© 1994 Heron Publishing—Victoria, Canada
Relationships between nutrient supply, nitrogen partitioning and growth in young Sitka spruce (Picea sitchensis)
M. F. Proe and P. Millard
Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2QJ, Scotland, U.K. / Received October 21, 1992
Summary
Budget studies have shown that internal cycling may contribute a large proportion of the annual nutrient supply required to
support new growth in trees. Use of budgets to quantify internal cycling only quantifies the net transfer of nutrients within
the plant. Differential partitioning of remobilized nutrients and current nutrient uptake could lead to errors in the interpretation
of results from these studies. We have quantified the dynamic relationships among tree growth, nutrient uptake and internal
cycling by labeling the current uptake of N in trees that received contrasting amounts of nutrient.
Two-year-old seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.) were grown in sand culture in a greenhouse for one year. The trees received nutrients in a balanced solution
at either a high (high-RAR) or a low (low-RAR) relative addition rate throughout the experiment. Current N uptake was labeled
with 15N from April 13 to July 25. Thereafter, trees were re-potted in clean sand and unlabeled N applied until November 13.
Overall growth was sustained for approximately 10 weeks before treatment effects were observed. Initially, no differences
in the partition of growth or remobilized N occurred, although partition of current uptake favored the roots of plants in
the low-RAR treatment. After 6 weeks, the partition of both growth and remobilized N altered in favor of roots of plants in
the low-RAR treatment. Nutrient supply had no effect on the amount or rate of N remobilization. No evidence was found to suggest
that N taken up in the current season and partitioned to preexisting shoots or roots is remobilized late in the season to
support growth of new shoots. However, some trees in the high-RAR treatment exhibited a second flush of growth later in the
season that was partially sustained by remobilization of 15N from current shoots formed earlier in the season. Use of 15N demonstrated differential partitioning of current uptake and remobilized N. The results highlight the limitations of simple
budget studies for quantifying internal cycling.
Keywords:
internal cycling, nutrient budgets, remobilization, root/shoot ratio.
