Alfalfa { M e d k a g o s a t i v a L) is a forage crop that establishes a symbiotic relationship with an N2-fixing bacterium { S i n o r h i z o - b i u m m e l i l o t i L). According to Udvardi and Day (1997), because legumes form this symbiotic association, there is an extra sink for exchange of additional C with the nodule to enhance N2 fixation in these plans.