Effects Of CO₂ Enrichment On Plant Biomass Production And Carbon Balance In The Leymus Chinensis Steppe

Atmospheric CO2 concentration has increased from approximately 280μmol mol-1 in the late 19th century to over 360 μmol mol-1 today, and is projected to double over present ambient concentration by the mid- to late- 21st century. Increases in atmospheric CO2 concentration along with projected rises in other 'greenhouse' trace gases are likely to cause a change in climate. Such predicted changes in global atmospheric CO2 concentration and climate have led to numerous investigations into how crops as well as natural ecosystems and their dominant species will be affected. Most of the earlier researches on the effects of elevated atmospheric CO2 on plants mostly involved relatively short-term experiments investigating direct effects on the physiology and growth of crop plants and of herbaceous plants from natural ecosystems. Extrapolating results from pot experiments to the field situation is very difficult since a plant's response to increasing atmospheric CO2 concentrations is markedly species specific and varied strongly according to growth conditions. Consequently, long-term measurements of ecosystem responses to elevated CO2 under natural conditions are needed. However, long-term, in situ experiments with elevated CO2 in native terrestrial ecosystem are rare. Studies in less anthropogenicly influenced systems, where the availability of growth resources and interactions among individuals varied greatly, revealed more varied responses to elevated CO2.Grasslands are interesting study objects for research of this-field for their relatively simple structure and composition, and relatively fast nutrient cycling rates. Till now, grassland types and regions in most studies are very limited. Most studies were conducted in tallgrass prairie in Kansas, annual grasslands in California, calcareous grasslands in NW Switzerland, shortgrass steppe in Colorado, and some pastures. This seriously constrains the understanding and predicting of responses of grassland ecosystems to CO2 enrichment. In China, grasslands cover about 40% of the total territory area. However, no experimental report has been conducted on the effect of elevated CO2 on grassland ecosystems.Twelve field open-top chambers were installed on the Leymus chinensis steppe site in typical steppe region of Inner Mongolia Plateau of China from early May until mid-September in 2000 and 2001 to evaluate the influences of elevated CO2 on the productivity, species composition, CO2 exchange and C pool of L. chinensis steppe. Six chambers were maintained at current CO2 concentration (ambient treatment), six at twiceambient CO2, i.e., approximately 600umol mol-1 (elevated treatment), and six nonchambered plots served as controls. Aboveground biomass was determined four times during the two experimental years by clipping method and plant community composition was censused simultaneously. Belowground biomass was also determined along with aboveground samplings by using ring-cutting method. Ecosystem CO2 exchange rates were measured using Li-cor 6400 photosynthesis system. C and N contents in plant and soil materials were measured to estimate C pool composition and nutrient conditions. Meanwhile, two pot cultivation experiments in the yard of Inner Mongolia Grassland Ecosystem Research Station were designed to investigate the dependence of CO2 effects on soil moisture levels and plant species. The two types of pot cultivation are: (1) intact monoliths from the fenced L. chinensis steppe plot; (2) pots filled with universal soil from L. chinensis steppe and planted with the seeds of Agropyron cristatum (Xeric grass), Medicago sativa (Legume) and Bromus inermis (mesic grass), respectively. Both types of pot cultivation were treated with two soil moisture levels xtwo CO2 levels. Two soil water levels were applied. In the wet treatment, 1000ml water was added every four days, corresponding to 60% more rainfall above average, while in the dry treatment, no supplementing water was given until plants began wilting with a total of 4000 ml water being added during the whole experiment. CO2...