COS 5-1 - Refining the role of nitrogen mineralization in mycorrhizal nutrient syndromes

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 (F_{1, 12} = 49.41, P < 0.0001) despite higher gross N mineralization (F_1,11.86 =11.32, P = 0.006) and higher microbial NH₄⁺ assimilation (F_1,15.77 =17.66, P = 0.0007) in ECM compared to AM stands. Additionally, we observed higher gross N mineralization rates, but lower net nitrification rates (F_1,12 = 266.91, P < 0.0001) and NO₃^- concentrations (F_{1, 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.