| Ecological stoichiometry, a new tool to study ecological processes from genes to the biosphere, has been successfully applied in ecological studies in recent years. Being three important elements in biogeochemical cycle and most ecosystems, the stoichiometry of carbon (C), nitrogen (N) and phosphorus (P) have received much attention. Most studies of C:N:P stoichiometry are about ecological interactions in aquatic ecosystems. Terrestrial ecosystems are also very important for the survival of human beings, for most human activities largely take place on land. The appearance of plants made it possible for human to live on this planet. Thus the C:N:P stoichiometry of plants in terrestrial ecosystems needs to be further investigated. Furthermore, C:N:P stoichiometry could indicate nutrient limitation, which could be more effectively testified by fertilization experiment. The objectives of this study are to elucidate the variation of vascular plant N:P stoichiometry in natural terrestrial ecosystems in China, to test if there exists some general pattern among evolutionary stages and plant functional groups (PFGs) from the view of N:P stoichiometry, and studied the stoichiometry properties of a steppe ecosystem from tissues, species, PFGs, root, soil and litter levels. N fertilization experiment, at rates of 0, 5, 15, 30, 50 and 80 g NH4NO3.m-2.yr-1, was carried in this ecosystem in two sites, which characterized two successionally distinctive communities; one had been fenced for 22 years till 2001 (site A) and another for 2 years (site B). P fertilization experiment, at rates of 0, 2, 4, 8, 16, and 32 g P2O5 m-2-yr-1 with 9 replicates was only performed in the site devoid of animal grazing for 22 years (site C).We use a recently compiled data set containing 1,603 species of Chinese vascular plants-a database that is at least 5 times lager than currently used data sets (ref. Elser et al 2000, n=332)-to analyze the variation of the N:P stoichiometry among plants of different evolutionary levels and functional groups (or growth forms). The N:P ratio of the 1,603 species we analyzed is almost identical (12.93 vs. 12.65) to the result independently reported by Elser et al. (2000) (332 North American plant species).Human activities on N:P stoichiometry was studied by analyzing the differenceof N and P concentrations and N:P ratios of wild plants growing in the eastern and the western parts of China along Hu's line. Our results showed that plants in the eastern part of China have significantly higher N:P ratio than those in the west. Tissue N and P concentrations have significantly lower that those in the east.1. Responses of the organ N and P concentrations, N:P ratios of Leymus chinensis to N and P fertilization showed that N fertilization significantly increased organs N concentration in both sites for two years, while P fertilization significantly increased the P concentration of all organs in site C in 2001. Rhizome P concentrations remained constant with N fertilization except that of site B in 2001. Culm P concentrations did not significantly affected by N fertilization except that of site A in 2001. Leaf N concentrations significantly positively increased with N fertilization rates except that of site B in 2000 in N-amended plots. Tissue N concentrations remained constant with P fertilization rates in site C in 2001. Tissue N and P concentrations were significantly positively correlated for two years except those of leaves in site B in N-amended plots. Tissue N and P concentrations were significantly correlated in site C. N fertilization had no significant effect on N:P ratios of different organs in both sites for two years except that of culm in 2000 in site A and leaf in 2000 in site B. Tissue N:P ratios decreased significantly with P fertilization rates.2. N and P contents of the four species (L. chinensis, Achnatherum sibiricum, Stipa grandis and Carex korshinskyi) were significantly correlated in N-amended plots. The aboveground biomass of I. chinensis, A. sibiricum, and S. grandis in both sites were generally tend...
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