Towards an ecosystem capacity to stabilise organic carbon in soils.

Soil organic carbon (SOC) accrual, and particularly the formation of fine fraction carbon (OC), has a large potential to act as sink for atmospheric CO. For reliable estimates of this potential and efficient policy advice, the major limiting factors for OC accrual need to be understood. The upper boundary of the correlation between fine mineral particles (silt + clay) and OC is widely used to estimate the maximum mineralogical capacity of soils to store OC, suggesting that mineral surfaces get C saturated. Using a dataset covering the temperate zone and partly other climates on OC 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 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 in coarse to 75 g kg in most fine-textured soils. We conclude that OC 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.