Abstract: Mycorrhizae are fungi that form symbiotic relationships with 90% of land plants, facilitating nutrient uptake in host plants in exchange for carbon. The predominant mycorrhizal guilds are arbuscular (AM)- and ecto- mycorrhizae (EM). AM and EM guilds differ in critical ways, such as accessing different types of soil nutrients e.g., nitrogen, with important implications for forests and how they influence climate. Mycorrhizal nutrient acquisition strategies can potentially influence ecosystem functioning and lead to a positive feedback cycle that could be used to forecast forest composition as climate changes. Therefore, we ask: how does dominance of AM vs. EM trees within a forest stand influence nitrogen fluxes within the soil on a local scale?
AM vs. EM trees’ abilities to access different types of soil nutrients have varying possible outcomes. EM fungi can access nutrients from organic matter; whereas AM fungi rely on other microbes to decompose organic matter to access the inorganic nutrients. A potential consequence of this differential access is a positive feedback strategy between the host plants, mycorrhizae, and soil nutrient status that promotes con-mycorrhizal guilds in the growth and recruitment of new trees. Since EM trees can access organic nutrients, they tend to produce tissues that are tough to decompose, leading to an increase in EM dominance (because they alone access the nutrients). Conversely, AM trees produce easier to decompose tissues, increasing soil nutrient availability, and AM dominance. We therefore hypothesized that concentration of ammonium and nitrate would be highest in the all-AM plots.
Field samples were taken at our local field site in Cornwall, NY across a gradient of nine plots ranging from all-EM to mixed to all-AM. Along this gradient, we measured soil fluxes of nitrate and ammonium from August to October using a discrete analyzer. Our results for nitrate and ammonium concentrations from August displayed little significance between all 3 stand types. Contrastingly, in October significant difference in nitrate (p-value 3.113e-05) and ammonium (p-value: 0.0001254) concentrations between AM, EM, and mixed stands was recorded. Significant inorganic nitrogen fluxes were only observed after the growing season has ended. This temporal difference indicates that nitrate and ammonium concentrations vary throughout the growing season across all forest stands with inorganic nitrogen accumulation in the soil in all-AM plots at the start of the fall. Further research into the mechanisms behind these patterns and the role of mycorrhizae would aid in our understanding of ecosystem ecology.
