Influence of Soil Moisture on Growth and Nitrogen Nutrition of Alfalfa

Abstract

Soil pot culture of alfalfa, deep-rooted leguminous plant, was firstly carried out to clarify the effect of ground water levels on the growth, nodule formation and nitrogen fixation. Then the inhibitory effect of soil moisture excess on the symbiotic nitrogen fixation was investigated with special attention to the oxygen concentration in soil atmosphere. 1) Growth of alfalfa at first cutting, aerial parts and underground parts, was restrained when the ground water level in soil pot rose higher than 10 cm. Nodule formation, nitrogen fixation and plant nitrogen concentration in aerial parts also decreased with the ascending of ground water level over 20 cm. 2) The nitrogen fixing activity of the second cutting alfalfa plants regrown after first cutting was markedly inhibited by the ascending of ground water level (10 cm). Namely, in the short period of the ascending of ground water level (1 day), the nitrogen fixing activity was not appreciably affected, while it almost diminished in the long period of the treatment (3-5 days). 3) The low oxygen concentration in culture solution resulted in the decrease of nodule formation, nitrogen fixing activity and specific nitrogen fixing activity of alfalfa grown in solution culture. Moreover, the specific nitrogen fixing activity of detached alfalfa nodules was also recognized to lower with the decrease of ambient O₂ concentration.