Simultaneous measurement of nitrogen fixation estimated by acetylene-ethylene assay and nitrate absorption by soybeans.

An apparatus was designed for simultaneous measurement of rates of N(2) fixation estimated by C(2)H(2)-C(2)H(4) assay (N(2)[C(2)H(2)] fixation) and NO(3) (-) absorption by roots of intact, nodulated soybeans (Glycine max [L.] Merr.). The principal design features include: (a) a gas-tight mist chamber in which nodulated roots can be exposed simultaneously to C(2)H(2) in the gas phase and to a liquid phase containing NO(3) (-) sprayed in a fine mist; and (b) provision for sampling the gas phase for C(2)H(4) determination, and the liquid phase for NO(3) (-) determination.We studied NO(3) (-) absorption by soybeans as affected by nodulation, NO(3) (-) concentration during assay, and previous N nutrition during growth in nutrient solution culture in controlled environment chambers. It was established that 0.5 mm NO(3) (-) nearly saturated the NO(3) (-) absorption system of both nodulated and unnodulated soybeans when the concentration dependence of NO(3) (-) absorption rate was measured just after flowering began. Nitrate absorption rates were measured after development of N stress in unnodulated plants, and during recovery from N stress in nodulated plants. The results suggested that the lower capacity for NO(3) (-) absorption of nodulated plants was a consequence of N stress during the period of nodule growth and development.Nitrogen [C(2)H(2)] fixation rates were compared in intact plants assayed in the mist chamber and in excised roots assayed in both the mist chamber and in glass jars. Excised roots had a lower N(2)[C(2)H(2)] fixation rate than intact plants. The decline observed during the first hour after shoot removal was more pronounced for glass jar-assayed excised roots than for mist chamber-assayed excised roots.We discuss the advantages of our method for assessing the capability of a nodulated legume to acquire nitrogen through both N(2) fixation and absorption and assimilation of NO(3) (-).