Tue, Aug 16, 2022: 5:00 PM-6:30 PM
ESA Exhibit Hall
Background/Question/MethodsNitrogen (N) availability constrains forest productivity worldwide. Predicting future patterns of N availability, however, is complicated by conflicting hypotheses. Increasing ambient carbon dioxide (iCO2) may increase ecosystem N demand beyond supply, resulting in N oligotrophication. Alternatively, atmospheric N deposition may saturate ecosystem N supply, resulting in N eutrophication. Under these environmental changes, ectomycorrhizal (EcM) fungi may either enhance or limit tree N availability relative to arbuscular mycorrhizal (AM) fungi. Unlike AM fungi, some EcM fungi can break down organic matter to access otherwise immobile N, conferring a potential advantage to EcM trees. If EcM fungi increase mycelial production as predicted under iCO2, increased N immobilization in their biomass may itself limit tree N availability. To test whether forests were becoming N oligotrophic or eutrophic under historical climate change, we analyzed N concentrations and δ15N among tissues from trees and fungi collected from 1871–2016 in Minnesota. To test whether EcM fungi confer an advantage or disadvantage to their tree partners, we evaluated whether EcM trees and fungi exhibited elevated N concentrations compared to AM trees and saprotrophic fungi over time, and if this was matched by elevated δ15N, a proxy indicating decreased N immobilization in EcM fungal mycelia.
Results/ConclusionsN concentrations and δ15N declined significantly in all groups except EcM trees, where the negative trend in N concentrations was only marginally significant (13.3 ± 6.4%; p = 0.064). This finding is consistent with reports of declining foliar N concentrations and δ15N as evidence for N oligotrophication and represents the first report of similarly declining fungal N concentrations and δ15N. Cumulatively, foliar N concentrations fell more than twice as much among AM trees (31.3 ± 5.1%; p < 0.001) than ECM trees (p = 0.031). Similarly, δ15N declined approximately twice as much among AM trees (6.01 ± 0.91‰; p < 0.001) than EcM trees (3.20 ± 0.92‰; p = 0.001) (p = 0.001) and saprotrophic fungi (7.71 ± 1.15‰; p < 0.001) than EcM fungi (3.45 ± 1.06‰; p = 0.001) (p = 0.006). Together, these results support that EcM fungi confer a nutritional advantage to their partners under century-scale N oligotrophication. This advantage may be due to decreased N immobilization in EcM fungal mycelia, consistent with elevated δ15N among EcM trees and fungi over time, and/or EcM fungal access to N otherwise bound in organic matter, consistent with elevated EcM fungal δ15N.
Results/ConclusionsN concentrations and δ15N declined significantly in all groups except EcM trees, where the negative trend in N concentrations was only marginally significant (13.3 ± 6.4%; p = 0.064). This finding is consistent with reports of declining foliar N concentrations and δ15N as evidence for N oligotrophication and represents the first report of similarly declining fungal N concentrations and δ15N. Cumulatively, foliar N concentrations fell more than twice as much among AM trees (31.3 ± 5.1%; p < 0.001) than ECM trees (p = 0.031). Similarly, δ15N declined approximately twice as much among AM trees (6.01 ± 0.91‰; p < 0.001) than EcM trees (3.20 ± 0.92‰; p = 0.001) (p = 0.001) and saprotrophic fungi (7.71 ± 1.15‰; p < 0.001) than EcM fungi (3.45 ± 1.06‰; p = 0.001) (p = 0.006). Together, these results support that EcM fungi confer a nutritional advantage to their partners under century-scale N oligotrophication. This advantage may be due to decreased N immobilization in EcM fungal mycelia, consistent with elevated δ15N among EcM trees and fungi over time, and/or EcM fungal access to N otherwise bound in organic matter, consistent with elevated EcM fungal δ15N.