Measurement of the remobilization of nitrogen for spring leaf growth of trees under field conditions.

Remobilization of N for leaf growth in the spring was studied in adjacent plots of Picea sitchensis and Acer pseudoplatanas. Before the onset of bud burst in April 1992, a solution of (15)NH(4) (15)NO(3) (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 (15)N would remain in the soil nitrate- and ammonium-N pools, and so the appearance of (15)N in the leaves would indicate when soil N uptake started to contribute to leaf growth. Leaf N present before the appearance of (15)N 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 sumer. 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 (15)N. 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 (15)N was recovered, indicating that remobilization of N accounted for approximately one third of leaf N.