Seasonal dynamics of carbon，nitrogen and phosphorus stoichiometry in Stipa bungeana

Abstract： Less is known about whether the Homeostasis Theory and Growth Rate Hypothesis work in higher plants. Our objective was to study the seasonal dynamics of carbon (C)、nitrogen (N) and phosphorus (P) contents and test the Growth Rate Hypothesis in wild higher plants. Additionally, we tried to explore the influence of grazing on plants homeostasis traits. This study was conducted at SemiArid Climate and Environment Obwervatory of Lanzhou University in Loess Plateau (3557 N，10409 E). We collected the aboveground biomass of Stipa bungeana in the grazed and exclosure treatments in different seasons and quantified the C, N and P contents in the leaf tissue. The specific growth rates of plant S. bungeana were calculated and seasonal dynamics of C∶N, C∶P and N∶P ratios were described. In the exclosure (1) the aboveground biomass of S. bungeana accumulated gradually, and the maximum specific growth rate occurred during the period from March to May. Meanwhile, during this period, plant grew with higher C∶N, C∶P, N∶P ratios. Combined, these results did not support the Growth Rate Hypothesis; (2) the C content in S. bungeana increased significantly from March to May, but there were no differences among the C contents from May to September. The maximum values of N and P contents occurred in March, but the N and P contents decreased gradually from May to September. From March to September, the C∶N and C∶P ratios increased gradually, but N∶P ratio increased firstly and then decreased. Finally, there were small variations in C content and N∶P ratios (3.65% and 6.60%, respectively) during these four seasons, which showed strong homeostasis in both above parameters. However, there were huge variations in N、P contents and C∶N、C∶P ratios, and all of these values were bigger than 30%. In the grazed treatment (1) the dynamics of C and N contents in S. bungeana were similar to those in the exclosure, but the P content in September was bigger than that in July. Additionally, grazing resulted in higher N and P contents compared to those in the exclosure, and lower C∶N and C∶P ratios in the grazed site. While, N∶P ratios were bigger in grazed site than those in the excosure in both May and July. (2) there were also small variations in C content and N∶P ratios during these four seasons in the grazed treatment, but the variation of N∶P ratios (21.30%) is markedly bigger than that in the exclosure (6.60%).