USE OF ~( 15) N DILUTION METHOD FOR SCREENING SOYBEAN LINES WITH HIGH YIELD AND HIGH NITROGEN FIXATION ABILITY Using 15N isotopic dilution method to quantify the biological nitrogen fixation in sugarcane EFFECT OF PLANTING PATTERNS ON DINITROGEN FIXATION OF ALFALFA AND TRANSFER OF N FIXED Nitrogen Fixation in Leymus chinensis Grassland in Northeast China N RESOURCE OF GRASSES AND N_2-FIXATION OF ALFALFA IN MONOCULTURE AND MIXTURE