The Study Of Carbon-Nitrogen Dynamics And The Ecological Effects Of Nitrogen Addition For Degraded Grassland Ecosystem

As important patterns for grassland management, grazing and fertilizing will have far-reaching effects on grassland ecosystem. In this paper, we selected three types of steppe ecosystems (including meadow steppe, typical steppe, desert steppe) and their Grazing-induced retrogression types distributted along ecological gradient(aridity) in Xilinguole League of Inner Mongolia, and studied their vegetation, roots and soil features Comprehensively. We discussed the effect of the similarities and differences regulars about ecological gradients and Grazing-induced retrogression on the steppe communities, energy, carbon and nitrogen characteristics. We performed nitrogen addition experiments to two types of grasslands under different utilization patterns in order to reveal the response of soil and vegetation to fertilization in Inner Mongolia typical steppe.The main results as follows:(1)Grazing-induced retrogression led to grassland ecosystem biomass (aboveground and underground) reduced, the ratio of root to shoot increased, biomass allocation changed, plant root intensively distributed in soil surface layer. It also led to high-energy PFGs dominance index reduced and low-energy PFGs dominance index increased. Similarly, grazing made the community's average caloric values and existing energy greatly reduced.(2)Along the ecological gradient, from the meadow steppe to typical steppe and then to desert steppe:Species richness, aboveground biomass and underground biomass is gradually decreased; the rate of the plant root distributed in soil surface layer decreased; high-energy PFGs dominance index reduced and low-energy PFGs dominance index increased; Community's existing energy showed downtrend with increasing the aridity of the environment(3) Grazing-induced retrogression led to the energy content of community and PFGs nitrogen increased, But it made the content of soil nitrogen decreased; Grazing-induced retrogression had no obvious effect on the carbon content of community and energy PFGs, but it made the content of soil carbon decreased. Grazing-induced retrogression led community and energy PFGs C/N ratio to decrease. Grazing-induced retrogression impacted on the contents of plant N in the desert steppe was much less than the typical steppe and the meadow steppe.(4) The content of carbon and nitrogen of community, soil, roots and all kinds of energy PFGs didn't have coincident regulars along the ecological gradient. The order of the content of N of community, root and soil is community> root> soil, whether in meadow steppe, typical steppe or desert steppe. When compared the content of carbon and nitrogen among different energy PFGs, we found that a higher content of N in low-energy PFG and the content of C showed high-energy PFGs> moderate-energy PFGs> low-energy PFGs (5)Grassland soils in different utilization patterns had different responses to nitrogen addition. It was not related between Nitrogen retained in the soil and added nitrogen gradient. Adding nitrogen would lead vegetation and plants nitrogen content to increase; however, there were not differences among nitrogen addition gradients.(6) The physiological indexes of different species the same species under different utilization patterns were different in response to nitrogen addition. It is closely related to plant species dominant position in plant community and competitive advantage for light resources.