Effects of shrub encroachment on soil aggregates and organic carbon vary in different grasslands in Inner Mongolia, China

Widespread shrub encroachment in grasslands can lead to changes in soil aggregates and soil organic carbon (SOC), especially in deep soils. Soil aggregates physicochemically protect SOC and thus affect soil carbon sequestration. Characterizing the changes in soil aggregates and their organic carbon associated with shrub encroachment is critical for evaluating the ecological consequences of shrub encroachment in different grasslands. In this study, we investigated soil aggregates of various sizes and their associated organic carbon at maximum soil depths of 5 m in shrub patches and neighboring grass matrix in two types (desert and typical) of grasslands in Inner Mongolia, China. The mean weight diameter (MWD) of the soil aggregates was similar between the shrub patches and grass matrix in both grasslands. However, the proportion of each aggregate size fraction to whole soil in the shrub patches was significantly lower than that in the grass matrix in the desert grasslands, while there was no significant difference between the shrub patches and grass matrix in the typical grasslands. In addition, the organic carbon content of aggregates in the shrub patches was greater than that in the grass matrix in deep soil layers (>50 cm) but was lower in the topsoil (0–10 cm depth) in the desert grasslands. However, the organic carbon content of aggregates at all soil depths was greater in shrub patches than in the grass matrix in typical grasslands. Our results suggest that the effects of shrub encroachment on the soil aggregate proportion and associated organic carbon vary at different soil depths and in different grassland types. The results of this study also suggest that it is necessary to determine the size structure and its relation to carbon content for soil aggregates to accurately predict SOC dynamics with shrub encroachment.