Dinitrogen fixation in soil and alfalfa nodules in the presence of nitrification inhibitors

Abstract

Laboratory and growth chamber studies were conducted to determine the effect of four nitrification inhibitors, 2-chloro-6-(trichloromethyl)pyridine (nitrapyrin), 5-ethoxy-3-trichloromethyl-1.2.4-thiadiazole (etridiazole), dicyandiamide (DCD) and potassium methyl xanthate (KMX) on asymbiotic N 2 fixation in soil and on nodulation and symbiotic N 2 fixation in alfalfa ( Medicago sativa) nodules. Nitrogenase activity in the soil and in nodules was determined by the C 2H 2 reduction assay. Plant dry wt was also used to assess the effect of the nitrification inhibitors on the Rhizobium —alfalfa symbiosis. The total number of nodules and the weights of fresh nodules were measured to assess the effects on nodulation. Of the chemicals tested, only KMX inhibited nitrogenase activity in soil. The decrease in nitrogenase activity caused by KMX at rates effective in inhibiting nitrification (6 and 12 μgg −1) was low and the effect disappeared between 16 and 32 days of incubation of KMX in soil. Nitrapyrin at 2 μ g ml −1 increased nitrogenase activity of excised alfalfa nodules, and DCD at 60 μ gml −1 decreased nitrogenase activity. Also, nitrapyrin at 2 μg ml −1 increased nodule numbers on alfalfa plants grown in nutrient solution. High rates of all four compounds (nitrapyrin and etridiazole, 20 μg ml −1 DCD and KMX, 60 μg ml −1) significantly reduced nodule numbers. Nitrapyrin and DCD applied at high rates also reduced nodule and plant weights. The results suggest that the nitrification inhibitors have the potential to affect N 2-fixation processes in soil and nodules, but that the effect is not likely to be of practical significance when the compounds are used at rates normally required to inhibit nitrification.