Nitrogen fixation by three species of leguminosae in the Canadian High Arctic Tundra

Abstract. Significant levels of nitrogenase activity (nitrogen fixation) were demonstrated in three species of Arctic legumes ( Oxytropis maydelliana, O. arctobia and Astragalus alpinus ) growing in high tundra at Sarcpa Lake, Melville Peninsula, N.W.T. Nitrogenase activity of intact plants was correlated with the number of nodules per plant, with field soil temperatures and limited by water shortage. Activity in freshly detached nodules showed a plateau of maximum activity between 10°C and 25°C and a near linear decline with temperature down to 0°C. Unusually, the segmented nodules of all three species are perennial in which growth and leghaemoglobin production resumes each spring from an overwintering apical meristem. Nodules are most numerous in the warmer soil stratum (2–10 cm. depth). Other studies indicate that the arctic rhizobia belong to a single cold-adapted species which has co-evolved with the legumes of tundra.