Effects Of Atmospheric Nitrogen Deposition On The Carbon Cycle In The Haibei Alpine Meadow Ecosystem

Qinghai-Tibet Plateau has unique climate and characteristics in climate change. The atmospheric nitrogen deposition has an increased uptrend in this region. To study the effects of atmospheric nitrogen deposition on the plant growth and development, the physiological and ecological processes of alpine ecosystems is vital for understanding and predicting the response of the alpine vegetation’s carbon cycle to the global change. In this study, based on the artificial nitrogen addition experiment platform with different atmospheric nitrogen deposition levels (0,1,2 and 4 g N/m2/yr) and forms (KNO3, (NH4) 2SO4 and (NH4) Cl), we investigated the effects of simulated atmospheric nitrogen deposition on leaf nitrogen content of five dominant species (Stipa aliena, Elymusnutans, Gentianastraminea, Oxytropis kansuensis and Deschampsia caespitosa), leaf photosynthetic parameters (maximum carboxylation rate, electron transfer rate, triose phosphate transfer rate, etc.) of Stipaaliena, and canopy leaf area index (LAI) in the Haibei alpine meadow ecosystem. We further used a photosynthesis model to simulate and analyze possible effects of photosynthetic parameters on gross primary productivity (GPP). Use CEVSA2 model to simulate the effects of nitrogen addition(1,2,4,5 and 15 g/m2/yr) on GPP, net primary productivity (NPP), vegetation carbon pool,soil heterotrophic respiration and soil carbon content in Haibei alpine meadow.Results show that nitrogen addition levels and forms have no significant effects on leaf nitrogen content of five dominant species, except for Oxytropis kansuensis, the leaf nitrogen content of which increased significantly when adding (NHU)2SO4 fertilizer. Simulated nitrogen depositions have significant effects on leaf photosynthetic parameters of Stipaaliena. Values of the maximum carboxylation rate (Vcmax), electron transfer rate, triose phosphate transfer rate, dark respiration rate at 25℃ with high nitrogen treatment are 39%,30%,28% and 164% higher than control. The decrease in GPP due to increasing dark respiration offset the increase in GPP due to increasing Vcmax after nitrogen addition, which caused GPP increased slightly. The observed data measured by AccuPAR LP-80 and LAI-2200 show that LAI has no significant change with both level and form of nitrogen addition. The contributions of changing specific leaf area, leaf nitrogen allocated to Rubisco, and the ratio of root to shoot to increased aboveground biomass remains to be further studied. Simulation results of CEVSA2 show that different levels of nitrogen addition increased GPP, NPP,vegetation carbon pool and soil carbon content in the Haibei alpine meadow ecosystem, but the increase was not significant. Nitrogen addition had no significant effect on soil carbon content. Nitrogen deposition is usually observed to decrease the decomposition and increase the soil carbon content. The simulation method of soil carbon and nitrogen interaction in CEVSA2 needs to be improved.