Understanding soil carbon sequestration following the afforestation of former arable land by physical fractionation

- Improved the understanding of the mechanisms of carbon sequestration in afforested ecosystems.
- Physical fractionation techniques were used to assess SOC changes in afforested soil.
- Afforestation with robinia was more effective on C sequestration than afforestation with hippophae.
- Carbon associated with minerals was the major mechanism for C sequestration in afforested soils.

To determine soil organic carbon (SOC) sequestration and storage mechanisms following the afforestation of arable land, soil samples were collected from a depth of 0-100 cm from cropland as well as six hippophae (Hippophae rhamnoides) and robinia (Robinia pseudoacacia) stands, which represented two afforestation chronosequences that were converted from arable land 13 and 39 yrs ago, respectively, in the Loess Hilly Region of China. The SOC in the whole soil profile was separated into four specific size/density fractions: coarse free (cf) particulate organic carbon (POC) inter-macroaggregates (> 250 μm), fine free POC (ffPOC) inter-microaggregates ?thyc=5?> (53-250 μm), intra-microaggregate POC (iPOC) and mineral-associated organic carbon (MOC) in silt + clay (< 53 μm). The concentrations of SOC in the whole soil profile and its fractions under robinia stands were generally on the order of 39 yrs > 25 yrs > 13 yrs, whereas under the hippophae stands, the order was 38 yrs = 28 yrs > 13 yrs. The concentrations of SOC in all of the fractions in each soil layer were significantly higher under afforested stands relative to cropland, especially in the topsoil layer (0-10 cm). At a soil depth of 0-100 cm, the SOC sequestration rates in the fractions under robinia over 39 yrs were ranked on the order of MOC (0.94 Mg C ha− 1 yr− 1) > iPOC (0.41 Mg C ha− 1 yr− 1) > cfPOC (0.35 Mg C ha− 1 yr− 1) > ffPOC (0.12 Mg C ha− 1 yr− 1), whereas the order under hippophae over 38 yrs was MOC (0.27 Mg C ha− 1 yr− 1) > cfPOC (0.18 Mg C ha− 1 yr− 1) > ffPOC (0.09 Mg C ha− 1 yr− 1) = iPOC (0.07 Mg C ha− 1 yr− 1). Furthermore, MOC accounted for 47.0% and 52.1% of SOC sequestration under hippophae 38 yrs after afforestation and under robinia 39 yrs after afforestation, respectively. Our results indicate that the afforestation of former arable land with robinia and hippophae in the loess hilly gully region could greatly increase SOC sequestration in all four fractions, especially the MOC.