Effects Of Soil Nitrogen Additions On Biomass Allocation Of Stipa Krylovii Community

Since the industrial revolution, human activities significantly increased the atmospheric sedimentation of nitrogen input the soil. As a matter of fact, grassland ecosystem plays an important role in the global nitrogen cycle. However it is not very clear that the changes in the nitrogen cycle will show us which kinds of specific consequences to grassland ecosystem as we know. Biomass allocation is the material basis of morphological plasticity of plants, and is an important mechanism for adaptation to the environment. It not only affects their access to light, nutrient and water absorption and other functions, but also affects the whole ecosystem carbon turnover, species competition and coexistence.In order to study the regular pattern of biomass allocation of Stipa krylovii steppe plant community, community aboveground/belowground biomass allocation and 5 main species stem-leaf allocation were analyzed in different nitrogen gradations, through nitrogen fertilized on natural grassland for 2 consecutive years. At the same time, the two hypotheses about biomass allocation mechanism were verified。1. Nitrogen additions altered the aboveground/belowground biomass allocation of Stipa Krylovii community. It could increase the total biomass of Stipa Krylovii community within a certain range, but the total biomass decreased with the nitrogen addition above a certain value. The level of community belowground biomass was high in growth stage, but decreased in maturity, while it's still a much high level for biomass of low nitrogen treatments. Besides, in both of these two stages, the aboveground biomass of low nitrogen treatment was greater than high nitrogen treatment, which was greater than control. The root:shoot ratio was reduced after nitrogen addition. Allocation of stratified belowground biomass showed gradually decreased from top to bottom, but the low nitrogen treatment distribution more in bottom levels.2. Nitrogen additions altered the stem-leaf allocation of 5 main species. Stipa Krylovii increased the stem biomass after nitrogen addition. The stem biomass showed gradually increased with the nitrogen gradations increased. The increasing extent was small and average. Cleistogenes squarrossa increased the stem biomass after nitrogen addition. The stem biomass showed gradually increased with the nitrogen gradations increased. The difference between Low nitrogen treatment and control were small. The increasing extent of high nitrogen treatment was significant. Carex korshinskyi increased the stem biomass after nitrogen addition. But the difference was not significant among treatments. Artemisia frigida increased the stem biomass after nitrogen addition. The stem biomass showed gradually increased with the nitrogen gradations increased. The increasing extent was average. Vegetative shoot of Agropyron cristatum decreased the stem biomass after nitrogen addition. The stem biomass showed gradually decreased with the nitrogen gradations increased. The growth law was unstable.3. Biomass allocation of Stipa krylovii steppe plant community consistent with the optimal allocation hypothesis.4. In the two existing nitrogen levels, the effects of nitrogen addition on key indicators (Soil organic matter content, Soil total Nitrogen values, and Soil total Phosphorus values) of soil nutrient were not obvious after 2 consecutive years.