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

ABSTRACT

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.

Keywords:

• photosynthesis
• chlorophyll
• nitrate
• ammonium
• nitrogen
• Juglans nigra