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Interactions between carbon isotope composition (δ13C), foliar nitrogen concentration (foliar N), and biomass accumulation (growth) merit further investigation in soybean (Glycine max (L.) Merr.) and other species as these metrics may be valuable for assessing moisture stress and for screening of drought-resistant varieties. To this end, we examined the response of six soybean genotypes to water-deficit stress in a greenhouse study. Two treatments were imposed: low soil moisture (5–10% volumetric water content) and high soil moisture (30–38% volumetric water content). Above-ground biomass accumulation, foliar N, and δ13C were measured at the end of the experiment. Leaf water potential and midday gas exchange (net assimilation, stomatal conductance, and transpiration) were measured multiple times throughout the experiment. All measurements were affected by water-deficit stress. Significant, but weak, positive relationships were found between δ13C and biomass accumulation in both soil moisture treatments. Foliar N was significantly, but weakly, correlated to growth in the high soil-moisture treatment, but not in the low soil-moisture treatment. The data suggest that selection for genotypes with higher δ13C values could result in improved biomass accumulation. The relationship between foliar N and δ13C was negative under high soil-moisture conditions and positive under low soil-moisture conditions. The relationships between δ13C and foliar N could be a highly informative metric to help understand the effects of water-deficit stress and may further indicate whether water or nitrogen acquisition is limiting in a specific environment, which should help in breeding improved soybean cultivars. [ABSTRACT FROM AUTHOR]