Rare earth elements have been used for 30 years in Chinese agriculture to improve growth and yield. Numerous scientific studies have shown improvements in physiology, mineral nutrition, and growth, though deleterious effects may also occur. Very few studies have been performed on woody species. We exponentially fertilized one-year old black walnut (Juglans nigra L.) seedlings with 0, 0.1, 1, 10, or 100 mg lanthanum (La) and 800 mg nitrogen (N) as NaNO3, (NH4)2SO4, NH4NO3, or no N. One month following final fertilization, growth, mineral nutrition, photosynthesis, chlorophyll, and nitrate reductase activity were assessed. Plants fertilized with the highest level of La had reduced fine root growth, concentrations of magnesium, calcium, nickel, and phosphorus, photosynthesis levels, and chlorophyll a content. Foliar La concentration showed an interaction effect, with three to four times greater concentration in plants fed at 100 mg La to those given 10 mg La for (NH4)2SO4 and NH4NO3 treatments. The results suggested no beneficial effects of La addition at levels used in this study and interactions between N source and La levels did not have an important impact on the growth, mineral nutrition, or physiology of black walnut seedlings.