University of Wisconsin Madison Madison, Wisconsin, United States
Background/Question/Methods
Due to their highly context-dependent nature, mycorrhizal symbioses have been challenging to integrate into broader frameworks of biogeochemical cycling and global change. There currently exists a latitudinal gradient with regards to mycorrhizal associations, where ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) systems dominate in northern and southern latitudes respectively. It is thought that this pattern derives from differing nutrient acquisition abilities, namely the ability of EM fungi to access nitrogen from organic matter, as well as associated carbon costs between the two guilds. However, as global changes continue to ensue, understanding what drives forest structure and how it is connected to belowground factors becomes increasingly important.
This study aims to identify how certain abiotic factors affect the total and relative colonization of EM and AM fungi in tree roots. A greenhouse experiment was conducted using P. deltoides and variable treatments of nitrogen source, soil moisture, and light intensity, including their representative combinations. The dually-colonized Populus deltoides can associate with both guilds simultaneously and therefore avoids the confounding effects of differing host trees. We predict that roots will exhibit a higher proportion of EM as opposed to AM colonization, when given a more complex nitrogen source, and when photosynthetic potential is high.
Results/Conclusions
The ratio of ecto- to arbuscular mycorrhizal colonization in Populus deltoides roots can be explained by nitrogen as well as an interaction between nitrogen and light. Control pots, giving no additional nitrogen treatment, had a higher EM:AM ratio than those with NPK fertilizers or leaves, and this was particularly prominent when light was high. These results are consistent with our hypothesis that there may be increased selection for the EM symbiosis when nitrogen is limited or tightly bound in recalcitrant organic material, as well as when light – and, therefore, perhaps carbon – is not potentially limiting. Additionally, results indicate that total mycorrhizal colonization (AM + EM colonization) is positively correlated with soil moisture, and this signal is maintained when considering additional symbionts, such dark septate endophytes and oomycetes. Given their connection to forest structure and resulting biogeochemical cycles, determining what drives AM and EM interactions is becoming increasingly important and may be crucial for predicting how our forests will respond to global change.
