FORAGE LEGUME ROOTS AND NODULES AND THEIR ROLE IN NITROGEN TRANSFER

Legumes can transfer significant amounts of symbiotically fixed N to n eighboring plants, and a putative pathway for N transfer is decomposit ion of fine roots (those that are not secondarily thickened) and nodul es. Our objective was to quantify the amount of N in living and dead r oots and nodules of alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.). These values were used with estimates of roo t length and nodule numbers that decomposed during a season to estimat e the size of this pathway of N transfer. Birdsfoot trefoil and effect ively and ineffectively nodulated alfalfa were grown in root observati on boxes filled with N-15-labeled topsoil (Typic Hapludolls). Roots an d nodules were removed according to age and analyzed for total N and C . Specific N content of fine roots older than 2 wk was 0.22 mg N m-1 i n alfalfa and 0.14 mg N m-1 in trefoil, and did not change with age. S pecific N content did not decline in dead, nondecomposed roots of N2-f ixing plants, indicating either that no significant N remobilization o ccurred or that N import equaled N export in these organs. Large, acti ve birdsfoot trefoil nodules (>2 mm) had twice the N content of large active alfalfa nodules. Dead nodules, observed in birdsfoot trefoil on ly, had significantly lower N content than live nodules, indicating N remobilization during senescence. Fine roots contained up to 69% of N from fixation in alfalfa and 47% in birdsfoot trefoil, and nodules con tained 89% and 94% fixed N, respectively. We conclude that alfalfa rel eases more N through decomposing roots than nodules, whereas birdsfoot trefoil contributes more N to the soil through decomposing nodules th an roots, but neither process could account fully for published estima tes of N transfer.