Storage and internal cycling of nitrogen in relation to seasonal growth of Sitka spruce

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Abstract
Three-year-old clonal cuttings of Picea sitchensis (Bong.) Carr. were grown for two years (1988–1989) in sand irrigated with a nutrient solution containing either 1.0 mol N m−3 (low N) or 6.0 mol N m−3 (high N) NH4NO3. In 1988, all the N provided was enriched with 15N to 4.95 atom % (labeled N). In 1989, N was supplied with 15N at natural abundance (unlabeled N). The recovery of unlabeled and labeled N in new foliage was used to quantify the internal cycling of N. In the high-N treatment, trees had two flushes of shoot growth and a period of rapid root growth, which coincided with the second flush of shoot growth in August. The timing of root growth and the first flush of shoot growth was similar in the low-N treatment, but there was no second flush of shoot growth and a greater proportion of biomass was recovered in roots. By November 1989, the root/needle dry matter ratio was 1.95 for the low-N trees and 1.36 for the high-N trees. Nitrogen was stored overwinter in roots and current-year needles. During the first six weeks of growth in the spring of 1989, stored N was remobilized for new foliage growth. Subsequent growth depended on root uptake of N. Remobilization of stored N was apparently not affected by the current N supply, because the amount of unlabeled N recovered in foliage produced in 1988 was the same for both N treatments. During 1989, the proportion of 15N remobilized from roots relative to that from leaves produced in 1988 was greater in low-N trees than in high-N trees. In the autumn of both years, there was rapid uptake of N into roots and current-year needles. The effects of N supply on tree growth and nitrogen use efficiency are discussed in terms of the capacity for both N storage and internal cycling.

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