Enhanced nitrogen (N) availability in temperate forests has altered ecosystem carbon (C) and N cycling. Recent research has shown that these alterations lead to reductions in belowground C allocation by trees and that the consequences of these reductions on soil C and nutrient cycling may vary by mycorrhizal type. We hypothesized that trees that associate with ectomycorrhizal fungi (ECM) would reduce C allocation towards roots and mycorrhizal fungi to a greater extent than trees that associate with arbuscular mycorrhizal fungi (AM) in response to > 25 years of N fertilization. We further hypothesized that N induced decoupling of roots and microbes in ECM trees would be evidenced by greater declines in extracellular enzyme activities. We measured belowground C allocation to fine root biomass and mycorrhizal colonization in 6 AM and 6 ECM dominated plots in the N fertilized and reference watersheds at the Fernow Experimental Forest in West Virginia, USA. We compared these to measurements of simple-C, complex-C, nitrogen, and phosphorus acquiring enzyme activities in organic horizon, bulk mineral, and rhizosphere soil fractions. N fertilization reduced fine root biomass and mycorrhizal colonization in both AM and ECM stands. We found more consistent reductions in enzyme activities in ECM soils than AM soils under N fertilization which may have been driven by greater declines in root-C transfer to soil microbes. This mechanism helps to explain variability in soil C cycling responses across N gradient and fertilization experiments and may prove useful in predicting the fate of soil C stocks in response to N deposition.

温带森林氮 (N) 可用性的增强改变了生态系统碳 (C) 和氮循环。最近的研究表明，这些变化导致树木地下碳分配的减少，并且这些减少对土壤碳和养分循环的影响可能因菌根类型而异。我们假设，与与丛枝菌根真菌（AM）相关的树木相比，与外生菌根真菌（ECM）相关的树木在超过25年的氮肥施肥后，会更大程度地减少对根和菌根真菌的碳分配。我们进一步假设，氮诱导的 ECM 树根部和微生物的解耦可以通过细胞外酶活性的更大下降来证明。我们在美国西弗吉尼亚州 Fernow 实验森林的施氮流域和参考流域的上午 6 点和 6 个 ECM 主导的地块中测量了地下碳分配给细根生物量和菌根定植的情况。我们将这些与有机地层、散装矿物质和根际土壤部分中的简单碳、复合碳、氮和磷获取酶活性的测量结果进行比较。施氮肥降低了 AM 和 ECM 林中的细根生物量和菌根定殖。我们发现，在施氮肥下，ECM 土壤中的酶活性下降幅度比 AM 土壤更一致，这可能是由于根部向土壤微生物转移碳的下降幅度更大所致。这种机制有助于解释氮梯度和施肥实验中土壤碳循环响应的变化，并且可能有助于预测土壤碳库对氮沉降的反应。