Nitrogen Retention of Terricolous Lichens in a Jack Pine Forest in Northern Alberta

Description

The Athabasca Oil Sands in Alberta, Canada is one of the largest point sources emitters of NOx in Canada and there
are concerns that elevated nitrogen (N) deposition will lead to widespread eutrophication impacts, including altered
species composition, similar to what has occurred in several parts of Europe. Atmospheric deposition rates as high as
25 kg N ha-1 yr-1 have been measured close to the industrial center. The role of the forest floor in regulating these
potential eutrophication effects was investigated following a 5-year enrichment study in which N was applied as
NH4NO3 above the canopy of a jack pine (Pinus banksiana Lamb) stand in northern Alberta close to Fort McMurray
at dosages ranging from 5 – 25 kg N ha-1 yr-1 in addition to background deposition of 2 kg N ha-1 yr-1. Chemical
analysis of lichen mats revealed that apical (upper) lichen tissue N concentration increased with treatment, as did the
necrotic tissue. When expressed as a pool, the fibric-humic (FH) material held the largest quantity of N across all
treatments due to its relatively large mass. Soil net N mineralization and net nitrification rates did not differ among N
inputs after five years of application. A 15N tracer applied to the forest floor showed that N is initially absorbed by the
apical lichen, FH material, and the foliage of the vascular plant Vaccinium myrtilloides in particular. After 2 years,
the FH 15N pool size was elevated and all other measured pools were depleted, indicating a slow transfer of N to the
FH material. Applied 15N was not detectable in mineral soil. The microbial functional gene ammonia monooxygenase
(amoA) was undetectable using PCR screening of mineral soil microbial communities in all treatments, and broad
fungal/bacterial qPCR assays revealed a weak treatment effect on fungal/bacterial ratios in mineral soil. This work
suggests that terricolous lichen mats, which form the majority of ground cover in upland jack pine systems, have a
large capacity to effectively retain elevated N deposition via the formation of stable humus.
Author Keywords: Biogeochemistry, Boreal Ecology, Lichen, Nitrogen Enrichment, Oil Sands