The Mechanism Of Speciesloss Under Addition Of Nitrogen And Phosphorus At Sub-alpine Meadow In Tibetan Plateau

Nitrogen deposition and Phosphorus input will cause the changes of species richness and influence the structure and functions of the impacted systems profoundly.We chose one site in Qinghai-Tibet Plateau sub-alpine meadows as the study object, with three kinds of treatments, which are N addition alone, P addition alone and N and P addition together. There were three levels in each treatments containing five replications. We studied that the impacts of elements addition on plant chemometrics and photosynthetic rate, then influence plant community aboveground primary production and the speciesrichness.The major results as following:1, N addition caused the N:C values of all six studied species significantly increased, while P addition caused the N:C values of most species significantly decreased. The N:C values of most species in the treatments of N and P addition together also increased. For N:P values, the values of most species increased in N addition treatments, while the values of most species decreased in P addition and N and P addition together treatments.2, The photosynthetic rates of Elymus nutans Griseb and Artemisia tangutica Pamp in the treatments of N addition and N and P addition together significantly increased. The photosynthetic rates of Legume species and four species of forbs had no significant differences among all treatments.3, The photosynthetic nitrogen-use efficiency(PNUE) of Grass increased significantly in P addition and N and P addition together treatments, while N addition significantly increased the PNUE and Legume. There were no consistent reflects for Forbs in PNUE. As to photosynthetic nitrogen-use efficiency(PPUE), P addition and N and P addition together significantly increased PPUE of Grass. The PPUE of Legume functional groups significantly increased in all treatments. N addition decreased PPUE of Forbs significantly, while the other two treatments increased PPUE significantly.4, The aboveground primary production of Grass functional groups in N addition treatments, and P addition has no significantly influence on every functional groups. For N and P addition together treatments, the aboveground primary production significantly increased for Grass functional groups, significantly decreased for Legume functional groups and decreased for Forbs functional groups, while the aboveground primary production of whole community significantly increased.5, N and P addition together significantly decreased the species richness of sub-alpine meadow systems in Qinghai-Tibet Plateau, especially for the elements addition level is15, whose species richness decreased41.9%(control treatments averaged41.8versus N15P15treatments averaged24.3). The species richness of Forbs functional groups represented the similar trend with species decreased45.9%when the elements addition level is15.The Legume functional group species richness dropped significantly only when N and P were added together and the level is15.There were no significant impacts on every functional groups and community species richness when added P to community. N addition decreased significantly the species richness of Forbs functional group relative to that of control treatments.Above all, we conclude that:1. Grass are sensitive to N addition treatments. The photosynthetic rates, biomass of Grass increased significantly after N addition and N and P addition together. At the same time the proportion of Grass in the total biomass also increased significantly from42.8%in control treatments to74.4%when N added. When N and P added together, the proportion of Grass increased to91.3%significantly.2. There are no significant differences in photosynthetic rates for Legume and Forbs functional groups when N added or N and P added together. But the biomass of Legumes and Forbs dropped significantly. So do the proportion of biomass of Legumes and Forbs in total biomass, from6.7%in control treatments to1%for Legumes, and dropped from50.4%in control treatments to8.7%.3. The influence of P addition are not significant to sub-alpine meadows community.4. The increase in photosynthetic rates of Grass caused the accumulation in biomass, and then triggered the decline of the species richness of Forbs functional groups via competition exclusion when N added and N and P added together.5. There are interaction between N and P elements, which has stronger impact on the species biodiversity and primary production than N or P added alone.