Studies On Soil N Mineralization, Nitration And The Relationship Of Above-ground And Below-ground In Alpine Meadows On The Eastern Qinghai-Tibetan Plateau

Alpine meadow and tundra accounted for only 7% of the land, but contributed 13% of the global soil carbon and 10% of the global soil nitrogen. Under the background of global warming, these grasslands will have a significant effect on the global carbon and nitrogen cycles. We want to study the effect of climate change on soil N cycling and plant diversity and better understand the controlling factors on soil nitrogen cycling and the distribution of soil organic carbon, nitrogen and phosphorus at the different spatial and temporal scales in these ecosystems. Plant productivity, species diversity, soil nitrogen mineralization and nitrification rates (0-15 cm depth) were measured using an in situ closed-top tube incubation across altitudes (3000,3500 and 4000 m) and aspects (North facing slope, South facing slope and Beach) from 2006 to 2008 on the alpine meadows of the eastern Qinghai-Tibetan plateau.The study results showed that:1) Soil water content increased with the altitude increase, soil water content in NF slope was higher than SF slope; Soil organic carbon, total nitrogen, C/N, available phosphorus and nitrogen at the both slope increased with altitude increase. Soil C/N, available nitrogen and phosphorus was higher in the SF slope than the NF slope; Soil organic carbon and nitrogen content in the SF slope > NF slope>beach, the order of soil available N was that SF slope> beach> NF slope, these indicated that the slopes by changing soil temperature and moisture indirectly influenced soil nutrient characteristics.2) Soil N mineralization and nitrification rates showed a same trend which the order was SF > NF> Beach. Because the higher soil temperature in the SF slope could favor soil microbial activity than the NF slope, the higher soil moisture in the NF slope and lower soil moisture of beach limited the activity of microorganisms, and it suggested that both soil temperature and moisture could be key factors on soil nitrogen mineralization rate and nitrification rate in the alpine meadows.3) The largest vegetation coverage and biomass were 93% and 330.2 g/m2, respectively, were found in the middle altitude. The biggest species richness also was found at SF slope in the middle altitude, this also supports the "hump" pattern of species diversity and biomass in altitude gradient.4) The above ground biomass in the grasses, noxious weed and legume functional groups decreased with the increasing of altitudes. The high proportion of forbs and noxious weeds indicated that the weeds invasion of serious in alpine meadow, leading to the functional structure of meadow system change, thus meadows system began to degradation or into the period of instability.5) RDA analysis showed that the grasses and legumes biomass and community biomass were significantly correlative with the soil microbial biomass carbon and nitrogen, but soil microbial biomass carbon and nitrogen were negatively correlated with Cmic/Nmjc and C/N. In addition, there were significantly positive correlation between the soil microbial biomass carbon, nitrogen and soil temperature, but with negatively related with soil moisture, these may explained that the grasses and legume functional group had great influence on the soil microbes carbon and nitrogen.6) In addition, species richness and the ground biomass were significantly positive correlation with soil available phosphorus, NH4+-N, it suggested that the available nitrogen and available phosphorus combined influencing the structure and function of the alpine meadow community.