Internal cycling of nitrogen, potassium and magnesium in young Sitka spruce

Potassium (K) and magnesium (Mg) are essential macro-nutrients, but little is known about how they are cycled within plants. Stable isotope studies have shown that the internal cycling of nitrogen (N) is independent of current nutrient supply in temperate tree species. This is ecologically significant because it allows trees to produce rapid shoot growth in spring independent of current soil N uptake. We used stable isotopes to quantify N, K and Mg in new shoots of Sitka spruce (Picea sitchensis (Bong.) Carr.) seedlings and to compare the relative contributions from current uptake and internal cycling. Two-year-old Sitka spruce seedlings were labeled with N-15, K-41 and Mg-26 in an abundant or a limited supply for one growing season. The trees were repotted in the subsequent dormant season to prevent further root uptake of enriched isotopes and provided with an abundant or a limited supply of unlabeled nutrients until they were harvested in early summer of the following year. The supply was switched for half the trees in the second year to create four nutrient regimes. Enrichment of N-15, K-41 and Mg-26 in current-year growth was attributed to internally cycled N, K and Mg uptake from the previous year. The internal cycling of N, K and Mg in new growth was significantly affected by the first-year nutrient treatments. The second-year nutrient supply affected the growth rates of the trees, but had no effect on the amounts of N, K or Mg contributed from internal cycling. Thus, internal cycling of K and Mg in Sitka spruce are, like that of N, independent of current nutrient supply.