Mon, Aug 15, 2022: 1:30 PM-1:45 AM
513D
Background/Question/Methods
Mycorrhizal associations of trees with either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi are frequently used to predict stand-level nitrogen (N) dynamics. Specifically, stands dominated by ECM-associated trees tend to have lower net N nitrification rates, nitrate (NO3-) leaching and gaseous N losses than stands dominated by AM-associated trees. Contrasting N acquisition strategies of ECM and AM fungi and their associated trees are proposed to alter decomposition dynamics and determine rates of N mineralization, thereby driving downstream N cycling processes. However, net N mineralization and nitrification rate measurements conflate production and consumption of inorganic N such that the drivers of distinct N cycling syndromes in ECM and AM stands remain uncertain. We investigated the role of gross N production and consumption pathways in driving mycorrhizal nutrient syndromes in ECM-versus AM-dominated temperate forest stands using the 15N pool dilution technique to measure gross N cycling rates in addition to more commonly measured net and potential N cycling rates. Using these data, we aimed to determine if net N mineralization patterns mask gross N mineralization patterns and to assess the effect of substrate limitation on nitrification and downstream gaseous N losses as N2O.
Results/Conclusions
We found that net N mineralization rates can mask patterns in gross NH4+ production and consumption with lower net N mineralization occurring (F1, 12 = 49.41, P < 0.0001) despite higher gross N mineralization (F1,11.86 =11.32, P = 0.006) and higher microbial NH4+ assimilation (F1,15.77 =17.66, P = 0.0007) in ECM compared to AM stands. Additionally, we observed higher gross N mineralization rates, but lower net nitrification rates (F1,12 = 266.91, P < 0.0001) and NO3- concentrations (F1, 12 = 39.98, P < 0.0001) ECM relative to AM stands, suggesting that nitrification can be inhibited by mechanisms other than limited NH4+ production. Finally, we found that gaseous N losses via denitrification generally correlated with NO3- availability such that controls on nitrification may have broader implications for N2O emissions across forest types. Overall, strong inorganic N demand by free-living microbes and soil acidity effects on nitrification may lead to the closed ecosystem N cycle characteristic of ECM forest stands compared to the open ecosystem N cycle of AM-dominated forest stands. We conclude that N mineralization does not play a central role in forming mycorrhizal nutrient syndromes as previously thought, and that soil pH may ultimately control nitrification and downstream processes.
Mycorrhizal associations of trees with either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi are frequently used to predict stand-level nitrogen (N) dynamics. Specifically, stands dominated by ECM-associated trees tend to have lower net N nitrification rates, nitrate (NO3-) leaching and gaseous N losses than stands dominated by AM-associated trees. Contrasting N acquisition strategies of ECM and AM fungi and their associated trees are proposed to alter decomposition dynamics and determine rates of N mineralization, thereby driving downstream N cycling processes. However, net N mineralization and nitrification rate measurements conflate production and consumption of inorganic N such that the drivers of distinct N cycling syndromes in ECM and AM stands remain uncertain. We investigated the role of gross N production and consumption pathways in driving mycorrhizal nutrient syndromes in ECM-versus AM-dominated temperate forest stands using the 15N pool dilution technique to measure gross N cycling rates in addition to more commonly measured net and potential N cycling rates. Using these data, we aimed to determine if net N mineralization patterns mask gross N mineralization patterns and to assess the effect of substrate limitation on nitrification and downstream gaseous N losses as N2O.
Results/Conclusions
We found that net N mineralization rates can mask patterns in gross NH4+ production and consumption with lower net N mineralization occurring (F1, 12 = 49.41, P < 0.0001) despite higher gross N mineralization (F1,11.86 =11.32, P = 0.006) and higher microbial NH4+ assimilation (F1,15.77 =17.66, P = 0.0007) in ECM compared to AM stands. Additionally, we observed higher gross N mineralization rates, but lower net nitrification rates (F1,12 = 266.91, P < 0.0001) and NO3- concentrations (F1, 12 = 39.98, P < 0.0001) ECM relative to AM stands, suggesting that nitrification can be inhibited by mechanisms other than limited NH4+ production. Finally, we found that gaseous N losses via denitrification generally correlated with NO3- availability such that controls on nitrification may have broader implications for N2O emissions across forest types. Overall, strong inorganic N demand by free-living microbes and soil acidity effects on nitrification may lead to the closed ecosystem N cycle characteristic of ECM forest stands compared to the open ecosystem N cycle of AM-dominated forest stands. We conclude that N mineralization does not play a central role in forming mycorrhizal nutrient syndromes as previously thought, and that soil pH may ultimately control nitrification and downstream processes.