Effects of nitrogen source, soil moisture, and light on the colonization of ecto- and arbuscular mycorrhizal fungi in the roots of dually-colonized Populus deltoides

Kathleen Thompson, Botany, University of Wisconsin, Madison, WI and Richard Lankau, Plant Pathology, University of Wisconsin, Madison, WI

Background/Question/Methods
Despite their prevalence and importance, mycorrhizal symbioses have been challenging to understand as a consequence of their highly context-dependent nature. 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, and both coexist in temperate zones. Theory and models suggest that this pattern derives from differing nutrient acquisition abilities and associated carbon costs between the two guilds, since EM fungi are able to mine for nitrogen from organic matter while AM fungi rely on freely available mineral nitrogen.
Certain species, such as Populus deltoides, are able to associate with both fungal guilds concomitantly – a relatively unique trait. This dually-colonized system is ideal for studying the relative costs and benefits of the two mycorrhizal strategies, as it avoids the confounding effects of differing host trees. Since forest function can depend on the dominant mycorrhizal type, understanding what drives forest structure and how it is connected to belowground factors becomes increasingly important as global changes continue to ensue.
This study aims to identify how certain abiotic factors, and factor interactions, affect the total and relative colonization of EM and AM fungi in tree roots. A greenhouse experiment was conducted using the dually-colonized Populus deltoides as well as variable treatments of nitrogen source (organic vs. mineral), soil moisture, and light intensity, including their representative combinations. We predict that roots will exhibit a higher proportion of EM as opposed to AM colonization (and uncolonized root) when given a more complex nitrogen source and when photosynthetic potential is high (high light and adequate moisture).

Results/Conclusions
Results indicate that EM colonization can be explained by nitrogen source, light intensity, and soil moisture. However, there are no higher-order interaction effects, and responses are instead additive. Greater EM colonization was observed in treatments receiving a more complex nitrogen source (leaves) than mineral nitrogen fertilizer, as well as in higher light intensities. Additionally, EM colonization was positively correlated with soil moisture. These results are consistent with our hypotheses and suggest that there may be increased selection for the EM symbiosis when nitrogen is bound in organic matter, as opposed to freely available in mineral form, as well as when light – and, therefore, perhaps carbon – is not potentially limiting. These results will be compared with AM colonization metrics to see if abiotic treatments explain any potentially differential colonization between the two guilds.