Purpose: The increase in CO₂ concentration significantly impacts the nitrogen cycle dynamics which is closely related to plant growth dynamics and can change microbial activities in terrestrial ecosystems. However, this is dependent on environmental factors affecting soil types, plant types and microbial systems etc. In this study, the effect of elevated CO₂ concentration on changes in nitrogen isotopic composition in soil and leaves of alfalfa, a nitrogen-fixing plant, and the response of the nitrogen cycle process are studied in the Loess Plateau of China.

Methods: Hexagonal open-top chambers were set up in the experimental area. The automatic control system was adopted to adjust the rate of CO₂ spraying. Alfalfa seeds (seedling) (20 seeds (seedlings) per pot) were planted and 50g of leaves were collected. Soil samples were pretreated before planting. The samples were washed and deactivated for determination of total nitrogen content and δ¹⁵N value. The content of NO₃-N and NH₄⁺-N in soil were determined. The NO₃^--N and NH₄⁺-N contents were also determined. The ¹⁵N isotope values in soil and plant leaves were pre-treated and then δ¹⁵N-NO₃^- was determined. The experimental data were initially organized and analyzed using Microsoft Excel 2019 and statistical analysis was performed using SPSS 27.0.

Results: The results showed that under the semi-arid climate conditions of the Loess Plateau, with the increase of CO₂, total inorganic nitrogen in soil decreased, the nitrate-nitrogen concentration and the δ¹⁵N are negatively correlated with the CO₂ concentration. It shows that with the increase of CO₂, both nitrification and denitrification in the soil will weaken with the decrease of microorganisms. However, the degree of denitrification weakening is greater than nitrification. The nitrogen level in alfalfa leaves gradually increased with the increase in CO₂ concentration.

Conclusion: This indicated that the rise in CO₂ concentration could promote alfalfa's nitrogen absorption and improve alfalfa's nitrogen utilization efficiency. This study provides a scientific basis for changes in the nitrogen cycle process of soil and plant in the Loess Plateau under the greenhouse effect.