter type (i.e., leaf vs. fine root litter) of a poplar tree (Populus simonii Carr., C3 plant) on soil respiration and the turnover of new vs. old soil C in surface (0–10 cm) and subsurface mineral soils (40–50 cm) collected from a marginal cropland planted to maize (Zea mays L., C4 plant) in a semi-arid region in northeast China. Our results showed that fine root rather than leaf litter addition helps to sequester SOC even though soil microbial respiration rates were stimulated by both leaf and fine root litter input. Neither leaf nor fine root litter addition stimulated mineralization of old soil C across the two soil layers, but more new C was incorporated into the soil with fine root addition as compared with leaf litter addition. Moreover, the subsurface soil had greater potential to sequester SOC as compared to the surface soil. Our results suggest that root rather than leaf litter input drives soil carbon sequestration on the marginal soil, especially in the subsurface soil, and planting deep-rooted trees with large belowground biomass production could be used to increase SOC sequestration in marginal croplands.