Effects of ¹⁵N‐nitrate fertilization on yield and dinitrogen fixation of vegetative pea plants

Abstract

The dry matter yields and total nitrogen contents in the vegetative shoots of pea plants (Pisum sativum L.), infected with an effective or ineffective strain of Rhizobium leguminosarum, and supplied with increasing amounts of ¹⁵N‐labelled nitrate fertilizer, were studied in a pot experiment with a sandy soil. In the ineffective strain treatment both dry matter yield and shoot nitrogen concentration increased considerably as the amount of ¹⁵N‐labelled nitrate fertilizer was increased from 0 to 400 mg N/pot. When plants had been infected with an effective strain the dry matter yield was independent of the nitrate application rate as was the tissue nitrogen concentration which was high in all cases, indicating that nitrogen supply had been sufficient. Only application of 100 mg N/pot in the effective strain treatment resulted in a slightly depressed nitrogen content of the shoot, indicating a suboptimal collaboration of nitrate uptake and assimilation, and dinitrogen fixation. Calculation of the amount of ¹⁵N‐labelled combined (NO₃‐)nitrogen which was taken up by the plants, based on the ¹⁵N content of the shoot and some simple assumptions, indicated that nodulation was initiated after depletion of nitrate‐nitrogen in the soil. The decreasing contributions of dinitrogen fixation to nitrogen accumulation from 65 to 8% as nitrate supply was increased thus reflect the points of time at which nitrogen nutrition of the plant was altered from nitrate uptake to dinitrogen fixation.

Key words:

• heavy nitrogen
• N‐fertilization
• nitrate
• nodulation
• pea
• Pisum sativum L.
• symbiotic N‐fixation