Variances of carbon storage in soil particle-sized fractions following conversion of cropland to forest in Loess Hilly Region

Abstract: With the implementation of Grain for Green Project in the Loess Hill region of China, a large amount of arable land has been replaced by shrubs and trees, resulting in increased storage of soil organic carbon (SOC). To elucidate the mechanisms of change in the SOC and differences in the storage of SOC among the afforested lands, we separated the whole SOC into three particle-sized fractions: sand associated carbon (sand-C, ≥53-2000 μm), silt associated carbon (silt-C, ≥ 2-53 μm) and clay associated carbon (clay-C, <2 μm). Soils were collected from cropland land and from areas with five tree species, robinia (Robiniapseudoacacia L), apricot(Armeniacasibirica), poplar (Populustomentosa), buckthorn (Hippophaerhamnoides L) and caragana (Caraganakorshinskii), which have grown on former cropland for 15 years. The result showed that compared with cropland, the content of SOC in whole soil at all afforested lands were significantly increased by 1.9-1.0 g/kg from top soil of 0-10 cm to soil depth of >40-60 cm, while the content of silt-C, clay-C and sand-C was increased by 1.0-0.6, 0.4-0.3, 0.5-0.1 g/kg, respectively. As a result, the stock of whole SOC and silt-C in 0-20 cm soil depth followed the increased ordercaragana = poplar< buckthorn < apricot Clay-C (29.3%) >Sand-C (13.8%). There was only 4%-8% of the whole SOC lost because that dissolved organic carbon and trace amounts of soil was lost during the process of fractionation. In conclusion, our results indicated that carbon absorbed by silt fraction was the major mechanism for SOC change and accumulation during conversion of cropland to forest. The afforestation of former arable land with robinia and apricotcould increase more SOC in whole soil and particle-sized fraction that other afforested lands. Thus planting robinia and apricot at former arable land could be the best ecological technology for carbon sequestration in the soils of the region.