| Livestock grazing is recognized as one of the main causes of vegetation in grassland. Typical steppes in the Loess Plateau in grazed and fenced respectively were selected as the investigative object.The plant biomass,content of carbon(C),nitrogen(N)and phosphorus (P) and the storages characteristics of the seasonal variation were studied.The stoichiometric homeostasis of C,N and P in Stipa Bungean.and Leymus secalinus(Georgi) Tzvel were explored,the relationship between,stoichiometric homeostasis and growth rate were also studied.Some conclusions could be drawn as follows:1 From May to July was high-speed growth phase for the studied grass.The growth rates of Stipa Bungean and Leymus secalinus(Georgi) Tzvel.in this phase were higher than average growth rate.The multiplier is 2.7(FG,fencing grassland),4.8(GG,grazing grassland) and 6.40(FG),4.2(GG),respectively.However,lower C:N,C:P,N:P ratio in the two grass did not appear in this time.It was inconsistent with Growth rate hypothesis(GRH).2 With the maturity of plant the C:N,C:P ratio in Stipa Bungean.and Leymus secalinus(Georgi) Tzvel in both grazed and fenced grasslands were increased,and they reached maximum value in September.The N:P ratio without interference was basically in a stable condition.The Variation Coefficient(CV) of Stipa Bungeana N:P was 0.066, which occupied only 16.04%and 15.38%in CV of N and P,respectively.The homeostasis of Stipa Bungeana was interfered by grazing,The CV of Stipa Bungeana N:P in fenced grassland was 0.213,it was still smaller than CV of N and P.Preliminary,it seemed that N:P could be used as ecological stoichiometric characteristics analysis on ecosystem.The CV of Leymus secalinus(Georgi) Tzvel N:P in grazing grassland was smaller than CV of N and P.3 Grassland biomass was mainly affected by moisture,temperature,grazing and other factors.Two years total biomass have the same seasonal dynamics with underground biomass.The maximum total biomass appeared in July and November in 2007.with the value of 359.79±27.20(FG) and 278.64±18.09(GG),respectively.In 2008,they appeared in July.The values were 371.00±32.67(FG) and 332.11±25.09(GG).The maximum of underground biomass appeared in November(2007) and July(2008).The maximum of aboveground biomass in fenced grassland appeared in July to September. Grasses Root were mainly concentrated in 0-30cm soil layer.Its biomass was 90%of total underground biomass with the greate variation in growing season.Grasses roots showed little variation in 30-50cm layer.Grazing significantly reduced community biomass of each component(P＜0.05).Underground/aboveground biomass ratio in grazing grassland were higher than that in fenced grasslands.4 The C content of plant were increased with plant maturity,its change range was 436.53～493.92 g/kg.Dilution effect were found in the content of N and P elements,with the increasing of biomass,they reduced.Nutrients may be transfered to underground in the end stage of plant growth.C,N and P content of root showed V-type seasonal variation. They were increased after September.C content Change range was 330.99～475.54 g/kg. Grazing showed no significant impact to plant C content(P＞0.05),but it significantly increased N and P content(P＜0.05).5 C,N and P storage of grassland showed the same seasonal dynamics with their biomass.C,N and P storage of grassland showed significant positive correlation with their biomass(0.990,0.899 and 0.936(FG),0.990,0.891 and 0.936(GG);P＜0.01,2007).The differences between fenced grassland and grazing grassland were caused by their biomass. The C,N and P storage of fenced grassland were higher than that grazing grassland(except N and P storage in July,2007).Grazing stimulated forage regrowthed,it could also increase the utilization of grassland.6 Stipa Bungean is constructive species in Loess Plateau,its live shoot biomass occupied 84.50%of total aboveground biomass.Other weeds occupied small proportion of live shoot biomass.The seasonal variation of aboveground biomass were affected by biomass of Stipa Bungeana.
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