The potential significance of ericoid mycorrhizal fungi in ombrotrophic peatland biogeochemistry

Northern peatlands have sequestered a huge amount of carbon through exceptionally low microbial activity which is partly attributed to their nutrient-poor conditions. Evergreen shrubs, a dominant species in ombrotrophic bogs, adapt to this nutrient-poor condition by developing organic nutrient acquisition strategies, mediated by ericoid mycorrhizal association. However, the mycorrhizal symbionts together with nutrient cycling have been omitted in peatland models, precluding our ability to simulate the significance of nutrient limitation in peatlands following environmental changes. To address this issue, we further developed the well-established peatland model MWM by incorporating a mechanistic mycorrhizal fungi model and both nitrogen and phosphorus cycles. The new model was adopted to simulate the fertilization effect on peatlands and evaluated against measurements from the long-term fertilization experiments at Mer Bleue, a raised ombrotrophic bog located in southern Ontario, Canada. The model successfully reproduced the observed dramatic changes with fertilization in mycorrhizal performance, vegetation composition and carbon cycle. Greater availability of inorganic nutrients diminished the role of mycorrhizal fungi in plant nutrient uptake. More assimilated carbon was allocated to shrub growth, which then inhibited the growth of sphagnum moss and ultimately posed a threat to the carbon-sequestration capacity of peatlands. Therefore, mycorrhizal activities, which have been overlooked in past peatland studies, could play a significant role in understanding how peatlands respond to increased nutrient deposition in the future.