Long-term grassland restoration exerts stronger impacts on the vertical distribution of labile over recalcitrant organic carbon fractions in Mollisols

Land use change influences soil organic carbon (SOC) distribution in soil matrix. Conversion of cropland to grassland or forestland has the potential to sequester more carbon and influence the carbon allocation. Soil carbon sequestration happens within different physical and chemical fractions. This study aimed to investigate the vertical distribution of SOC fractions under 29-yr of cropland, grassland, and forestland of a typical Mollisol and to derive specific factors for SOC fraction changes associated with land use conversion. Soil samples within 100 cm soil depth were fractionated by density and humic methods. Biomass of litter residues and plant roots were also measured. Together with plant root biomass, the contents of total SOC and organic carbon (OC) fractions decreased along soil profile under three land uses. At each soil depth, total SOC and OC fractions followed the same order of grassland > forestland > cropland except for higher OC contents in heavy fraction, fulvic acid, and humin in cropland than those in forestland at the 0-10 cm layer. Compared with forestland and cropland, 29-yr grassland exerted stronger effects on the profile distribution of labile OC (water-soluble and light OC fractions) than recalcitrant OC (humic and heavy fractions), significances were observed at 0-80 cm layers for the labile OC and at 0-40 cm layers for the recalcitrant OC (P < .05). Larger root biomass and better soil structure in grassland might facilitate such changes. Grassland restoration, with greater C sequestration potential and larger liable OC contents, may help improve soil quality better than forestland and cropland in this Mollisol region.