Supplemental Nitrogen Effects on Alfalfa Regrowth and Nitrogen Mobilization from Roots

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Recent studies suggest that alfalfa (Medicago sativa L.) root carbohydrate concentrations are not correlated closely with shoot regrowth, and that root N reserves may influence shoot regrowth after harvest. The objectives of this greenhouse study were to (i) determine the effects of supplemental N on alfalfa regrowth and N uptake, (ii) quantify N remobilization from roots to regrowing shoots, and (iii) determine if certain root N pools, including vegetative storage proteins (VSPs), were utilized preferentially as N sources during early shoot regrowth. Four cultivars with contrasting fall dormancy reaction were defoliated and supplied with 85 mg N (kg soil)−1 as 15N‐enriched (NH4)2SO4 or KNO3. Samples were taken immediately and once weekly for 28 d. After 28 d, plants were again defoliated and shoots, crowns, and roots of one‐half of the plants sampled immediately. Intact crowns and roots of remaining plants were transplanted into sand, and shoots, crowns, and roots sampled 10 d after transplanting. Taproot N from these samples was partitioned into four specific pools. Nitrate‐N uptake was greater than NH4‐N uptake at 7 and 28 d after N application. Ten days after transplanting, 25% of crown and root total N and 35% of crown and root 15N was remobilized to regrowing shoots. Fifty‐five percent of total root N was soluble in phosphate buffer. The buffer‐soluble N pool was fractionated into a low molecular weight N pool (≈30% of total root N), a non‐VSP protein N pool (≈20% of total root N), and a VSP‐enriched protein N pool (≈5% of total root N). The low molecular weight N and VSP‐enriched protein N pools in roots were utilized preferentially as N sources for regrowing shoots 10 d after transplanting. Accumulation of these root N pools may be important for rapid early shoot regrowth after defoliation.