Effects Of N Addition On C, N, P Stoichiometry And Photosynthetic Characteristics Of Stipa Bungeana And Leymus Secalinus In The Grassland On Loess Plateau

To study the effects of nitrogen deposition on plant metabolism in grassland ecosystem, four levels of N addition was conducted for 2 years to simulate nitrogen deposition in a natural grassland on the Loess Plateau. Changes in the individual plant biomass, contents of carbon(C), nitrogen(N) and phosphorus(P), the C:N:P stoichiometry, photosynthetic characteristics of Stipa bungeana and Leymus secalinus were investigated to beter understand the response of C:N:P stoichiometry, N, P resorption efficiencies, photosynthetic characteristics, leaf senescence of the plants to N addition, as well as the correlations between them. The main conclusions are as follows:1 The individual plant biomass of Stipa bungeana and Leymus secalinus increased firstly and then decreased with N addition. The maximum value was observed for N2 treatment(9.2g N/m2·a). The individual plant biomass in 2010a was higher than that in 2009a.2 Different levels of N addition did not influence the C content of plants, while the N contents of the two plants increased significantly with N addition. The P content in the two plants showed different patterns, which decreased for Stipa bungeana and increased for Leymus secalinus with the increase of N addition. The C contents of plants did not change at the seasonal scale for all treatments. With change of season, the N contents of two plants increased firstly and then decreased in 2009a, while it increased gradually in 2010a. Both of the N and P contents of the two plants were higher in 2009a than that in 2010a.3 The C:N mass ratio of the two plants decreased gradually with the increase of N addition, while the N:P mass ratio showed an increasing trend. No consistent trends were found in the C:P mass ratio with N addition for the two plants, which increased for Stipa bungeana but decreased for Leymus secalinus. The N:P mass ratio of Stipa bungeana and Leymus secalinus in 2009a were greater than 16. The N:P mass ratio for NO in 2010a was less than 14, N1 was between 14 and 16, while that of N2 and N3 treatment were greater than 16.4 Stipa bungeana had higher resorption efficiencies for N(60.35%) and P (71.75%), but it decreased significantly with N addition. With the highest level of N addition, the N and P resorption efficiencies of were reduce 12.68% and 18.78% that of control,respectively. Higher N, P resorption efficiencies was very important for Stipa bungeana to survive in the barren soil.5 The chlorophyll content of the two plants increased with the increase of N addition, which was constantly higher than that of the control until October. This indicated that N deposition could delay leaf senescence of Stipa bungeana.6 N addition led to the increases of the photosynthetic rate, chlorophyll content and N content of the plant leaves. This indicated that moderate N deposition contributed to the plant photosynthesis. The stomatal conductance and transpiration rate of the plants increased with the increasing N addition within the whole experiments, except for Stipa bungeana in 2009a.