Xiaogang Tong¹, Xinhui Han², Jiao Li¹

　　¹ College of Natural Resources and Environment, Northwest A&F University

　　² College of Agronomy, Northwest A&F University

　　In China and many other countries, tree plantation (afforestation) is prevalent on formeragricultural lands with poor soil fertility and productivity. The gross areas of afforestation on former arable lands were 32 million hectares until 2010 in China, which is the largest tree plantation area in the world. This widespread shift in ecosystem structure has strong potential to alter key ecosystem processes that could affect C cycles at the ecosystem and regional scales, resulting in increased storage of soil organic carbon (SOC). However, previousstudies have generally addressed the changes to SOC in the whole soil profile and have been limited in evaluating the mechanisms contributing to the stabilization and accumulation of SOC in these afforested soils.

　　To determine 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 (Hippophaerhamnoides) and robinia (Robiniapseudoacacia) stands, which represented two afforestation chronosequences that were converted from arable land 13 and 39 years 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 (53–250 µm), intra–microaggregate POC (iPOC) and mineral–associated organic carbon (MOC) in silt + clay (<53 µm).Whole-soil SOC stocks at the 100 cm soil depth increased by 60 % and 218 % over 39 yrs. under hippophae and 38 yrs. under robinia afforestation in areas that were once croplands, respectively. The potential causes of these increased pool sizes include higher rates of litter and root production in afforested areas, protection of organic matter by stabilization within soil macro- and microaggregates and protection of matter associated with mineral particles. An increased free POC fraction inter-macroaggregate contributed to 25-41 % C accumulation in the whole soil profile, which was directly derived from litter and root residues, whereas a small proportion of the increased C was observed in a physically protected C fraction (12-22 %) with increased microaggregate mass following the afforestation of former arable land. The largest mass of silt and clay among the soil fractions (>85 %) and surface adsorption of mineral particles determined why MOC contributed to the largest proportion (47-52%) of C accumulation in the whole soil profile. Consequently, SOC associated with minerals was the major mechanism for C sequestration in afforested soils. The results of this study suggest that the conversion of cropland to forest may potentially act as a sink for atmospheric CO₂ and the establishment of deciduous trees (robinia) on former cropland, which would be a more efficient afforestation practice for carbon sequestration in the study area.