The POM-C / MAOM-C ratio as a compliance indicator for sustainable soil organic carbon management of arable soils in Central Europe

Soil organic matter consists of components that differ in their specific stabilization/decomposition dynamics, and turnovers. From a simplified viewpoint, two fractions in particular can be distinguished from one another. Particulate organic matter (POM) is predominantly unbound in the soil matrix. The POM decomposition rate is defined by its inherent chemical recalcitrance and occlusion within aggregates. Mineral-associated organic matter (MAOM) is significantly smaller and is protected from decomposition by its adsorption to mineral surfaces. MAOM-C has therefore significantly longer mean residence times in soil than POM-C. Since the soil organic carbon (SOC) stocks are determined by C input/output balances, it is important to decrease C output quantities by increasing the long-term stabilization of OC within the MAOM-C stocks. However, MAOM-C cannot be enriched indefinitely. It is limited by the amount of clay and fine silt particle surfaces it can adsorb to and to the general land-use management. We investigated the validity of a POM-C/MAOM-C ratio indicator on 25 long-term field experiments in Central Europe to evaluate the sustainability of SOC management measures. We found that the POM-C/MAOM-C ratio might be used to assess the sustainability of agricultural management in before/after management change comparisons. Accordingly, a sharply increasing ratio indicates that the change in management does not adequately affect the long-term MAOM-C storage of soil. Moreover, we found a dependence between the POM-C/MAOM-C ratio and the MAOM-C sequestration deficits in soils, where arable soils with a POM-C/MAOM-C ratio indicator > 0.35 are close to MAOM-C saturation. If these observations are repeatable on further arable soils, the POM-C to MAOM-C ratio of 0.35 could be used as a management target to avoid organic over-fertilization and N loss, especially in coarse-textured soils. Thereby, the indicator might help to optimize SOC management and sequestration on arable soils and support climate change mitigation strategies.