Refining the role of nitrogen mineralization in mycorrhizal nutrient syndromes

Abstract

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.