Dynamics of soil physical and chemical properties and vegetation succession characteristics during grassland desertification under sheep grazing in an agro-pastoral transition zone in Northern China

A number of field experiments were conducted from 1986 to 2003 to investigate the dynamics of soil physical and chemical properties as well as vegetation succession characteristics during the grassland desertification process under sheep grazing. Results indicate that fine silt (0.01–0.001 mm) removal and medium sand (0.5–0.25 mm) increase occurred early in the desertification process resulting in coarser surface soil. The fine sand (0.25–0.05 mm) was the major (33.7–68.4%) soil fractions element throughout the process. Changes in soil fractions were associated with a decline in soil fertility as the natural grassland shifted to a desertified landscape. The organic matter concentration decreased significantly by 94%, 89% and 69%, respectively, in the 0–5, 5–10 and 10–30 cm soil layers. The desertification effects on total soil N followed the same trend as the organic matter. Total soil P and K concentrations decreased only slightly and were consistent early in the desertification process. Soil bulk density increased companied with the decline of soil porosity and compaction as the desertification process continued. Species diversity declined both in the plant community and the soil seed bank, and species richness decreased by 56%. Three successional stages were identified, with bunchgrass communities being the first, followed by the growth of rhizomous grasses, and then sandy species and annual plant communities. In conclusion, grassland desertification was accompanied by severe soil erosion, soil nutrition decline and species diversity losses.