Chronic nitrogen (N) deposition from anthropogenic emissions alter N cycling of forests in Europe and in other impacted areas. It disrupts plant/microbe interactions in originally N-poor systems, based on a symbiosis of plants with ectomycorrhizal fungi (ECM). ECM fungi that are capable of efficient nutrient mining from complex organics and their long-distance transport play a key role in controlling soil N mineralization and immobilization, and eventual nitrate (NO₃⁻) leaching. Current meta-analyses highlight the importance of ECM biomass in securing the large soil N pool. At the same time, they point to the adverse effect of long-term N input on ECM fungi. The functioning of N-poor and N-overloaded forests is well understood, while the transient stages are much less explored. Therefore, we focused on the spruce-forest dominated catchment at Gårdsjön (Sweden) that received N addition of 40 kg N ha⁻¹yr⁻¹ over 24 years (a cumulative N input of >1200 kg N ha⁻¹) but still loses via runoff only <20% of annual N input (deposition + addition) as NO₃⁻. We found that, compared to the control, the N-addition catchment had a much larger soil microbial biomass. The N addition did not change the fungi/bacteria ratio, but a larger share of the bacterial community was made up of copiotrophs. Furthermore, fungal community composition shifted to more nitrophilic ECM fungi (contact and short exploration type ECM species) and saprotrophs. Such a restructured community has been more active, possessed a higher specific respiration rate, enhanced organic P and C mining through enzymatic production and provided faster net N mineralization and nitrification. These may be early indications of alleviation of N limitation of the system. We observed no signs of soil acidification related to N additions. The larger, structurally and functionally adapted soil microbial community still provides an efficient sink for the added N in the soil and is likely to be one of the explanations for low NO₃⁻ leaching that have stabilized in the last decade. Our results suggest that a microbial community can contribute to effective soil N retention in spite of the partial relative retreat (20–30%) of nitrophobic ECM fungi with large external mycelia, provided the fungal biomass remains high because of replacement by other ECM and saprotrophic fungi. Furthermore, we assume that N retention of similar C-rich boreal forests (organic soil molar C/N ~35) is not necessarily threatened by a large cumulative N dose provided N enters at a moderate rate, does not cause acidification and the soil microbial community has time to adapt through structural and functional changes.

人为排放物中的慢性氮（N）沉积改变了欧洲及其他受灾地区森林的氮循环。基于与外生菌根真菌（ECM）的植物共生，它破坏了最初的N贫乏系统中的植物/微生物相互作用。ECM真菌是从复杂的有机物和他们的长途运输，对控制土壤氮素矿化和固定化，并最终硝酸盐关键作用（NO能够有效养分流失的₃ ^-）的浸出。当前的荟萃分析突出了ECM生物量在确保大量土壤氮库中的重要性。同时，他们指出长期氮输入对ECM真菌的不利影响。氮贫和氮过载森林的功能已广为人知，而过渡阶段则少得多。因此，我们集中研究了瑞典Gårdsjön的云杉林为主的流域，该流域在24年中增加了40 kg N ha ^-1 yr ^-1的氮（累计氮输入量大于1200 kg N ha ^-1），但仍然流失通过仅径流<20％的年N个输入（沉积+添加）作为NO的₃ ^-。我们发现，与对照相比，氮添加集水区的土壤微生物量更大。氮的添加不会改变真菌/细菌的比例，但是细菌群落中较大的一部分是嗜营养菌。此外，真菌群落组成转移到更多的亲硝酸ECM真菌（接触和短期探索型ECM物种）和腐生菌。这样的重组社区更加活跃，拥有更高的比呼吸速率，通过酶促生产增加了有机磷和碳的开采，并提供了更快的净氮矿化和硝化作用。这些可能是减轻系统N限制的早期迹象。我们没有观察到与添加氮有关的土壤酸化迹象。较大的₃ ^-浸出已经在过去十年稳定。我们的研究结果表明，尽管具有较大外部菌丝体的疏水性ECM真菌发生部分相对退缩（20–30％），但微生物群落仍可促进土壤氮的有效保留，但前提是由于被其他ECM和腐养菌替代而使真菌生物量仍然很高菌类。此外，我们假设，如果氮以适度的速率进入，不会造成相似的富含碳的北方森林（有机土壤摩尔C / N〜35）的氮滞留，不一定会受到大量累积氮剂量的威胁，不会引起酸化和土壤微生物社区有时间通过​​结构和功能的变化来适应。