Effects Of Nitrogen And Silicon Addition On Plant Community Structure And Edaphic Nitrogen Of Alpine Meadow

Nitrogen is the main limiting element in alpine meadow. Previous studies suggested that adding exogenous nitrogen can significantly increase the aboveground biomass, reduce the species richness and affect the stability of the ecosystem. Silicon has been recognized as a modified element for plant. Its effects are increasing the stem lodging resistance and, reinforcing resistance of plant. There is at present almost no research on silicon fertilizer in the alpine meadow. The objective of this research is to analyze the above-ground biomass, edaphic nitrogen species richness of alpine meadow by increasing different combinations of nitrogen and silicon in the Qinghai-Tibetan Plateau, China.The main conclusions are as follows:(1)Our research results that the above-ground biomass of the alpine meadow community significantly increased with nitrogen and silicon fertilizer for two years in succession, and the increasing from nitrogen fertilizer for above-ground biomass is higher than silicon, indicating nitrogen is the main limiting factor for this. Adding nitrogen and silicon fertilizer at the same time could further increasing the above-ground biomass, and it reached the maximum value at the moderate concentration of silicon(8 g/m2).(2)The response of adding nitrogen and silicon was differed for different functional groups. The biomass of gramineous functional group increased most after two-year adding nitrogen alone. The biomass of weed and cyperaceae function group was significantly increased in 2014, while no significant change in 2015. There is the same result in three functional groups when adding silicon alone, with the above-ground biomass significantly increased in the medium and high concentrations of silicon. The same situation happened in adding nitrogen and silicon together. Thus, the biological function of nitrogen and silicon fertilizer hold the optimum concentration at the community level, it is should be the result of mutual balancing for varieties competitive.(3)In 2014 and 2015, species richness was significantly reduced under nitrogen, while no significant effects with silicon. Species richness has been improved when adding with nitrogen and silicon. To some extent, silicon could relief the tendency of reducing species richness due to nitrogen. It reached the maximum value at the moderate concentration of silicon(8 g/m2).(4)In 2014 and 2015, adding nitrogen fertilizer had significantly improved the abundance of gramineous functional group and reduced the abundance of weed functional group. There was a significant increase in the abundance of cyperaceae function group with nitrogen addition in 2014, while no significant change in 2015. Silicon fertilizer had significantly increased the abundance of three functional groups in 2014 and 2015. Adding nitrogen and silicon fertilizer at the same time could further increasing the abundance of three functional groups, and it reached the maximum value at the moderate concentration of silicon(8 g/m2).(5)In two-year-fertilizer study, the above-ground biomass were improved with the species richness synchronously with the silicon addition alone, but the relationship between the above-ground biomass and the species richness showed a negative correlation with the silicon and nitrogen together. It meant although adding silicon fertilizer could synchronously improved the species richness and the above-ground biomass, which exhibited positive correlation, the negative correlation caused by nitrogen was still in dominant.(6)There was no significant effect on the content of soil total nitrogen, but a significant increase of soil ammonium nitrogen and nitrate nitrogen with fertilizer addition and the increase caused by nitrogen fertilizer was more than silicon fertilizer. The maximum content of soil available nitrogen in 2014 is higher than the maximum value of 2015. The change of soil available nitrogen content was consistent with the above-ground biomass. This showed that through increasing the soil available nitrogen content and adding fertilizer improved the above-ground biomass of plant community.