Response Of Plant Community To Nitrogen An Phosphorous Additions In Sub-alpine Meadow Of The Qinghai-Tibetan Plateau

The nitrogen entering natural ecosystems has been increasing with the growing of nitrogen deposition and artificial fertilization. The increase in nitrogen entering natural ecosystem causes the change of species composition, including species loss and the change in the relative abundance of functional groups. Therefore, nitrogen is acknowledged to be the main factor affecting plants survival, growth and reproduction in the terrestrial ecosystem. But, many recent field experiments and simulations show that other elements, such as phosphorus, water, light, etc., are the limiting factors of plant communities and ecosystems as well. As a result, the study of species competition for resources is changing from a single resource limitation research to multiple resource limitation research, especially to the research on the effect of nitrogen addition on other elements.Elements addition leads to the decline in species diversity and increase in productivity, which indicates that the resource limitation and species competition determines the coexistence of species in the plant community. Traditional view believes that nitrogen limitation is the main factor affecting natural ecosystem. However, nitrogen and phosphorus limitation has much stronger influence on natural ecosystem than single nitrogen or phosphorus limitation, which indicates that, ecosystem is affected by multiple resource limitation. Besides nitrogen and phosphorus limitation further affects the relationship between species diversity and productivity.Qinghai-Tibet plateau is vulnerab and sensitive to external disturbance, especially to elements addition under the background of nitrogen deposition and human economic activity.The objective of this article was to investigate the single and combination effects of nitrogen (N) and phosphorus (P) addition on aboveground plant productivity and species richness in sub-alpine meadows in the Qinghai-Tibetan Plateau, Gannan, China. The results as following:1. Aboveground plant productivity significant increased with nitrogen(N) addition alone, while species richness showed opposite trend, phosphorus(P) addition alone did not show significant influence on aboveground plant productivity or species richness.2. Nitrogen (N) addition alone led to significant increase in grass biomass but decrease in legume biomass, phosphorus (P) addition alone did not have significant effects on grass biomass or legume biomass.3. Aboveground plant productivity and species richness were more strongly affected by addition of nitrogen (N) and phosphorus (P) than nitrogen (N).4. Aboveground plant productivity linearly decreased with species richness and light penetration, while species richness linearly increased with light penetration. According to these results, we draw the following conclusions:1. Effects of N, P additions on species richness were not equivalent. Nitrogen was the primary limiting resource for species richness and productivity.2. Grass was much more sensitive to N limitation relative to other function groups.3. Nitrogen and phosphorus interaction strongly affected aboveground plant productivity and species richness.4. Aboveground plant productivity and community coverage increased with fertilization, which led to the decrease of light penetration in community canopy. Therefore, small individuals of all species died under intense light competition which led to local extinction of species which were initially rare or stochastic species loss, thereby supporting light competition hypothesis.