Carbon cycle confidence and uncertainty: Exploring variation among soil biogeochemical models

• Published in: Global change biology 2018-04, Vol.24 (4), p.1563-1579
• Main Authors: Wieder, William R., Hartman, Melannie D., Sulman, Benjamin N., Wang, Ying‐Ping, Koven, Charles D., Bonan, Gordon B.
• Format: Article
• Language: English
• Subjects: Biogeochemistry; Carbon; Carbon - chemistry; Carbon Cycle; carbon sinks; cell respiration; climate; Climate Change; Climate models; earth system models; Evaluation; Forces (mechanics); Frameworks; freeze-thaw cycles; Freeze-thawing; Freezing; geographical distribution; global change; Heterotrophic Processes; Mathematical analysis; microbial models; Microorganisms; Models, Theoretical; Organic matter; Organic soils; Soil; Soil - chemistry; Soil improvement; Soil Microbiology; Soil organic matter; Soil stabilization; Soil temperature; Soils; Stabilization; Stocks; structural uncertainty; Temperature; Temperature effects; Time Factors; turnover time; Uncertainty
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.13979

Emerging insights into factors responsible for soil organic matter stabilization and decomposition are being applied in a variety of contexts, but new tools are needed to facilitate the understanding, evaluation, and improvement of soil biogeochemical theory and models at regional to global scales. To isolate the effects of model structural uncertainty on the global distribution of soil carbon stocks and turnover times we developed a soil biogeochemical testbed that forces three different soil models with consistent climate and plant productivity inputs. The models tested here include a first‐order, microbial implicit approach (CASA‐CNP), and two recently developed microbially explicit models that can be run at global scales (MIMICS and CORPSE). When forced with common environmental drivers, the soil models generated similar estimates of initial soil carbon stocks (roughly 1,400 Pg C globally, 0–100 cm), but each model shows a different functional relationship between mean annual temperature and inferred turnover times. Subsequently, the models made divergent projections about the fate of these soil carbon stocks over the 20th century, with models either gaining or losing over 20 Pg C globally between 1901 and 2010. Single‐forcing experiments with changed inputs, temperature, and moisture suggest that uncertainty associated with freeze‐thaw processes as well as soil textural effects on soil carbon stabilization were larger than direct temperature uncertainties among models. Finally, the models generated distinct projections about the timing and magnitude of seasonal heterotrophic respiration rates, again reflecting structural uncertainties that were related to environmental sensitivities and assumptions about physicochemical stabilization of soil organic matter. By providing a computationally tractable and numerically consistent framework to evaluate models we aim to better understand uncertainties among models and generate insights about factors regulating the turnover of soil organic matter. Given shifts in theoretical underpinnings of mechanisms responsible for soil organic matter stabilization and vulnerability, we need to better understand uncertainties among models and generate insights about factors regulating the turnover of soil organic matter at regional to global scales. We created the soil biogeochemical testbed to provide a computationally tractable and numerically consistent framework to evaluate models. Our results highlight structural uncer