Soil fauna : key to new carbon models 2 3

49 Soil organic matter (SOM) is key to maintaining soil fertility, mitigating climate change, 50 combatting land degradation, and conserving aboveand below-ground biodiversity and 51 associated soil processes and ecosystem services. In order to derive management options for 52 maintaining these essential services provided by soils, policy makers depend on robust, predictive 53 models identifying key drivers of SOM dynamics. Existing SOM models and suggested 54 guidelines for future SOM modelling are defined mostly in terms of plant residue quality and 55 input and microbial decomposition, overlooking the significant regulation provided by soil fauna. 56 The contribution of soil fauna activities to SOM decomposition can be as high as 40%, as they 57 control almost any aspect of organic matter turnover, foremost by regulating the activity and 58 functional composition of soil microorganisms and their physico-chemical connectivity with soil 59 organic matter. We suggest that inclusion of soil animal activities (plant residue consumption and 60 bioturbation altering the formation, depth, hydraulic properties and physical heterogeneity of 61 soils) can fundamentally affect the predictive outcome of SOM models. Understanding direct and 62 indirect impacts of soil fauna on nutrient availability, carbon sequestration, greenhouse gas 63 emissions and plant growth is key to the understanding of SOM dynamics in the context of global 64 carbon cycling models. We argue that explicit consideration of the soil fauna is essential to make 65 realistic modelling predictions on SOM dynamics and to detect expected non-linear responses to 66 global change, and we suggest that guidelines for future SOM modelling should implement the 67 role of soil fauna. Finally, we briefly introduce the new COST (European Cooperation in Science 68 and Technology) Action ES 1406 (KEYSOM – Soil fauna: key to new carbon models) which 69 brings together biogeochemists and soil ecologists from 21 EU countries. KEYSOM provides a 70 research network for improved SOM models by implementing the role of the soil fauna as a basis 71 for sustainable soil management. An interdisciplinary platform of experimentalists and modellers 72 will assure better access to experimental data, identify the most burning research gaps and inform 73 decision makers. 74 75