Can Fe oxides stabilise organic carbon in soil?

Soil aggregates are known to play an important role in preserving soil organic carbon (SOC). Poorly crystalline Fe oxides are one of the main aggregating agents in tropical soils. The interaction between soil organic matter (OM) and Fe oxide surface leads to the formation of an organo-mineral complex which is one of the main mechanisms of SOC preservation. The effect of Fe oxide amendment on soil aggregate formation as well as the preservation of SOC in temperate soils is largely unknown. Therefore, the main objective of this research project is to investigate these effects through a number of laboratory studies. Soils were amended with different levels of Fe oxides (0%, 0.5, 1%, 2% and 4%). An initial adsorption study confirmed that Fe oxide amendment can adsorb dissolved organic carbon (DOC) and significantly reduce the concentration of DOC in five different soil types (P≤0.05). This result gave a justification for further plant growth experiment for 8 weeks. Soils for the plant growth experiment were amended with the same levels of Fe oxides, organic matter (OM) (0 t/ha, 1.5 t/ha, 3 t/ha, 6 t/ha and 12 t/ha), and a combination of these different levels of Fe oxides + OM. After 8 weeks, Fe oxide amendment caused an increase in soil macro-aggregates followed by a significant increase in SOC in macro-aggregate fractions ((P≤0.05). In addition, the masses of resistant soil organic carbon (rSOC) fractions and SOC concentration in rSOC fractions significantly increased due to Fe oxide amendment (P≤0.05). The effects of OM and Fe oxide+ OM were only significant on SOC concentration in macro-aggregates. These results showed that Fe oxide amendment can certainly be a useful tool to stabilise SOC through the formation of stable soil aggregates which could mitigate the impacts of global warming.