Towards an ecosystem capacity to stabilise organic carbon in soils

Soil organic carbon (SOC) accrual, and particularly the formation of fine fraction carbon (OC_fine), has a large potential to act as sink for atmospheric CO₂. For reliable estimates of this potential and efficient policy advice, the major limiting factors for OC_fine accrual need to be understood. The upper boundary of the correlation between fine mineral particles (silt + clay) and OC_fine is widely used to estimate the maximum mineralogical capacity of soils to store OC_fine, suggesting that mineral surfaces get C saturated. Using a dataset covering the temperate zone and partly other climates on OC_fine contents and a SOC turnover model, we provide two independent lines of evidence, that this empirical upper boundary does not indicate C saturation. Firstly, the C loading of the silt + clay fraction was found to strongly exceed previous saturation estimates in coarse-textured soils, which raises the question of why this is not observed in fine-textured soils. Secondly, a subsequent modelling exercise revealed, that for 74% of all investigated soils, local net primary production (NPP) would not be sufficient to reach a C loading of 80 g C kg^-1 silt + clay, which was previously assumed to be a general C saturation point. The proportion of soils with potentially enough NPP to reach that point decreased strongly with increasing silt + clay content. High C loadings can thus hardly be reached in more fine-textured soils, even if all NPP would be available as C input. As a pragmatic approach, we introduced texture-dependent, empirical maximum C loadings of the fine fraction, that decreased from 160 g kg^-1 in coarse to 75 g kg^-1 in most fine-textured soils. We conclude that OC_fine accrual in soils is mainly limited by C inputs and is strongly modulated by texture, mineralogy, climate and other site properties, which could be formulated as an ecosystem capacity to stabilise SOC.