Nitrogen Uptake During Fall, Winter and Spring Differs Among Plant Functional Groups in a Subarctic Heath Ecosystem

Nitrogen (N) is a critical resource for plant growth in tundra ecosystems, and species differences in the timing of N uptake may be an important feature regulating community composition and ecosystem productivity. We added 15N-labelled glycine to a subarctic heath tundra dominated by dwarf shrubs, mosses and graminoids in fall, and investigated its partitioning among ecosystem components at several time points (October, November, April, May, June) through to the following spring/early summer.

Soil microbes had acquired 65 ± 7% of the 15N tracer by October, but this pool decreased through winter to 37 ± 7% by April indicating significant microbial N turnover prior to spring thaw. Only the evergreen dwarf shrubs showed active 15N acquisition before early May indicating that they had the highest potential of all functional groups for acquiring nutrients that became available in early spring.

The faster-growing deciduous shrubs did not resume 15N acquisition until after early May indicating that they relied more on nitrogen made available later during the spring/early summer. The graminoids and mosses had no significant increases in 15N tracer recovery or tissue 15N tracer concentrations after the first harvest in October.

However, the graminoids had the highest root 15N tracer concentrations of all functional groups in October indicating that they primarily relied on N made available during summer and fall. Our results suggest a temporal differentiation among plant functional groups in the post-winter resumption of N uptake with evergreen dwarf shrubs having the highest potential for early N uptake, followed by deciduous dwarf shrubs and graminoids.