Mycorrhizal Networks Inhibit Decomposition in Boreal Bogs

English Abstract:

Nitrogen (N) deposition has resulted in the loss of mycorrhizal fungi from diverse sites around the globe. Mycorrhizal fungi are symbiotic organisms that associate with the roots of the majority of plants, providing nutrients in exchange for host plant photosynthates. N enrichment inhibits mycorrhiza formation in plants associating with fungi specialized in N acquisition, leading to a decline of the symbiosis. I hypothesized that the inhibition of mycorrhizal networks will allow saprotrophic fungi to degrade carbon (C) more freely in ombrotrophic peatlands. With ample supplies of C provided directly by host plants, mycorrhizal fungi do not need to break down complex C substrates and instead, they focus their scavenging efforts on obtaining essential nutrients. In order to assess potential repercussions of mycorrhizal decline due to N deposition, I experimentally excluded the networks of mycorrhizal fungi while simultaneously fertilizing with ammonium nitrate. This manipulative experiment was conducted in two bogs in Alberta Canada, one mature and the other recently burned. Mycorrhizal colonization responses to N fertilization and peat litter decomposition responses to mycorrhizal network exclusion were assessed in-situ for Picea mariana seedlings growing in native Sphagnum peat. My results agreed with the hypothesis of N-induced reductions in mycorrhizal colonization diversity. Morphotype density (number of different mycorrhizal phenotypes per tree) was reduced (p < 0.0001) at the highest N addition level (20 kg N ha^-1 yr^-1). Mycorrhizal network exclusion resulted in 1% higher litter decomposition in the recently burned bog over two years. This research illustrates the importance of mycorrhizal symbiosis in boreal bog Picea mariana N acquisition and peatland C sequestration.