Rates and Controls of N2 Fixation in Sphagnum spp. along the Hydrological Gradient – Beaver Pond to Bog Transition at Mer Bleue, Ontario, Canada

Abstract:

Many northern bogs with low atmospheric N inputs acquire N only via N₂-fixation. Little is known about rates and controls on N₂-fixation in bogs. The aim of this study was to: 1) test the important ecological drivers for N₂-fixation, 2) investigate seasonal and temporal patterns of N₂ fixation, and 3) to estimate current N₂-fixation rates at Mer Bleue bog.

We used acetylene reduction assay (ARA) to measure N₂-fixation from June-October 2013 and 2014 (currently ongoing field season) along a hydrological gradient (beaver pond, hollows and hummocks). The highest ARA rates in 2013 growing season occurred in the pond in floating Sphagnum cuspidatum mats (50.3 ± 12.9 μmol m^-2 d^-1 Mean ± Std Err) which were up to 2.5 times latger than the rates found in the hummock with the lowest water table depth throughout the season. Two rain events during the summer 2013 increased ARA rates in all plots by 1 to 4 times, suggesting that moisture availability may play a crucial role on N₂ fixation potential in the field. We are currently investigating the role of moisture, temperature, PAR and nutrient content (N, phosphorous and metals) on ARA along the gradient. In addition, we are using ¹⁵N₂ enrichment method to estimate N₂ fixation rates and compare them to ARA method at Mer Bleue bog.