Responses Of Plant Traits And Soil Microbial Biomass C, N, P To Nitrogen Addition In Mature And Degraded Leymus Chinensis Steppe Ecosystems In Inner Mongolia Plateau

Nitrogen (N) is one of the most limiting resources to the growth of plants and net primary productivity (NPP) in terrestrial ecosystems. However, the available N has been significantly changed by widespread nitrogen enrichment through industrial and agricultural processes. This lissens N limitation, accelerates processes of nutrient cycling and impacts the biodiversity and ecosystem functioning. Impacts vary considerably among ecosystems depending on the parent material, climate conditions, topography, and vegetation composition. Field studies on the effects of nitrogen enrichment on ecosystem functioning have been done mostly in the North American and European artificial grasslands, whereas little is known about the response of Eurasia steppe ecosystems to nitrogen fertileization.We established a filed experiment to examine ecosystem responses to N-enrichment in the typical steppe region of Inner Mongolia grassland, which represents the largest continuous tract of the Eurasia steppe ecosystems. Two contrasting sites, i.e. undisturbed, mature Leymus chinensis community in which grazing ceased in 1979 and degraded community in which grazing ceased in 1999 when our experiment began, were selected for our nitrogen addition experiment.Responses of aboveground biomass, population density, specific leaf area, leaf nitrogen content, total chlorophyll, and the chlorophyll a/b ratio for six dominant and sub-dominant plant species to nitrogen addition rates were measured in 2005. Our results showed that: it is a suite of functional traits including specific leaf area, mass-based leaf nitrogen, total chlorophyll, and the chlorophyll a/b ratio that interact to determine responses of plant species to nitrogen addition rates. Leymus chinensis has the highest plasticity in specific leaf area, leaf nitrogen and total chlorophyll content among the six species. As a result, both the population density and aboveground biomass of L. chinensis increased significantly with nitrogen addition rates. The increasing dominance of Achnatherum sibiricum is attributed to high plasticity in specific leaf area and the mass-based leaf nitrogen and chlorophyll content along the nitrogen addition gradient. Compared with L. chinensis and A.