Nitrogen (N) fertilization has enhanced the forest land carbon (C) sink by increasing the amount of C stored in soils, possibly through reductions in decomposition. Established differences in nutrient acquisition strategies between trees that associate with arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi have been shown to influence the magnitude of this N effect on decomposition and soil C stocks. However, N deposition is declining across many temperate North American forests and little is known about how mycorrhizal-associated strategies in trees may impact short-term recovery. To examine divergent nutrient acquisition responses to N between AM and ECM systems, we developed a conceptual framework based on the idea that N fertilization reduces the C cost of N acquisition. In this framework, under N fertilization, ECM trees shift from N mining to N foraging and AM trees shift from mycorrhizal foraging to root foraging. We expanded on this framework by hypothesizing that initial recovery occurs across a spectrum, where nutrient foraging strategies either (1) persist in their N fertilized state, (2) return to the ambient state, or (3) shift to a new steady state. We tested this framework by examining fine root biomass and morphology, mycorrhizal colonization, and soil enzyme activities in organic horizon, bulk mineral, and rhizosphere mineral soils in AM and ECM dominated plots during the last year of a ~ 30 year N fertilization experiment and 1-year after fertilization ceased at Bear Brook Watershed, in Maine USA. Overall, our results indicate that N fertilization disrupted the organic N mining nutrient economy of ECM trees by reducing fine root biomass and mycorrhizal colonization and altering root morphology to improve N foraging. In contrast, AM trees appeared to shift from mycorrhizal foraging toward root foraging by reducing mycorrhizal colonization while maintaining root biomass. While AM and ECM mycorrhizal colonization in the fertilized plots remained lower than the ambient plots during the year after fertilization ceased, the rapid recovery of roots in fertilized ECM soils back to a level similar to those of the control soils was mirrored by ECM rhizosphere and organic horizon enzyme recovery. The ECM bulk mineral and all of the AM soil enzymes activities remained at their N fertilized levels. Although these are short-term recovery responses, our results suggest that the recovery of enzyme activities in the majority of ECM soil fractions, but not the AM soils may destabilize stored soil C in ECM stands that decades of N deposition have enhanced.

氮 (N) 施肥通过增加土壤中储存的碳量（可能是通过减少分解）来增强林地碳 (C) 汇。与丛枝菌根 (AM) 和外生菌根 (ECM) 真菌相关的树木之间养分获取策略的既定差异已被证明会影响这种 N 对分解和土壤碳储量的影响程度。然而，北美许多温带森林的氮沉积量正在下降，而且人们对树木中的菌根相关策略如何影响短期恢复知之甚少。为了检查 AM 和 ECM 系统之间对 N 的不同养分获取响应，我们基于施氮降低 N 获取 C 成本的想法开发了一个概念框架。在这个框架下，在施氮肥下，ECM 树从 N 挖掘转向 N 觅食，AM 树从菌根觅食转向根觅食。我们通过假设初始恢复发生在一个范围内来扩展这个框架，其中营养物觅食策略（1）持续在它们的施氮状态，（2）返回到环境状态，或（3）转移到一个新的稳定状态。我们通过检查在约 30 年 N 施肥试验和 1 - 在美国缅因州的熊溪流域停止施肥一年后。总体，我们的结果表明，施氮通过减少细根生物量和菌根定植以及改变根系形态以改善 N 觅食，破坏了 ECM 树的有机 N 开采养分经济。相比之下，AM 树似乎通过减少菌根定植同时保持根生物量，从菌根觅食转向根觅食。虽然施肥地块中 AM 和 ECM 菌根定植在施肥停止后的一年内仍低于周围地块，但 ECM 根际和有机物反映了施肥 ECM 土壤中的根迅速恢复到与对照土壤相似的水平地平线酶回收。ECM 散装矿物质和所有 AM 土壤酶活性保持在施氮水平。虽然这些是短期的恢复反应，