The effects of nitrogen deposition on mycorrhizae and soil food webs in a clear-cut dominated by Populus tremuloides

Nitrogen (N) deposition is increasing globally and may have serious impacts on terrestrial ecosystems by modifying flow of energy within them. Its effects on soil organisms such as fungi and fauna that are involved in nutrient cycling are unclear. Ecto-(EM) and arbuscular (AM) mycorrhizal fungi typically supply plants with nutrients in systems that have a “closed” nutrient cycle (especially N). These fungi may be drastically affected when N deposition alleviates N limitation to the plant. Both types of mycorrhizal fungi occur in roots of aspen (Populus tremuloides Michx.) and mechanisms underlying their coexistence are not well understood since they typically prefer different soil conditions. EM fungi prefer higher organic matter and lower pH conditions than AM fungi. These conditions typically change with soil depth.; Two studies were performed in a northern Ontario boreal forest dominated by aspen that was clear cut 1 to 3 years earlier. The first was to test the hypothesis that two types of mycorrhizae (i.e. AM and EM) associated with aspen are separated along a soil depth gradient. The objective of the second study was to investigate the effects of simulated N deposition on mycorrhizae and fungal based food webs associated with aspen. The soil depth study compared the vertical distribution of EM and AM fungal colonization in aspen roots at three rooting depths (i.e. 0–5cm, 5–10cm and >10cm) to edaphic properties in the adjacent soil. EM fungal colonization was more abundant in the shallow organic soils and AM colonization was more abundant in deeper mineral soils. This suggests that EM and AM fungi involved in dual associations are distributed differently in the soil profile and illustrates a mechanism for coexistence.; The N fertilization study lasted two growing seasons and emulated high (i.e. 19 kg N/ha/yr) and 5 x high N (i.e. 95 kg N/ha/yr) deposition levels occurring in Ontario. Both levels of N significantly decreased arbuscular, AM, EM, and total fungal colonization in aspen roots. Elevated N also reduced AM hyphal length, increased Glomus spore abundance, and altered the mycophagous microarthropod distribution in the soil. However, N enrichment did not change hyphal length of other fungi (a proportion being non-mycorrhizal fungi) or abundance or diversity of microarthropods. Results suggest that elevated N (even 19 kg N/ha/yr) can alter the route of energy flow from a closed mutualistic mycorrhizal-based food web to a more open opportunistic decomposer/parasite based food web.; These studies demonstrate that EM and AM associated with aspen are preferentially partitioned at different soil depths and are very sensitive indicators of N deposition.