What limits carbon sequestration in soils?

Carbon (C) sequestration in soils has been discussed as important climate mitigation option with the potential to generate negative emissions. Agriculture requires such negative emissions since some of their greenhouse gas emissions are unavoidable and require compensation to achieve net zero. Expectation of soils contribution to climate mitigation need to come down from theoretical assumptions to realistic estimates. In order to do so the limitations for soil C sequestration need to be analysed and discussed. Here we present a framework with case studies looking at limitations that are i) intrinsic due to the soils´ ability to stabilize SOC on mineral surfaces (C saturation) and the current state of high SOC stocks ii) constraints by net primary productivity and biomass availability, and iii) restrictions due to limited land area and increasing global demand for food, feed and fibre from agricultural production. For the start of this analysis we used data of the first German Agricultural Soil Inventory comprising more than 3000 sites. In total 34% of agricultural topsoils (0-10 cm depth) in Germany contain high SOC stocks with more than 4% soil organic matter. In particular soils with ground water influence and grassland land-use contain high SOC stocks, which need to be maintained first before further SOC accumulation can be achieved. C saturation was frequently discussed as reason for preventing further built up of stabilised SOC in C-rich soils. However, based on data from long-term field experiments and the national soil inventory we challenge the perception that C saturation is a limiting factor for soil C sequestration in our soils.

Biomass is required to maintain and enhance SOC. However, the quality and form of biomass influences the effectiveness for SOC formation. Roots are more important than above ground biomass. This shifts the view of C-management to below ground. Above ground biomass, such as straw, maybe harvested without harms to SOC stocks and used in industrial processes or converted to biochar. Strongly limited is the land area on which measures for SOC built-up can be implemented without compromising other ecosystem services. Avoiding leakage of greenhouse gas emissions due to measures for SOC sequestration are a major challenge. With the example of cover crops as agricultural management option we illustrate these limitations and discuss how some of the limitations for SOC sequestration could be removed.