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Forest stands dominated by ectomycorrhizal (ECM) associated trees often have more closed nitrogen (N) cycling than stands dominated by arbuscular mycorrhizal (AM) associated trees, with slower N mineralization in ECM stands thought to suppress inorganic N cycling. However, most estimates of N mineralization come from measurements of net processes, which can lead to an incomplete view of ecosystem N retention and loss. To explore the mechanisms driving mycorrhizal N cycling syndromes, we measured gross N production and assimilation rates and net and potential N flux rates in paired N addition (from NH4SO4 and NaNO3) and control plots within ECM and AM-dominated stands. We observed greater gross N mineralization and microbial ammonium assimilation in ECM compared to AM stands, suggesting that increased microbial N demand drove lower net N mineralization rates in ECM stands. We found lower nitrification rates in ECM compared to AM stands and no effect of N addition on nitrification in ECM stands. Therefore, the low soil pH or high C:N ratios found in those stands, not limited ammonium supply, may have suppressed nitrification. Finally, potential denitrification rates and nitrous oxide fluxes were lower in ECM compared to AM stands with no effect of N addition, suggesting that denitrification is controlled by the endogenous supply of nitrate from nitrification, not exogenous nitrate inputs. Overall, we conclude that N mineralization may not play a central role in forming mycorrhizal nutrient syndromes, and that acidic conditions in ECM stands may ultimately control nitrification and the potential for ecosystem N loss.

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Forest stands dominated by ectomycorrhizal (ECM) associated trees often have more closed nitrogen (N) cycling than stands dominated by arbuscular mycorrhizal (AM) associated trees, with slower N mineralization in ECM stands thought to suppress inorganic N cycling and N loss. However, most estimates of N mineralization come from measurements of net processes, which can lead to an incomplete view of ecosystem N retention and loss. Here, we measured gross N production and assimilation rates, net N flux rates, and potential N flux rates to test the following hypotheses: (1) net N mineralization rates mask patterns in gross N cycling; (2) ammonium supply does not limit nitrification in ECM soils; (3) N cycling processes downstream of nitrification are limited by nitrate availability. We observed greater gross N mineralization and microbial ammonium assimilation in ECM stands compared to AM stands, suggesting that increased microbial N demand drove lower net N mineralization rates in ECM stands. We found no effect of N addition on nitrification in ECM stands, suggesting that ammonium supply does not limit nitrification. Finally, potential denitrification rates and denitrification-derived nitrous oxide fluxes were lower in ECM stands compared to AM stands with no effect of N addition, suggesting that denitrification is controlled by the endogenous supply of nitrate from nitrification, not exogenous nitrate inputs. Overall, we conclude that acidic soil conditions in ECM stands suppress nitrification to create closed N cycling such that increasing AM dominance may lead to greater ecosystem N losses only when those shifts increase soil pH.