Net primary production rather than saturation of mineral surfaces limits soil carbon sequestration

Accrual of soil organic carbon (SOC), and especially the formation of mineral associated organic carbon (MAOC), has a large theoretical potential to act as sink for atmospheric CO₂. For reliable estimates of this potential and efficient policy advice, the major limiting factors need to be understood. The positive correlation between the content of fine mineral particles (silt + clay) and the content of MAOC is widely used to estimate a general maximum MAOC storage capacity of soils, based on the notion that mineral surfaces get C saturated. However, recent literature raised doubts about the concept of C saturation and it remains unclear, if and to what extent the mineral capacity of soils to stabilise C limits SOC accrual. Here, using large scale soil datasets and the model RothC, we provide two independent lines of evidence, that the upper boundary line of the correlation between MAOC and silt and clay does not resemble a maximum mineralogical SOC stabilisation capacity. 1. In coarse-textured soils, the C loading of the silt and clay fraction was found to strongly overshoot the mentioned upper boundary line and thus exceed previous C saturation estimates. 2. A global modelling exercise revealed that only for 28.8 % of all soils, local net primary production (NPP) would be sufficient to reach a C loading of 80 g C kg^-1 silt and clay, which is currently assumed to be the maximum capacity of soils to stabilise C. This proportion decreased strongly with increasing silt and clay content, which revealed that high C loadings can hardly be reached in more fine-textured soils. We conclude that SOC accrual is mainly limited by C inputs and strongly modulated by texture, mineralogy and climatic conditions. Taken together, those factors could be used to be formulate an ecosystem capacity to stabilise SOC. However, a potential C saturation point of MAOC appears still unknown and of no relevance for strategies of climate mitigation via SOC sequestration.