[Characteristics of vegetation, soil, and their coupling of degraded grasslands]

This paper presented the characteristics of the vegetation, soil, and their coupling in the gradient of grazing pressure of the degraded alpine grassland in Sunan county and of the typical arid grassland in Huanxian county, Gansu province. The abundance index of species, Simpson index, Shannon-Wiener index, Cody index, nutrient functional diversity index and life type diversity index increased, while the Bray-Curtis index and nitrogen fixation diversity index decreased with the reduced grazing intensity in both types of degraded grassland. With reduced grazing pressure, the contents of total nitrogen, available nitrogen and organic matter in both grasslands and the content of available phosphorus in arid grassland at 0-40 cm soil depth were increased, and that of total phosphorus in alpine grassland was decreased, while phosphorus concentration in arid grassland showed no regular pattern of change. The total and available nitrogen, available phosphorus and the ratio of available phosphorus to total phosphorus and of organic carbon to nitrogen were all higher in alpine grassland than in arid grassland. The grey correlativity of vertical distribution pattern between soil elements and living root biomass was positively related to grazing pressure, while the coupling intensity of soil-herbage subsystem was increased with decreased grazing pressure. There existed a significant correlation between poor soil elements and abundance of poisonous and poor-nutritional plants in both degraded grasslands. The degradation process of grassland resulted not only from the degeneration of vegetation and soil subsystems, but also from the disappearance of coupling between both subsystems and the development of their discordance. The ecological process could be quantified by coupling intensity and discordance intensity. It is concluded that the studied arid grassland was degraded more seriously than alpine grassland.