Effects of Potassium Fertilization on Nitrogen Fixation and Nodule Enzymes of Nitrogen Metabolism in Alfalfa

Studies with four-year-old ‘Vernal’ alfalfa (Medicago saliva L.) fertilized yearly from planting with 673 kg/ha K as K₂SO₄ or KC1 and 448 kg/ha K as K₂SO₄ were made to determine the effects of K on nodulation, acetylene reduction, and nodule enzyme activities. The high rates of K fertilization increased nodule number and acetylene reduction rates over control plants, although the greatest increase in nodule mass and acetylene reduction rates was with the 673 kg/ha K as K₂SO₄ treatment. This indicates that Cl and/or S may alter or mask the effects of K fertilization. Increases in acetylene reduction rates on a plant basis with K fertilization were due to increases in nodule mass and not increases in activity per unit weight of nodule tissue. With all treatments a positive linear correlation (P <0.01, r = 0.854) was found between nodule number and acetylene reduction rate. The greatest increase in shoot weight per plant over control plants was with the 673 kg/ha K as KC1 treatment. Positive linear correlations were found between rates of acetylene reduction and the activities of enzymes involved in ammonia assimilation (NAD-glutamate dehydrogenase, glutamine synthetase, NADP-glutamate dehydrogenase), amino acid interconversions (glutamate oxaloacetate transaminase), C supply (NADP-isocitrate dehydrogenase) and energy transduction (glucose-6-phosphate dehydrogenase, malate dehydrogenase). Due to lower nodule protein concentrations and higher nodule masses in the K treatments, specific activities and, in general, activities on a per plant basis of enzymes assayed from K treatments were much higher than those of the control treatment. Malate dehydrogenase and NADP-glutamate dehydrogenase specific activities were significantly higher than control plants for both 673 kg/ha K treatments. There were few differences between treatments for enzyme data calculated on a gram fresh-weight basis. Yields of the two 673 kg/ha K treatments were nearly equal at third cut and were significantly higher than those of control plants. These data suggest that K fertilization increases alfalfa yields by increasing nodulation and N₂-fixation.