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

Abstract

To determine soil organic carbon (SOC) sequestration and storage mechanisms following the afforestation of arable land, soil samples were collected from a depth of 0–100cm 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 39yrs 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 (>250μm), fine free POC (ffPOC) inter-microaggregates ?thyc=5?> (53–250μm), intra–microaggregate POC (iPOC) and mineral–associated organic carbon (MOC) in silt+clay (<53μm). The concentrations of SOC in the whole soil profile and its fractions under robinia stands were generally on the order of 39yrs>25yrs>13yrs, whereas under the hippophae stands, the order was 38yrs=28yrs>13yrs. 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 (0–10cm). At a soil depth of 0–100cm, the SOC sequestration rates in the fractions under robinia over 39yrs were ranked on the order of MOC (0.94MgCha−1yr−1)>iPOC (0.41MgCha−1yr−1)>cfPOC (0.35MgCha−1yr−1)>ffPOC (0.12MgCha−1yr−1), whereas the order under hippophae over 38yrs was MOC (0.27MgCha−1yr−1)>cfPOC (0.18MgCha−1yr−1)>ffPOC (0.09MgCha−1yr−1)=iPOC (0.07MgCha−1yr−1). Furthermore, MOC accounted for 47.0% and 52.1% of SOC sequestration under hippophae 38yrs after afforestation and under robinia 39yrs 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.