Abstract: The effects of elevated CO₂ on terrestrial ecosystem processes are highly related to the N cycling in the ecosystem. In two closed chambers with different CO₂ concentration (ambient CO₂ 355～370 μmol·mol^-1, and elevated CO₂ 700 μmol·mol^-1), pot experiments were conducted to study the responses of Onobrychis viciaefolia Scop and soil nitrogen contents to elevated CO₂. The results showed that compared with ambient CO₂, elevated CO₂ increased the plant dry biomass by 25.1% (P<0.01), but decreased the plant N concentration by 25.3% (P<0.001), resulting in little change of total plant N. After three months experiment, soil total N, NO₃^--N and NH₄⁺-N under both ambient and elevated CO₂ conditions decreased, while soil microbial biomass N increased significantly. Soil NH₄⁺-N concentration had little difference under ambient and elevated CO₂ conditions, but under elevated CO₂ condition, soil NO₃^--N concentration decreased significantly, while microbial biomass N increased significantly. By the end of the experiment, the total N content of whole soil-plant ecosystem had a slight increase, especially under elevated CO₂ condition, which was probably due to the higher N-fixation capability of O. viciaefolia Scop under CO₂ enrichment.

Key words: Eringonidium graminicola Sundevall, Adonia variegata Goeze, Nabis sinoferus Hsiao, Predation function, Helicoverpa armigera Hubner