Carbon–Nitrogen Coupling in Agroecosystems under Elevated CO₂ Scenarios: A Review

Plants assimilate carbon mostly by the process of photosynthesis, fixing atmospheric CO₂ into organic materials like carbohydrates. These materials not only form the structural components of plant biomass but also substrates that energize soil microbial populations. Carbon–nitrogen (C–N) coupling is a key element controlling agroecosystem productivity, soil fertility, and the earth's climate. Elevated atmospheric CO₂ levels (eCO₂) promote photosynthesis and biomass production, while augmenting nitrogen (N) requirement in a way that tends to trigger nutrient dilution effects and decreased nutritional quality of crops. This review compiles conceptual foundations, empirical reactions, management practices, and issues underpinning C–N coupling under eCO₂. It emphasizes the dual function of eCO₂ to stimulate plant growth while amplifying N limitation, modifying belowground processes, and impacting greenhouse gas (GHG) processes. Management practices like integrated nutrient management, conservation tillage, legume integration, and microbial interventions hold potential for augmenting C–N balance. Yet, there are uncertainties arising from soil heterogeneity, climate interactions, and limitations of existing models. Future work should focus on long-term multifactor experiments, advanced modeling structures, and innovations in breeding to balance productivity, nutritional value, and environmental stability. Enhancing C–N coupling in eCO₂ is critical for sustainable agriculture, mitigation of climate change, and food security at the global level. Weak coupling of carbon and nitrogen cycles can contribute to nutrient dilution in crops, loss of soil fertility, and enhanced nitrous oxide (N₂O) emissions, which could reduce otherwise expected climate change mitigation.