Carbon Sequestration Potential Promoted by Oxalate Extractable Iron Oxides through Organic Fertilization

Carbon sequestration in paddy soils through organic fertilization is of great importance for soil quality improvement in subtropical Asia. This study explored the effect of oxalate extractable iron oxides (Feo) on soil organic carbon (SOC) sequestration in response to different fertilization treatments. Soil samples were collected from 0-to 10-cm, 10-to 20-cm, and 20-to 40-cm depth intervals in June 2016 after a wheat (Triticum aestivum L.) harvest. This study involved five treatments: control with no fertilizer (CK), chemical fertilizer (NPK), 50% chemical fertilizer plus manure (NPKM), 100% chemical fertilizer plus straw (NPKS), and 30% chemical fertilizer plus manure organic-inorganic compound fertilizer (NPKMOI). Organic fertilization significantly (P < 0.05) increased the SOC content and the Feo concentration compared with chemical fertilization alone. The specific C mineralization rate (SCMR, rate per unit SOC) increased with increasing soil depths, suggesting that SOC at the 20-to 40-cm depth was more labile than that from 0 to 20 cm. The percentage of SOC present as microbial biomass carbon (MBC) was significantly (P < 0.001) positively correlated with SCMR, indicating that soil microorganisms influenced the potential SOC mineralization. Furthermore, the alternation of drying and wetting in paddy soils drives the biogeochemical cycles of iron and SOC, during which organic fertilization promotes the accumulation of Feo possibly by forming organo-iron complexes as indicated by the higher Feo content in NPKM than in NPK. The Feo was significantly (P < 0.001) positively correlated with SOC and the percentage of aromatic C, indicating that Feo may play an important role in preserving SOC, especially the aromatic compounds. Therefore, the enhancement of Feo by organic fertilization, especially organic manure that is enriched in aromatic compounds, improved the SOC sequestration potential in the rice (Oryza sativa)-wheat rotation system.