Relationships among Fires, Fungi, and Soil Dynamics in Alaskan Boreal Forests

Fires are critical pathways of carbon loss from boreal forest soils, whereas microbial communities form equally critical controls over carbon accumulation between fires. We used a chronosequence in Alaska to test Read’s hypothesis that arbuscular mycorrhizal fungi should dominate ecosystems with low accumulation of surface litter, and ectomycorrhizal fungi should proliferate where organic horizons are well-developed. This pattern is expected because ectomycorrhizal fungi display a greater capacity to mineralize organic compounds than do arbuscular mycorrhizal fungi. The sites were located in upland forests near Delta Junction, Alaska, and represent stages at 3, 15, 45, and 80 years following fire. Soil organic matter accumulated 2.8-fold over time. Fire did not noticeably reduce the abundance of arbuscular mycorrhizal fungi. In contrast, ectomycorrhizal colonization required up to 15 years to return to pre-fire levels. As a result, dominant mycorrhizal groups shifted from arbuscular to ectomycorrhizal fungi as succession progressed. Bacterial functional diversity was greatest in the oldest sites. Altogether, microbes that can mineralize organic compounds (i.e., ectomycorrhizae and bacteria) recovered more slowly than those that cannot (i.e., arbuscular mycorrhizae). Potential net N mineralization and standing pools of ammonium-N were relatively low in the youngest site. In addition, glomalin stocks were positively correlated with arbuscular mycorrhizal hyphal length, peaking early in the chronosequence. Our results indicate that microbial succession may influence soil carbon and nitrogen dynamics in the first several years following fire, by augmenting carbon storage in glomalin while inhibiting mineralization of organic compounds.