Abstract: The physical protection of soil aggregates is an important mechanism of soil carbon sequestration. In this study, shrub-grassland and shrubless-grassland in East margin of Qinghai-Tibet Plateau were investigated. We used wet sieving proposed by Cambardella and Elliott and a settling siphon to separate soil aggregates, which were then measured. Data obtained included: Soil aggregate stability, differentiation of different forms of Fe and Al oxides, and the relationship between Fe and Al oxides and soil aggregate stability. It was found that: Shrub encroachment significantly reduced (P<0.05) the proportion of microaggregates (<0.053 mm), but had no significant effect on soil aggregate stability (P>0.05). Shrub encroachment significantly increased the content of dithionthite-citrate-bicarbonate extractable Al oxides in macroaggregates (>0.25 mm), which was increased by 14.09% (P<0.05). Shrub encroachment also significantly increased the content of dithionthite-citrate-bicarbonate extractable Fe and Al oxides in microaggregates (<0.25 mm), which was increased by 9.75% and 15.31%, respectively (P<0.05). However, shrub encroachment significantly reduced the content of complex Fe and Al oxides in microaggregates (<0.25 mm), the content of sodium pyrophosphate extractable Fe and Al oxides decreased by 31.03% and 9.41%, respectively (P<0.05). Boosted regression tree analysis indicated that the free Fe and Al oxides had the strongest effect on soil aggregate stability, while the complex Al oxides had the weakest effect.The results indicate that shrub encroachment is helpful to enhance soil aggregate stability and promote the sequestration of soil organic carbon.

Key words: Qinghai-Tibet alpine grassland, shrub encroachment, aggregate stability, Fe and Al oxides, boosted regression tree