Four-year elevated atmospheric CO2 increases soybean seed yield and reshapes fatty acid composition without consistently altering crude oil content.

Elevated atmospheric CO2 concentration (eCO2) can stimulate crop productivity, but whether soybean oil-related responses remain stable under long-term CO2 enrichment remains unclear. We conducted a four-year open-top chamber experiment to examine seed yield, crude oil content, fatty acid composition, and oil quality indices in two soybean cultivars. Elevated CO2 consistently increased seed yield by 18-54%, with an average increase of 31%, but did not consistently increase crude oil content. Linoleic acid remained the dominant fatty acid, followed by oleic, palmitic, linolenic, and stearic acids, indicating that the basic soybean oil profile was conserved. However, eCO2 generally reduced saturated fatty acids and increased the unsaturated fatty acids/saturated fatty acids (UFA/SFA) ratio, whereas the oleic/linoleic and ω-6/ω-3 ratios showed substantial cultivar- and year-dependent variation. These results suggest that future CO2 enrichment may enhance soybean productivity while inducing complex, trait-specific changes in oil composition and quality-related attributes.