Winter legume cover crops are a key component of nutrient management in organic agroecosystems. Accurate estimates of N fixation are important for determining the balance of N inputs and outputs and the potential for N loss from these systems. Our first estimates of N fixation for Vicia faba (bell beans) and Vicia dasycarpa (woollypod vetch) using the 15N natural abundance method were unexpectedly high across all field histories and fertility levels, e.g., 70-98 %Ndfa for vetch, using oats (Avena sativa) as the reference crop. We also observed a negative relationship between d15N of the non-fixing reference crops and time since last compost addition but no relationship between legume d15N and compost history, indicating different N pool use by the legumes and reference crop. In a greenhouse experiment, we measured active nodule weight as a proxy for fixation activity to ask if fertility had an effect on early crop N fixation. Preliminary analysis showed active nodules in early bell bean and vetch growth (3.5 wk for bell bean, 2.5 wk for vetch), independent of fertility levels (0, 75 and 200 kg N/ha added to field soil). Evidence of early fixation activity may support the validity of the high %Ndfa estimates. We also present data from a second greenhouse experiment, in which we determined %Ndfa by 15N natural abundance for vetch grown in field soil with six different management histories and four fertility treatments. These results may help us further determine the potential for high N fixation even under high available soil N conditions.
