The growth of plants in elevated CO2 (eCO2) compared to current atmospheric CO2 concentrations typically reduces the concentration of nitrogen (N) in their tissues, especially leaves.  The causes of such N decreases are not fully understood, but are likely due to multiple factors.  For example, eCO2 can sometimes decrease the N-uptake rate (NUR) of roots (on a mass or area basis); however, the effects of eCO2 on NUR, both direction and magnitude, are inconsistent across studies.  To investigate why effects of eCO2 on NUR are variable, we re-analyzed NUR data from seven different studies of ours, some published and some not, each examining a single species (tomato, barley, or wheat) provided N as nitrate, ammonium, or both, and at low and/or high levels depending on study.  We sought to determine if the direction (increase or decrease) and magnitude of eCO2 effects on NUR were correlated with effects on other plant traits.  We found that effects of eCO2 on NUR were strongly and linearly correlated with effects of eCO2 on root %N and protein (r2 = 0.74 and 0.72, respectively), but less so with shoot-to-root mass ratio (r2 = 0.37) and shoot %N (r2 = 0.23).  These results suggest that whether eCO2 increases or decreases NUR depends on if it increases or decreases root N status, which is highly variable, and they provide an explanation for past observations that showed variable effects of eCO2 on NUR.