| The elevated CO2 concentration, drought stress and nitrogen deposition have significant effect on the structure and function of the terrestrial ecosystem. The simulation of elevated CO2, drought stress and nitrogen deposition experiments are significant in predict the response of terrestrial ecosystem under the future global climate change. The present research chose Bothriochloa ischaemum, which are the climax community of hervaceous vegetation on the sunny slopes in the natural succession on the abandoned cropland, as study subjects. A pot experiment was conducted to evaluate the effects of elevated CO2, drought stress and nitrogen application on the photosynthetic characteristics, plant biomass, photosynthetic product, mineral elements and their allocations, the rhizosphere soil chemical properties and microbial properties. There were two CO2 concentration accomplished using artificial atmospheric phenomena simulator, three water treatment controlled by artifical weighting, and three nitrogen application rate supplied as NH4NO3. The purpose of the present research was to provide scientific basis for deeper understanding the response of terrestrial vegetation on the Loess Plateau under the global climate change. The main results are as follows:1. Elevated CO2 concentration and nitrogen addition significantly increased the SPAD value, maximum net photosynthetic rate, and light compensation point; while these parameters were all significantly decreased by drought stress. CO2 concentrations, soil water conditions, and nitrogen addition treatments had highly significant interaction effects on photosynthetic capaticy of B. ischaemum. Nitrogen addition was the most important factor that affects the photosynthesis characteristics of B. ischaemum. Elevated CO2 concentration and nitrogen addition had compensation effect on the photosynthetic reduction of B. ischaemum induced by drought stress under the background of global climate change.2. Elevated CO2 concentration and nitrogen addition significantly increased the biomass and root to shoot ratio of B. ischaemum, while drought stress significantly decreased the biomass and root to shoot ratio of B. ischaemum. And there were significantly interaction effects of the three factors on the biomass and its allocation of B. ischaemum. Nitrogen addition decreased soluble sugar, increased starch and non-structural carbohydrates; elevated CO2 concentration increased soluble sugar, starch and non-structural carbohydrates; drought stress significantly increased the content of soluble sugar, while the starch and non-structural carbohydrates significantly decreased. The nitrogen content in the whole plant of B. ischaemum was significantly increased with nitrogen addition; elevated CO2 concentration significantly increased the nitrogen content of the aboveground part, while had no impact on the nitrogen content of the root part; drought stress significantly decreased the nitrogen content of the B. ischaemum. Elevated CO2 concentration, drought stress and nitrogen addition had significantly interaction effects on the nitrogen content of the aboveground part of B. ischaemum. Nitrogen addition and drought stress significantly decreased the phosphorous content of the B. ischaemum; elevated CO2 concentration decreased the phosphorous content of the root part of B. ischaemum. Overall, elevated CO2 concentration and nitrogen addition provided the source of soluble sugar when plant suffers drought stress, which was benefit for the normal growth of plant and for the carbon and nitrogen balance in the plant.3. Elevated CO2 concentration, drought stress and nitrogen addition had no significant influence on the content of soil organic carbon, total nitrogen, and total phosphorous of the rhizosphere and bulk soil of B. ischaemum. The total phosphorous content in the rhizosphere soil was significantly lower than the bulk soil under the treatment of CAN1 and CAN2. The nitrate nitrogen content and ammonium nitrogen content in the rhizosphere soil was lower than in the bulk soil, and there were significant difference under the treatment of N1 and N2. B. ischaemum had higher absorption rate of nitrate nitrogen than ammonium nitrogen. Nitrogen addition significantly decreased the ammounium nitrogen content in the rhizosphere soil, while the nitrate nitrogen content in the rhizosphere soil, nitrate nitrogen content and ammonium nitrogen content in the bulk soil were all significantly increased with the nitrogen addition. Drought stress also significantly increased the nitrate nitrogen content and ammonium nitrogen content in the rhizosphere soil and bulk soil. The soil water dissolved organic carbon(DOC) was slightly lower in the rhizosphere soil than in the bulk soil under different treatment, and the bulk soil had significantly higher DOC content than rhizosphere soil in the N2 treatment. The rhizosphere soil had significantly higher water dissolved ammonium nitrogen content(NH4+), but significantly lower total soil dissolved nitrogen content(TSN), water dissolved nitrate nitrogen content(NO3-) and water dissolved organic nitrogen content(DON) than the bulk soil. Nitrogen addition increased TSN and NO3- in the rhizosphere soil and bulk soil, and increased DON in the rhizosphere soil. Elevated CO2 concentration significantly decreased the DON in the rhizosphere soil in the treatment of N0 and N1. The ratio of DON/TSN in the bulk soil was significantly decreased with the nitrogen addition, while this ratio in the rhizosphere soil had no significant change with the nitrogen addition. Elevated CO2 concentration decreased the ratio of DON/TSN in the rhizosphere soil. The ratio of DOC/DON in the rhizosphere soil was significantly decreased with nitrogen addition, while elevated CO2 concentration significantly increased this ration under the treatment of N0 and N1.4. The basal respiration(BR) was significantly higher in the rhizosphere soil than bulk soil. Nitrogen addition significantly increased BR, while elevated CO2 and drought stress decreased BR. The substrate-induced respiration(SIR) showed no significant difference in the rhizosphere soil and bulk soil. Elevated CO2 and nitrogen addition had no significant influence on SIR in the rhizosphere soil, while drough stress significantly decreased the SIR in the rhizosphere soil. The soil microbial functional diversities showed no significant difference in the rhizosphere soil and bulk soil under elevated CO2, drought stress and nitrogen addition. The bulk soil had higher untilization rate for carbohydrates and carboxylic acids than the rhizosphere soil. The total PLFA content, G+/G- ratio, and F/B ratio was significantly higher in the rhizosphere soil than the bulk soil. The total PLFA content in the rhizosphere soil significantly increased with nitrogen addition; elevated CO2 concentration increased the fungi PLFA content and F/B ratio, decreased G+/G- ratio in the rhizosphere soil; drought stress decreased total PLFA content in the rhizosphere soil, and decreased F/B ratio in the rhizosphere soil and bulk soil. |
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