DINITROGEN FIXATION BY 7 LEGUME CROPS IN ALASKA

Information on amounts of N-2 fixed by legumes in subarctic regions is lacking. We determined the N-2-fixation potential of seven legume spe cies 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.), fababe an (Vicia faba L.), lentil (Lens culinaris Medik.), pea (Pisum satipum L.), and white lupin (Lupinus alba L.) on a neutral (pH 7.2) and an a cid (pH 5.4) soil during two field seasons. A Lime variable was includ ed at the acid soil location. Dinitrogen fixation was estimated by the N-15 isotope dilution method and by the total-plant N difference meth od. Three non-N-2-fixing crops and uninoculated analogs at one locatio n were used as reference crops. Agreement among reference crops and be tween methods of estimating N-2 fixation was adequate for most applica tions, but differences of >50 kg ha(-1) were sometimes observed. Maxim um N-2 fixation estimates by the isotope dilution method were 58 kg ha (-1) for alfalfa, 121 for red clover, 109 for sweetclover, 204 for fab abean, 72 for lentil, 121 for pea, and 162 for white lupin. Liming usu ally 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.