No saturation of soil carbon under long-term extreme manure additions

Abstract Background and aims According to the carbon (C) saturation concept, the capacity of soils to accumulate stabilized organic C is limited by the number of binding sites on mineral surfaces. The concept and its application are highly debated. Therefore, we aimed at testing this theory using field experimental data. Methods Soils were sampled from four long-term field experiments with different amounts of organic fertilisation going up to extreme high C inputs (20 Mg C ha −1 yr −1 ) five times higher than in common agricultural practice. Soils were fractionated by particle size to obtain sand-sized, coarse silt and fine silt plus clay fractions. Results We found a linear relation between C input and soil organic carbon stocks (SOC) even with vast amounts of organic C inputs to the soil at three experimental sites. Across all experiments, C stocks in the sand-sized fraction increased on average by 146%, whereas C stocks in the fine silt plus clay fraction (< 20 µm) increased by just 17% without distinct saturation behaviour. The C sequestration efficiency (amount of C retained as SOC per amount of C input) tended to increase with initial SOC content which is not in line with the saturation theory. Conclusion The experiments were subject to C inputs via organic fertilisation that would and should rarely be reached in agricultural practice due to negative side effects. Even under these artificial conditions experiments did not show a distinct saturation behaviour. Initial SOC stocks or SOC in the mineral-associated fraction did not appear to limit the potential of soils to sequester additional SOC. It can be concluded that C sequestration is mainly limited by the availability of C inputs from biomass.