Soil aggregation, aggregate stability, organic carbon and nitrogen in different soil aggregate fractions under forest and shrub vegetation on the Loess Plateau, China

Revegetation has been reported as one of the most effective counter measures to reduce soil and water erosion on the Loess plateau in China. Soil aggregate stability and the distribution of organic carbon and nitrogen in different aggregate fractions would be affected by different plant communities. The objectives of this study were to elucidate the effects of different plant communities on soil aggregate stability and the distribution of organic carbon and nitrogen in different aggregate fractions in order to prove that the different plant covers enhance soil aggregate stability. Six kinds of soil samples under forest (Quercus liaotungensis, Populus davidiana, Pinus tabulaeformis, Bothriochloa, a 14 year abandoned land, and a 19 year bare fallow soil. Four kinds of soil samples under shrub land (the 24 year old Caragana Korshinskii Kom.; the 14 year old C. Korshinskii Kom., 3 year old abandoned grazing land and traditional slope cropland which is claimed by the farmers for production with very low fertilizers) were collected from the hilly–gully area on the Loess Plateau, which was divided into 0–10 cm and 10–20 cm. We investigated soil aggregate stability and soil aggregate fractions by ultrasonic fractionation (USAS), and the distribution of organic carbon and nitrogen in different fractions under forest and afforested land, as key indicators for soil remediation through revegetation. The results showed that soil organic carbon (Corg) and total nitrogen (Nt) were strongly increased under forest and artificial shrub land compared to cropland and bare fallow land and were higher in the surface layers (0–10 cm) than in the subsurface (10–20 cm). Soil aggregate stability (SAS) was quite low under bare fallow land and cropping land soils, in comparison with the 4 forest communities. The three main fractions of soil aggregates, obtained by ultrasonic fractionation, were < 63 μm, 63–100 μm and 100–250 μm, which represented approximately 60%, 10% and 10%, respectively. In all land uses, macro-aggregates, 1000–630 μm > 630–250 μm had a higher Corg content than micro-aggregates, 250–100 μm > 100–63 μm ≫ 63 μm. In comparison to cropland and bare fallow land, forest and artificial shrub can protect and enhance the Corg, Nt and soil aggregate stability. The higher C:N ratio of macro-aggregate fractions indicates a rapid turnover of soil organic carbon in the top soil of forest sites and even on bare fallow and crop land. The narrow range of C:N ratio in micro-aggregates indicates that soil organic carbon in micro-aggregates is more stable than that in the macro-aggregates. We concluded that revegetation of eroded soils accelerates soil remediation and rehabilitation.