Nitrogenase activity and vegetative regrowth of alfalfa grown alone and in mixture with grass after successive harvests

Abstract

A field experiment was conducted with the purpose of establish the behavior of the nitrogenase activity and nodule mass of alfalfa plants and the effect that some grass species may have on the rate of legume N2-fixation during many successive harvest cycles, when both grown in mixed stands. The results have shown that nitrogenase activity of alfalfa (Medicago sativa L.) is not significantly affected when grown in association with timothy (Phleum pratense L.) or bromegrass (Bromus inermis Leyss.). It was concluded that mixed cultures of alfalfa with timothy or bromegrass have no effect on alfalfa N₂-fixation (acetylene reduction). The initial decline in nitrogenase activity within two days of harvest, and subsequent fast recovery of nodule activity after harvest associated with the maintenance of the nodule weight of alfalfa suggest that the alfalfa plants have an adaptive mechanism to the stress of shoot removal. This may explain why the capacity of N₂-fixation by alfalfa is only temporarily impaired with fast recovery rate in comparison to other forage legumes.