Biogeochemical Integration Department, Max Planck Institute for Biogeochemistry, Jena, Germany.
International Max-Planck Research School Global Biogeochemical Cycles, Jena, Germany.
Glob Chang Biol. 2020 Jul;26(7):3978-3996. doi: 10.1111/gcb.15114. Epub 2020 May 18.
The magnitude of the nitrogen (N) limitation of terrestrial carbon (C) storage over the 21st century is highly uncertain because of the complex interactions between the terrestrial C and N cycles. We use an ensemble approach to quantify and attribute process-level uncertainty in C-cycle projections by analysing a 30-member ensemble representing published alternative representations of key N cycle processes (stoichiometry, biological nitrogen fixation (BNF) and ecosystem N losses) within the framework of one terrestrial biosphere model. Despite large differences in the simulated present-day N cycle, primarily affecting simulated productivity north of 40°N, ensemble members generally conform with global C-cycle benchmarks for present-day conditions. Ensemble projections for two representative concentration pathways (RCP 2.6 and RCP 8.5) show that the increase in land C storage due to CO fertilization is reduced by 24 ± 15% due to N constraints, whereas terrestrial C losses associated with climate change are attenuated by 19 ± 20%. As a result, N cycling reduces projected land C uptake for the years 2006-2099 by 19% (37% decrease to 3% increase) for RCP 2.6, and by 21% (40% decrease to 9% increase) for RCP 8.5. Most of the ensemble spread results from uncertainty in temperate and boreal forests, and is dominated by uncertainty in BNF (10% decrease to 50% increase for RCP 2.6, 5% decrease to 100% increase for RCP 8.5). However, choices about the flexibility of ecosystem C:N ratios and processes controlling ecosystem N losses regionally also play important roles. The findings of this study demonstrate clearly the need for an ensemble approach to quantify likely future terrestrial C-N cycle trajectories. Present-day C-cycle observations only weakly constrain the future ensemble spread, highlighting the need for better observational constraints on large-scale N cycling, and N cycle process responses to global change.
在 21 世纪，陆地碳（C）储存的氮（N）限制程度非常不确定，因为陆地 C 和 N 循环之间存在复杂的相互作用。我们使用集合方法来量化和归因于 C 循环预测中的过程级不确定性，方法是分析一个由 30 个成员组成的集合，该集合代表了在一个陆地生物圈模型框架内发表的关键 N 循环过程（化学计量、生物固氮（BNF）和生态系统 N 损失）的替代表示。尽管在模拟的当前 N 循环中存在很大差异，主要影响北纬 40°以北的模拟生产力，但集合成员通常符合当前条件的全球 C 循环基准。对于两个代表性浓度途径（RCP 2.6 和 RCP 8.5）的集合预测表明，由于 N 限制，由于 CO 施肥导致的陆地 C 储存增加减少了 24±15%，而与气候变化相关的陆地 C 损失则减少了 19±20%。因此，由于 N 循环，在 2006 年至 2099 年期间，陆地 C 吸收量的预测减少了 19%（从减少 37%到增加 3%），对于 RCP 2.6 减少了 21%（从减少 40%到增加 9%）。集合的大部分差异源于温带和北方森林的不确定性，并且主要由 BNF 的不确定性（对于 RCP 2.6，减少 10%至增加 50%，对于 RCP 8.5，减少 5%至增加 100%）决定。然而，关于生态系统 C:N 比的灵活性以及控制生态系统 N 损失的区域过程的选择也起着重要作用。本研究的结果清楚地表明，需要采用集合方法来量化未来陆地 C-N 循环轨迹的可能性。当前的 C 循环观测仅对未来集合的差异产生微弱的约束，这突出了对大规模 N 循环以及 N 循环过程对全球变化的响应进行更好的观测约束的必要性。
