Dinitrogen Fixation by Seven Legume Crops in Alaska

Abstract

Abstract Information on amounts of N 2 fixed by legumes in subarctic regions is lacking. We determined the N 2 ‐fixation potential of seven legume species under field conditions in subarctic Alaska. Seasonal N 2 fixation was estimated for alfalfa ( Medicago sativa L.), red clover ( Trifolium pratense L.), yellow sweetclover ( Melilotus officinalis Lam.), fababean ( Vicia faba L.), lentil ( Lens culinaris Medik.), pea ( Pisum sativum L.), and white lupin ( Lupinus alba L.) on a neutral (pH 7.2) and an acid (pH 5.4) soil during two field seasons. A lime variable was included at the acid soil location. Dinitrogen fixation was estimated by the 15 N isotope dilution method and by the total‐plant N difference method. Three non–N 2 ‐fixing crops and uninoculated analogs at one location were used as reference crops. Agreement among reference crops and between methods of estimating N 2 fixation was adequate for most applications, but differences of >50 kg ha −1 were sometimes observed. Maximum N2 fixation estimates by the isotope dilution method were 58 kg ha −1 for alfalfa, 121 for red clover, 109 for sweetclover, 204 for fababean, 72 for lentil, 121 for pea, and 162 for white lupin. Liming usually did not significantly affect N 2 fixation. Nitrogenase activity, as measured by the acetylene reduction assay, peaked at or soon after flowering in some legumes; in others, it either plateaued or continued to increase after flowering until the end of the growing season.