Technical Abstract: The atmospheric carbon dioxide concentration ([CO2]) has risen by 30% in the last 250 years and is predicted to rise an additional 50% by 2050. This extremely rapid rise in [CO2] portends critical uncertainty over both carbon uptakes by terrestrial ecosystems, and food production. Although photosynthesis and growth are typically stimulated when plants are grown at elevated [CO2] the stimulation often disappears with time. The observed and modelled attenuation are nearly always associated with the inability of plants to acquire sufficient nitrogen, resulting in the commonly observed reduction in leaf nitrogen content. Here we show, in two consecutive seasons, that unfertilized fields of soybean grown at elevated [CO2] were able to overcome an early season reduction in leaf nitrogen content because additional carbohydrate from increased photosynthesis at elevated [CO2] resulted in improved nitrogen assimilation in subsequent growth. Results from this model field system suggest that nitrogen limitation may not restrict increases in soybean production with rising [CO2] and may not prevent increased carbon uptake into terrestrial ecosystems that include legumes.