Impact of different forest types on stability and organic carbon of soil aggregates

Based on the field survey and laboratory analysis, we studied the soil aggregates stability and soil organic carbon characteristics in two soil layers, i.e., 0-20cm and 20-40cm, under five different forest types, namely Eucalyptus robusta, Pinus tabuliformis, Cunninghamia lanceolata, Phyllostachys heterocycla, and natural forestland. 1) In the 0-20cm layer, the mass percentage of soil water stable aggregates under the five different forest types firstly decreased and then increased along with decreasing aggregate size, and particle size 0.5-0.25mm had the lowest percentage. In both layers, soil aggregates were dominated by particle size>0.25mm, and the mass percentage of macro-aggregates decreased as the soil went deeper. 2) The mean weight diameter (MWD) and geometric mean diameter (GMD) of soil in different forests had similar variation tendency: it was higher in the 0-20cm layer than that in the 20-40cm layer. When comparing the MWD and GMD in the same layer, both of the factors were the highest in natural forestland, while the lowest in Eucalyptus robusta forest. However, the fractal dimension (D) of the soil in the Eucalyptus robusta forest was the highest, but lowest in the Cunninghamia lanceolata forest. 3) The content of organic carbon in soil aggregates of each particle size in five forest types decreased with increasing depth in soil and was distributed mainly in the grain size of 2-5mm. In 0-20cm soil layer, with decreasing particle size of soil aggregates, the content of organic carbon firstly increased, then decreased for the particle size of 5-2mm and finally increased, while contribution ratio of organic carbon firstly decreased, and increased finally. According to variance analysis of MWD and GMD of different forest types and soil layers, soil in natural forest had better structure and its soil water-stable aggregates had better stability than the other four types of plantations, while Eucalyptus robusta forest was the lowest. The stability of soil water-stable aggregate decreased with increasing depth in soil. The content of organic carbon in natural forest soil aggregates was the highest, while that in Eucalyptus robusta forest was the lowest.