Articles | Volume 13, issue 15
Research article
10 Aug 2016
Research article |  | 10 Aug 2016

Underestimation of boreal soil carbon stocks by mathematical soil carbon models linked to soil nutrient status

Boris Ťupek, Carina A. Ortiz, Shoji Hashimoto, Johan Stendahl, Jonas Dahlgren, Erik Karltun, and Aleksi Lehtonen

Abstract. Inaccurate estimate of the largest terrestrial carbon pool, soil organic carbon (SOC) stock, is the major source of uncertainty in simulating feedback of climate warming on ecosystem–atmosphere carbon dioxide exchange by process-based ecosystem and soil carbon models. Although the models need to simplify complex environmental processes of soil carbon sequestration, in a large mosaic of environments a missing key driver could lead to a modeling bias in predictions of SOC stock change.

We aimed to evaluate SOC stock estimates of process-based models (Yasso07, Q, and CENTURY soil sub-model v4) against a massive Swedish forest soil inventory data set (3230 samples) organized by a recursive partitioning method into distinct soil groups with underlying SOC stock development linked to physicochemical conditions.

For two-thirds of measurements all models predicted accurate SOC stock levels regardless of the detail of input data, e.g., whether they ignored or included soil properties. However, in fertile sites with high N deposition, high cation exchange capacity, or moderately increased soil water content, Yasso07 and Q models underestimated SOC stocks. In comparison to Yasso07 and Q, accounting for the site-specific soil characteristics (e. g. clay content and topsoil mineral N) by CENTURY improved SOC stock estimates for sites with high clay content, but not for sites with high N deposition.

Our analysis suggested that the soils with poorly predicted SOC stocks, as characterized by the high nutrient status and well-sorted parent material, indeed have had other predominant drivers of SOC stabilization lacking in the models, presumably the mycorrhizal organic uptake and organo-mineral stabilization processes. Our results imply that the role of soil nutrient status as regulator of organic matter mineralization has to be re-evaluated, since correct SOC stocks are decisive for predicting future SOC change and soil CO2 efflux.

Short summary
We evaluated the soil carbon stock estimates of Yasso07, Q, and CENTURY soil carbon models, used in national greenhouse gas inventories in Europe, Japan, and USA, with soil carbon stock measurements from Swedish Forest Soil National Inventories. Measurements grouped according to the gradient of soil nutrient status revealed that the models underestimated for the Swedish boreal forest soils with higher site fertility. We discussed mechanisms of underestimation and further model developments.
Final-revised paper