Define optimum carbon levels in soils in view of multiple soil functions

Soil organic matter (SOM) content is key for a healthy and high-quality agricultural soil and drives soil processes controlling both crop yield and environmental losses. An increase in soil organic matter or carbon (SOC) levels is seen, both by many conventional farmers and by policy makers, as a desirable objective. Better plant nutrition due to retention of nutrients (N, P, S, micronutrients), ease of cultivation, penetration and seedbed preparation, greater aggregate stability, reduced bulk density, improved water holding capacity and enhanced porosity have all been associated with increased amounts of SOC. A critical threshold for SOC below which the soil becomes less fertile and sustainable is however missing. Consideration of such critical levels involves assessment of the quantitative evidence, i.e. the nature of SOM and the properties it confers on soils, whether justifiable limits can be set for a range of soil types, climatic conditions, or land management/cropping practices and, finally, whether there are any dis-benefits from an increase in SOM levels in soils. Using quantitative relationships derived from literature we assessed the contribution of SOC to the aforementioned soil functions and properties, and linked this contribution to critical targets for the soil functions evaluated. Using this we derived an optimum SOC range for agricultural soils and we illustrate the potential benefits of changes in SOC for most common agricultural systems across Europe.