A tipping-point in carbon storage when forest expands into tundra is related to mycorrhizal recycling of nitrogen

Tundra ecosystems are global belowground sinks for atmospheric CO2. Ongoing warming-induced encroachment by shrubs and trees risks turning this sink into a CO2 source, resulting in a positive feedback on climate warming. To advance mechanistic understanding of how shifts in mycorrhizal types affect long-term carbon (C) and nitrogen (N) stocks, we studied small-scale soil depth profiles of fungal communities and C-N dynamics across a subarctic-alpine forest-heath vegetation gradient. Belowground organic stocks decreased abruptly at the transition from heath to forest, linked to the presence of certain tree-associateds ectomycorrhizal fungi that contribute to decomposition when mining N from organic matter. In contrast, ericoid mycorrhizal plants and fungi were associated with organic matter accumulation and slow decomposition. If climatic controls on arctic-alpine forest lines are relaxed, increased decomposition will likely outbalance increased plant productivity, decreasing the overall C sink capacity of displaced tundra.

Methods
Soil samples were taken in 24 plots representing four vegetation types along a subarctic-alpine birch forest-heath tundra ecotone in Northern Sweden. Soil cores were split into fine-scaled layers, which were analysed for various biochemical characteristics and for fungal communities based on ITS2 meta-barcoding. Please find all details on sampling and analyses in the associated paper.

冻土苔原生态系统是全球大气二氧化碳（CO₂）的地下碳汇。当前由气候变暖驱动的灌木与树木扩张，正将这一碳汇转化为二氧化碳源的风险不断加剧，进而对气候变暖形成正反馈效应。为从机制层面深入理解菌根类型（mycorrhizal types）的转变如何影响长期碳（C）与氮（N）储量，我们针对亚北极-高山森林-灌丛植被梯度下的小型土壤深度剖面，分析了真菌群落与碳氮动态变化。在从灌丛到森林的过渡带，地下有机碳储量骤降，这与特定树源外生菌根真菌（ectomycorrhizal fungi）的存在密切相关——这类真菌在从有机质中获取氮时会促进有机质分解。与之相反，欧石楠类菌根（ericoid mycorrhizal）植物与真菌则与有机质积累及缓慢分解过程相关。若对北极-高山林线的气候约束被解除，有机质分解速率的提升将远超植物生产力的增长，进而降低被取代的苔原生态系统的整体碳汇能力。

方法
本研究在瑞典北部的亚北极-高山桦木林-灌丛苔原生态交错带布设了24个样地，涵盖4种植被类型并采集土壤样品。将土壤岩芯拆分为高精度分层样本，针对多种生物化学特征以及基于ITS2元条形码（ITS2 meta-barcoding）的真菌群落进行分析。有关采样与分析的全部细节，请参见相关研究论文。