Alberta Bog and Poor Fen Responses to Experimental Nitrogen Addition Lead to New Conceptual Frameworks of N Cycling
Abstract
North of Fort McMurray, Alberta, Canada, oil sands development releases NOx and SOx into the atmosphere, resulting in increasing N and S deposition to surrounding ecosystems. In a bog and a poor fen 100 km south of the oil sands region, where background N deposition is about 2 kg N ha-1 yr-1, we experimentally added N (as NH4NO3 in simulated rainfall) to replicated plots at 0, 5, 10, 15, 20 and 25 kg N ha-1 yr-1 for 5 years. In the bog, increasing N addition led to: 1) an increase in biological N2-fixation at N additions up to 3 kg N ha-1 yr-1, followed by an inhibition of N2-fixation at N additions above 3 kg N ha-1 yr-1, 2) decreased net primary production (NPP) and cover of Sphagnum fuscum, the dominant moss, 3) increased NPP and cover of ericaceous shrubs, and 4) increased root biomass and production. In the poor fen, increasing N addition led to: 1) an inhibition of biological N2-fixation, 2) a stimulation of Sphagnum magellanicum NPP with no effect on NPP of S. fuscum or S. angustifolium, 3) a decreased cover of S. fuscum and a concomitant increased cover of S. angustifolium and S. magellanicum, 4) increased ericaceous shrub cover, and 5) increased root biomass and production. In the bog and poor fen, at background N deposition, biological N2-fixation represents 95 and 87 %, respectively, of new N inputs (from fixation plus deposition); at N additions of 7.6 and 20.9 kg N ha-1 yr-1, respectively, atmospheric deposition and N2-fixation are equal; at higher N addition levels, atmospheric N deposition becomes increasingly dominant as a source of new N. In the bog and poor fen, uptake of new N inputs is dominated by Sphagnum mosses at N deposition values up to 18.6 and 15.9 kg N ha-1 yr-1, respectively; at higher N deposition, uptake of new N inputs is dominated by vascular plants. We did not observe increases in porewater NH4+-N, NO3- -N, or DON at either the bog or poor fen, even at the highest N addition treatments. At both sites, responses to increasing N deposition did not follow the "triphasic response" framework, developed for European bogs. We propose new conceptual frameworks describing bog and poor fen responses to N deposition in northern Alberta and recommend that N critical loads be set for 3 kg N ha-1 yr-1 or lower.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B23L2501W
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0475 Permafrost;
- cryosphere;
- and high-latitude processes;
- BIOGEOSCIENCES;
- 4950 Paleoecology;
- PALEOCEANOGRAPHY