Elevated CO2 Alters Plant–Soil Nutrient Dynamics and Exacerbates Nutrient Dilution: A Field Comparison of Legume and Non-Legume Crops

Elevated CO2 (eCO2) is expected to enhance crop productivity but may simultaneously reduce nutritional quality through nutrient dilution. However, responses among crop species with contrasting functional traits and the associated plant–soil responses remain insufficiently understood. Using a multi-crop Free-Air CO2 Enrichment (FACE) platform, we investigated the effects of eCO2 on per-plant biomass and yield-component traits, nutritional quality, soil properties, and mineral element dynamics in four representative crops with contrasting functional traits: soybean, peanut, maize, and sweet potato. Elevated CO2 significantly increased several per-plant biomass and yield-component traits, with the magnitude of these responses varying among crop species and soybean showing pronounced increases in multiple measured traits. Protein concentration decreased significantly by 11.47% in maize and also declined in sweet potato, whereas it remained stable in soybean and increased in peanut, demonstrating crop-specific differences in nutritional responses to eCO2. Bulk soil responses also differed among crop types, with soybean and sweet potato showing significant decreases in soil pH and crop-specific changes in the total concentrations of Zn and Cu; however, these changes were not consistently accompanied by corresponding increases in elemental concentrations in the harvested plant tissues. These findings indicate that changes in total soil elemental concentrations do not necessarily correspond to higher elemental concentrations in edible organs. Overall, the four crop species exhibited distinct responses to eCO2, highlighting the importance of integrating crop nutritional quality, nutrient dynamics, and plant–soil interactions when developing adaptive agricultural management strategies under future climate change.