Effect Of Nitrogen And Phosphorous Additions On Species Diversity And Plant Height In The Sub-alpine Meadow

The eastern edge of Qinghai-Tibetan Plateau is going through distinct atmospheric nitrogen deposition with an upward trend and rising soil temperature under the warming process in recent years. Both these have led to an increase in the available nitrogen(N) and phosphorus(P) in soils. At present, many researchers have shown that plant richness declined at high soil fertility mainly due to light competition. However, very few studies were to explore that how the species diversity change at the level of function groups in the region. And the reasons for the decrease in species richness after resource additions remain controversial. Plant height is a major determinant of a plant’s ability to compete for light. Differences in height growth between plant species represent differences in ability to compete for light. Our study was conducted in a sub-alpine meadow in the eastern edge of Qinghai-Tibetan Plateau. Ten different fertilization treatments were set up to investigate the single and combination effects of N and P addition on species loss, canopy height, the differences in height growth and aboveground productivity at the level of community and function groups. Then we explained the underlying mechanisms for species loss based on plant height and differences in height growth between different function groups. Finally we discussed the correlation between species richness and community productivity.The results we obtained were as follows:1. The species richness was decreased by11.8%、27.1%、31.0%、14.4%、29.7%and56.8%in different fertilization treatments of N5、N10、N15、N5P5、N10P10and N15P15respectively.80%of the lost species are forbs. The species number of legumes was decreased significantly with different fertilization treatments of N15、N10P10and N15P15, while grasses exhibited no changes in all the treatments. P addition alone had negligible effects on species richness.2. The PAR was decreased by57.3%、66.9%、71.6%、56.0%、77.8%and82.5%with different treatments of N5、N10、N15、N5P5、N10P10and N15P15respectively. The PAR was decreased by approximately34%with fertilization treatments of P alone but didn’t show a downward trend with increasing fertilization amount of P.3. The canopy height of grasses and canopy height differences between grasses and other two functional groups increased significantly along with increasing fertilization amount of N. Canopy height difference between grasses and forbs was greater than that with legumes.4. Community productivity increased significantly along with increasing fertilization amount of N as well as the productivity of grasses. The productivity was more strongly affected by addition of N and P than N alone. The percentage of community productivity that grasses contributed for increased in a similar but greater trend. The sub-alpine meadow community was dominated by grasses.5. There was a significantly linear negative relationship between community productivity and species richness. The increase in community density after fertilization was paralleled by the increase in community productivity, which resulted in the decline of PAR. Then individuals of short-statured or slower-growing species at the bottom of the canopy were excluded because of a disadvantage in light competition.According to the analysis results, we can reach the following conclusions:1. Species richness significantly decreased with N addition, while P addition alone had negligible effects. Species richness was more strongly affected by addition of N and P than N alone.2. The function groups differed in the decline of species richness. Forbs were much more easily eliminated comparing with other function groups.3. Light competition exclusion, leaded by a more unequal height distribution increased with fertilization between functional groups, is the primary cause for species loss after fertilization.4. The decline in species richness and increase in community productivity are two synchronized responses for fertilization.