Measurement of the remobilization of nitrogen for spring leaf growth of trees under field conditions
1994
Millard, P. (The Macaulay Land Use Research Institute, Aberdeen, U.K.)
Remobilization of N for leaf growth in the spring was studied in adjacent plots of Picea sitchensis and Acer pseudoplatanus. Before the onset of bud burst in April 1992, a solution of 15NH4 15NO3 (enriched to 7.5 atom %) was applied to the soil, to supply 20 g N m-2 over the total area of each plot. It was assumed that 15N would remain in the soil nitrate- and ammonium-N pools, and so the appearance of 15N in the leaves would indicate when soil N uptake started to contribute to leaf growth. Leaf N present before the appearance of 15N from the fertilizer was assumed to be remobilized N. Growth of P. sitchensis needles continued for 45-50 days after bud burst, but their N content showed no consistent pattern throughout the spring and early summer. The majority of needle N was already present at the initial samplings, immediately after bud burst, and any increases in needle N content occurred concomitantly with uptake of fertilizer 15N. Acer pseudoplatanus leaves also grew for the duration of the experiment and their N content increased for the first 35 days after bud burst, reaching approximately 15 mg N leaf-1. Acer pseudoplatanus leaves had approximately 5 mg N leaf-1 before any 15N was recovered, indicating that remobilization of N accounted for approximately one third of leaf N
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