Effects of Increasing CO₂ Concentration on Crop Growth and Soil Ammonia-Oxidizing Microorganisms in a Fababean (Vicia faba L.) and Wheat (Triticum aestivum Yunmai) Intercropping System

Elevated carbon dioxide (eCO₂) levels can enhance crop yields but may simultaneously reduce quality, impacting both macronutrient and micronutrient concentrations, and potentially decreasing protein content in cereal grains. This study examined the effects of elevated CO₂ (eCO₂) and nitrogen (N) fertilization on crop growth, yield, and soil nitrogen cycling through a glass greenhouse experiment using Eutric Regosol soil. The experimental design incorporated two CO₂ gradients: ambient CO₂ (aCO₂) at approximately 410 ppm during the day and 460 ppm at night, and eCO₂ at approximately 550 ppm during the day and 610 ppm at night. Additionally, two nitrogen fertilization treatments were applied: no fertilizer (N0) and 100 mg N kg⁻¹ dry weight (DW) soil (N100). Crops were cultivated under two cropping systems: the monoculturing of fababean (Vicia faba L.) or wheat (Triticum aestivum Yunmai) and the intercropping of both species. The results demonstrated that eCO₂ significantly enhanced the growth and yield of both fababean and wheat, particularly when nitrogen fertilization was applied. Nitrogen fertilizer application did not always enhance crop yield, considering the complexity of nitrogen management under elevated CO₂ conditions. Furthermore, the intercropping of fababean and wheat presented multiple advantages, including improved crop yields, enhanced soil health, and increased ecosystem services. These findings suggest that intercropping can serve as a sustainable strategy to boost productivity and ecosystem resilience in the face of climate change. The changes in nitrogen application and CO₂ concentration affect the gene copy number of ammonia-oxidizing bacteria and archaea, which may affect the nitrogen cycling process in soil. There are complex interactions between crop biomass, nitrogen accumulation, transpiration rate, photosynthetic rate and stomatal conductance with soil properties (e.g., pH, organic matter, nitrogen content) and microbial community structure. The interaction between CO₂ concentration, nitrogen application level and crop intercropping pattern had significant effects on crop growth, soil properties and microbial communities. Future research should prioritize investigating the long-term effects of intercropping on soil productivity and the development of management strategies that optimize the benefits of this cropping system.