Physical protection of soil organic carbon in Swedish arable soils in relation to oxalate-extractable aluminum.

Stabilizing soil organic carbon (SOC) is essential for maintaining soil structure and carbon sequestration. Soil aggregation through organic-mineral associations helps to protect SOC from microbial decomposition physically. Recent research has shown that the chemical interactions between SOC and reactive aluminum (Al) phases may be central for this aggregation in acidic soils and, hence, for the protection of SOC. However, the role of reactive Al phases in protecting SOC on a national level in Sweden is unclear. We analyzed 100 topsoil samples from the Swedish national monitoring program ‘Swedish soil and crop monitoring inventory’ taken between 2001 and 2017 in a grid covering all major agricultural areas in Sweden and determined the Al and iron content after chemical extraction using ammonium oxalate acid. We also estimated silt-sized aggregation from the differences in particle size distributions measured with a laser diffraction analyzer (Partica LA-950 V2, Horiba) between mechanically and chemically dispersed samples. Preliminary results show that SOC is indeed positively correlated with oxalate extractable Al on a national scale in Sweden, while correlations with clay and oxalate extractable iron are much weaker. The results derived from this study can help to determine the relationship between reactive Al phases, soil aggregation, structure, and the potential for carbon sequestration in Swedish arable soils.