Simulating Global Terrestrial Carbon and Nitrogen Biogeochemical Cycles With Implicit and Explicit Representations of Soil Microbial Activity Journal of Advances in Modeling Earth Systems

Nutrient limitation is widespread in terrestrial ecosystems. Accordingly, representations of nitrogen (N) limitation in land models typically dampen rates of terrestrial carbon (C) accrual, compared with C‐only simulations. These previous findings, however, rely on soil biogeochemical models that implicitly represent microbial activity and physiology. Here we present results from a biogeochemical model testbed that allows us to investigate how an explicit versus implicit representation of soil microbial activity, as represented in the MIcrobial‐MIneral Carbon Stabilization (MIMICS) and Carnegie‐Ames‐Stanford Approach (CASA) soil biogeochemical models, respectively, influence plant productivity, and terrestrial C and N fluxes at initialization and over the historical period. When forced with common boundary conditions, larger soil C pools simulated by the MIMICS model reflect longer inferred soil organic matter (SOM) turnover times than those simulated by CASA. At steady state, terrestrial ecosystems experience greater N limitation when using the MIMICS‐CN model, which also increases the inferred SOM turnover time. Over the historical period, however, warming‐induced acceleration of SOM decomposition over high latitude ecosystems increases rates of N mineralization in MIMICS‐CN. This reduces N limitation and results in faster rates of vegetation C accrual. Moreover, as SOM stoichiometry is an emergent property of MIMICS‐CN, we highlight opportunities to deepen understanding of sources of persistent SOM and explore its potential sensitivity to environmental change. Our findings underscore the need to improve understanding and representation of plant and microbial resource allocation and competition in land models that represent coupled biogeochemical cycles under global change scenarios.

养分限制（nutrient limitation）广泛存在于陆地生态系统中。相较于仅考虑碳循环的模拟试验，当前陆面模型（land models）对氮限制的表征通常会抑制陆地碳固存速率。然而，此前的相关研究结论均基于隐式表征微生物活动与生理特性的土壤生物地球化学模型（soil biogeochemical models）。本研究基于生物地球化学模型测试平台（biogeochemical model testbed），探究了土壤微生物活动的两种不同表征方式——分别为显式表征的微生物-矿物碳稳定化模型（MIcrobial‐MIneral Carbon Stabilization, MIMICS）与隐式表征的卡内基-埃姆斯-斯坦福方法（Carnegie‐Ames‐Stanford Approach, CASA）土壤生物地球化学模型——对初始状态及历史时期内植物生产力、陆地碳氮通量的影响。当采用统一边界条件（boundary conditions）驱动模型时，MIMICS模型模拟得到的更大土壤碳库（soil C pools），对应更长的推断土壤有机质（soil organic matter, SOM）周转时间，这一结果长于CASA模型的模拟结果。在稳态条件下，采用MIMICS-CN模型时，陆地生态系统面临的氮限制更强，同时该模型推断的SOM周转时间也更长。但在历史时期，高纬度生态系统（high latitude ecosystems）中升温诱导的SOM分解加速，会提升MIMICS-CN模型中的氮矿化速率，这会缓解氮限制并加快植被碳固存速率。此外，由于SOM化学计量比（SOM stoichiometry）是MIMICS-CN模型的涌现属性（emergent property），本研究为深化对持久性土壤有机质（persistent SOM）来源的理解、探索其对环境变化的潜在敏感性提供了研究方向。本研究结果表明，在全球变化情景（global change scenarios）下构建耦合生物地球化学循环的陆面模型时，亟需提升对植物与微生物资源分配及竞争关系的理解与表征精度。