FORAGE LEGUME ROOTS AND NODULES AND THEIR ROLE IN NITROGEN TRANSFER

Legumes can transfer significant amounts of symbiotically fixed N to neighboring plants, and a putative pathway for N transfer is decomposition of fine roots (those that are not secondarily thickened) and nodules. Our objective was to quantify the amount of N in living and dead roots and nodules of alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.). These values were used with estimates of root length and nodule numbers that decomposed during a season to estimate the size of this pathway of N transfer. Birdsfoot trefoil and effectively and ineffectively nodulated alfalfa were grown in root observation boxes filled with N-15-labeled topsoil (Typic Hapludolls). Roots and 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 in alfalfa and 0.14 mg N m-1 in trefoil, and did not change with age. Specific N content did not decline in dead, nondecomposed roots of N2-fixing plants, indicating either that no significant N remobilization occurred or that N import equaled N export in these organs. Large, active birdsfoot trefoil nodules (>2 mm) had twice the N content of large active alfalfa nodules. Dead nodules, observed in birdsfoot trefoil only, 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 contained 89% and 94% fixed N, respectively. We conclude that alfalfa releases more N through decomposing roots than nodules, whereas birdsfoot trefoil contributes more N to the soil through decomposing nodules than roots, but neither process could account fully for published estimates of N transfer.