Growth, Nutrition, and Photosynthetic Response of Black Walnut to Varying Nitrogen Sources and Rates

Black walnut (Juglans nigra L.) half-sib 1+0 seedlings were exponentially fertilized with ammonium (NH4+) as ammonium sulfate [(NH4)2SO4], nitrate (NO3−) as sodium nitrate (NaNO3), or a mixed nitrogen (N) source as ammonium nitrate (NH4NO3) at the rate of 0, 800, or 1600 mg N plant−1 and grown for three months. One month following the final fertilization, N concentration, growth, and photosynthetic characteristics were assessed. Compared with unfertilized seedlings, N addition increased plant component N content, chlorophyll content, and photosynthetic gas exchange. Net photosynthesis ranged from 2.45 to 4.84 μmol m−2 s−1 for lower leaves but varied from 5.95 to 9.06 μmol m−2 s−1 for upper leaves. Plants responded more favorably to NH4NO3 than sole NH4+ or NO3 − fertilizers. These results suggest that N fertilization can be used to promote net photosynthesis as well as increase N storage in black walnut seedlings. The NH4NO3 appears to be the preferred N source to promote black walnut growth and physiology