Nutrient Addition Leads to Higher Methane Emissions in a Boreal Bog in Response to Changes in Vegetation and Microtopography

English Abstract:

Atmospheric nitrogen (N) deposition has led to nutrient enrichment in wetlands globally, affecting plant community composition, carbon cycling, and microbial dynamics. Nutrient-limited boreal bogs are long-term sinks of carbon dioxide (CO₂), but sources of methane (CH₄), an important greenhouse gas. We hypothesized that CH₄ emissions would increase in response to nutrient addition owing to changes in vegetation and microtopography. We fertilized Mer Bleue Bog, a Sphagnum moss and shrub-dominated ombrotrophic bog near Ottawa, Ontario, for 10-15 years with N as NO₃ and NH₄ at 5, 10 and 20 times ambient N deposition (0.6-0.8 g N m^-2 y^-1), with and without phosphorus (P) and potassium (K). Treatments were applied to triplicate plots (3 x 3 m) from May – August 2000-2014 and control plots received distilled water. We measured methane (CH₄) flux with static chambers from early June until the end of July 2014. Depth to water table and as well as peat/air temperature were measured concurrently with CH₄ sampling. Seasonal mean methane flux ranged from 7 to 150 mg CH₄ m^-2 d^-1 with emissions from the 5NPK and 20NPK treatments 3 and 11 times higher than controls, respectively. Water table position was closer to the peat surface in these treated plots owing to loss of Sphagnum moss and compression of the peat. Laboratory incubations of peat samples showed that the rates of potential methane production under anaerobic conditions in samples from 30 – 40 cm were three times larger under the 5NPK and 20NPK treatments than the controls. Potential rates of methane consumption under aerobic conditions showed no significant differences among fertilizer treatments nor between 0-10 and 30-40 cm depths. These results suggest that increased methane emissions are the result of a complex suite of feedbacks among vegetation change, microbial dynamics and hydrology in response to nutrient addition.