The challenge of selecting an appropriate soil organic carbon simulation model: A comprehensive global review and validation assessment.

Promotion of soil organic carbon (SOC) sequestration as a potential solution to support climate change mitigation as well as more sustainable farming systems is rising steeply. As a result, voluntary carbon markets are rapidly expanding in which farmers get paid per tons of carbon dioxide sequestered. This market relies on protocols using simulation models to certify that increases in SOC stocks do indeed occur and generate tradable carbon credits. This puts tremendous pressure on SOC simulation models, which are now expected to provide the foundation for a reliable global carbon credit generation system. There exist an incredibly large number SOC simulation models which vary considerably in their applicability and sensitivity. This confronts practitioners and certificate providers with the critical challenge of selecting the models that are appropriate to the specific conditions in which they will be applied. Model validation and the context of said validation define the boundaries of applicability of the model, and are critical therefore to model selection. To date, however, guidelines for model selection are lacking. In this review, we present a comprehensive review of existing SOC models and a classification of their validation contexts. We found that most models are not validated (71%), and out of those validated, validation contexts are overall limited. Validation studies so far largely focus on the global north. Therefore, countries of the global south, the least emitting countries that are already facing the most drastic consequences of climate change, are the most poorly supported. In addition, we found a general lack of clear reporting, numerous flaws in model performance evaluation, and a poor overall coverage of land use types across countries and pedoclimatic conditions. We conclude that, to date, SOC simulation does not represent an adequate tool for globally ensuring effectiveness of SOC sequestration effort and ensuring reliable carbon crediting.