N resource of grasses and N2-fixation of alfalfa in mono-culture and mixture

The N behavior in alfalfa and gramineous forage grasses, tall fescue, siberian wild rye, wheat grass and awnless brome were studied in potting and pasture experiments in 1986-1988 by using 15N isotope dilution technique. Comparison was made between the mixed culture and mono-culture. The % Ndff and %Ndfs of grasses were decreased by 14.19% and 20.76% respectively, while %Ndfa of alfalfa was increased by 20.22% in mixed culture as compared with mono-culture. The 15N and soil N uptake data revealed that this enhancement was largely due to a lower competitive ability for soil N by alfalfa than by grass in mixed stands, causing the alfalfa to depend more on atmospheric N2 fixation. 20.62%of N of grasses in mixed culture was from the N2-fixation by alfalfa, causing N level in root-sphere of alfalfa decreasing, which was considered to be one of the reasons that %Ndfa increased in mixed culture. N transfer may be carried out by the decomposition of roots and nodules of alfalfa plants