Stable Carbon Isotope Compositions In C₃ ,c₄ Herbaceous Plants And Their Responce To Changing Temperature

Stable carbon isotope compositions in plants (δ¹³C) are useful in providing theory and study basis to abstract the information of paleoenviroment and paleoecology. However, the study in the relationship betweenδ¹³C and temperature is still too little under the background of the intensification of"greenhouse effect". What's more, the results gotten by these mere studies are various and the explanation to them are still not convincing. Therefore, the temperature coefficient ofδ¹³C and the mechanism of the effect played by temperature onδ¹³C are not clear and need to be solute.In this present study, five temperature ranks are set with five controlled environment growth cabinets, under which four C₃ species and three C₄ species are planted. A collection of plant samples are made simultaneously along the 400 mm rainfall line from east to west in north China. Combination of two methods is used to analysis the responce ofδ¹³C in herb of C₃ and C₄ to the variance of temperature. Efforts are made to explain the effect mechanism from the view of plant physiology and nutrient contents as well. The objectives of this survey are to obtain temperature coefficient ofδ¹³C and to reveal the mechanism of the effect by temperature onδ¹³C, which can, furthermore, provide theory basis to the study of the effects onδ¹³C made by environmental factors and its quantitative application in paleoenviroment and paleoecology. The main conclusions are as follows:1. The value intervals ofδ¹³C of C₃ and C₄ plants were -28.31‰ -32.32‰and -14.39‰ -18.27‰, respectively, in controlled experiment, and were -24.32‰^-32.76‰and -10.50‰^-15.58‰, respectively, in field samples. No intersection was found betweenδ¹³C of C₃ and C₄ plants, which suggested that the 13C composition is mainly influenced by the photosynthetic pathway. By comparison, in controlled experiment, the range ofδ¹³C was smaller in C₃ plants and the values ofδ¹³C were more negative in C₄ plants than those obtained by field samples. The difference of target plants may be the main reason.2. It was showed by controlled experiment that there was a weak ascending trend ofδ¹³C in C₃ and C₄ plants overall, with a rate of -0.017‰/℃and -0.036‰/℃, respectively, while the changing trend of individual species were variant. With the analysis results from field plant samples, a significant and positive relationship was found betweenδ¹³C in C₃ plants and annual mean temperature, with a notable coefficient of 0.11‰/℃, while no correlation was found betweenδ¹³C in C₄ plants and temperature.3. In controlled experiment,δ¹³C in C₃ plants changed remarkable with the variance of ci/ca, while the responce ofδ¹³C in C₄ plants were much weaker, which suggested that it was the vital factor of ci/ca for carbon fraction in C₃ plants, while in C₄ plants,φwas the most important one.δ¹³C of C₃ and C₄ plants showed negative correlations with the specific leaf area (SLA).4. According to the results in situ, no correlationship was found between SLA and temperature, showing that the SLA was not the cause to the variations of plantδ¹³C. No significant relationships were exhibited by leaf nitrogen contens of C₃ and C₄ plants with the variant temperature, while soil nitrogen content decreased with temperature increasing. The low temperature, which lowered the applicability of soil N, might be the main reason. Responses of N content based on mass (Nmass) and on area(Narea) to temperature were detected. It was showed that as temperature increased, increasing and no significant trend were showed by Nmass and Narea. This may be accounted by the non-relationship of SLA and temperature, the range of annual mean temperature of the differences between species. The relationships betweenδ¹³C and leaf N content were also species- specific, which suggested that both N content and the N use efficiency in photosynthesis should be taken into considerate in the use of the relationship between N content andδ¹³C in leaf.5.δ¹³C in C₃ plants increased with altitude significantly, and decreased with latitude and longitude significantly, with a rate of 1.3‰/km, 0.11‰and 0.097‰, respectively. Temperature was the key factor to influence the 13C composition of C₃ plants in this study area. While no significant relationship was found betweenδ¹³C in C₄ plants and geographic variences.