Use of stable isotopes to quantify nitrogen, potassium and magnesium dynamics in young Scots pine (Pinus sylvestris)

Abstract

Two-yr-old Scots pine (Pinus sylvestris) seedlings were grown in sand culture for 1 yr with a generous supply of a balanced nutrient solution. Trees were repotted into clean sand in February 1998 and given either a reduced or adequate nutrient supply containing enriched "&N, %"K and #'Mg to label nutrient uptake during spring 1998. Trees doubled their biomass during the experiment. Whole-tree net photosynthesis was reduced by 43% after 95 d in trees that received the lower nutrient supply (P 0n001), although differences in biomass between the two treatments were less pronounced. Remobilization contributed 83, 82 and 52% of the N, K and Mg, respectively, used to support growth of new tissues in trees that received reduced nutrient supply. Those receiving the higher nutrient supply still obtained 44-59% of nutrients used for spring growth of new tissues from remobilization. Current nutrient supply had no significant effect on the amount of N or Mg remobilized to new tissues but K remobilization was less in trees that received the lower nutrient supply (P l 0n025). The importance of remobilization in young trees and problems associated with quantifying internal cycling of nutrients are discussed.

Nitrogen turnover in the leaf litter and fine roots of sugar maple

“…Internal N reserves are critical for new shoot growth in spring, because conditions for root N uptake are not optimal when buds break (Menino et al 2007). Several studies utilizing 15 N have demonstrated that new shoot growth in spring preferentially utilizes stored N (Proe et al 2000, Menino et al 2007) and our results confirm these observations. At our site, the 15 N tracer peaked in leaf litter one year after application.…”

“…Our results and those from other 15 N studies (Proe et al 2000, Menino et al 2007) suggest the use of stored N to construct new shoots in the spring may be a physiological mechanism that has evolved in response to cold soils and a poorly developed vascular system in the early spring. Early in the spring when cold soils limit diffusion in the soil and the metabolic processes required to actively transport N into the root system, the determinate shoots partially differentiated in the buds the prior year expand rapidly in the absence of a fully functional transpiration system.…”

“…The peak one year after the tracer addition is because the majority of N used in construction of shoots in 1998 was stored N (rather than tracer N). As the growing season progresses, shoots of deciduous trees typically rely more upon soil N than N in storage proteins (Proe et al 2000, Menino et al 2007). The fact that the 15 N recovery in fine roots peaked in the year the 15 N was applied suggests that the new roots of sugar maple are not built from stored N, but instead are built from mineral N. However, there are few studies quantifying the seasonality of 15 N in fine roots, and reliable information about the remobilization of N from fine roots still eludes the scientific community.…”

“…For example, young nonbearing orange trees are highly dependent on new inputs of N from the soil (Menino et al 2007). Proe et al (2000) report that current N supply has no significant impact on the amount of N remobilized to support new shoot growth, and as the growing season progresses, uptake of N from the soil becomes increasingly important for the formation of N reserves deployed the following spring. In mature almond trees, new N taken up from the soil represents ;50% of total canopy N (Weinbaum et al 1987).…”

“…We still do not understand if N is retranslocated from fine roots before they die and the existing evidence is equivocal. Millard (1996), Proe et al (2000), and Luyssaert et al (2005) all discuss problems with estimates of nutrient resorption from foliage during senescence if the estimates are not derived from the use of tracer techniques. Equivalent data for fine roots that would enable such a discussion are not available, and our continuous time course for the pool dilution of 15 N in fine roots is problematic compared to that for leaf litter.…”