Effect of Long-Term Straw Returning on the Soil Organic Carbon Bound to Iron Oxides in Black Soil and Fluvo-Aquic Soil

The purpose of this study was to explore the differences in the content and properties of different types of iron oxide bound organic carbon in different dryland soils and their responses to long-term straw returning.

Based on the long-term fertilization experiments of black soil and fluvo-aquic soil in Gongzhuling, Jilin Province and Zhengzhou, Henan Province since 1990, soil samples under no fertilization (CK), single application of chemical fertilizer (NPK) and chemical fertilizer +straw returning (NPKS) for 29 years were collected, and the complex iron (Fe_PP), amorphous iron oxides (Fe_HH) and crystalline iron oxides (Fe_DH) were obtained through continuous extraction method. The differences in the content of different forms of iron oxides and their bound organic carbons were explored. Combined with ultraviolet-visible spectroscopy, the differences in the properties of iron oxide-bound organic carbon were also accessed.

Compared with CK and NPK treatments, the soil organic carbon content under NPKS treatment in black soil was significantly increased by 18.1% and 14.9%, respectively, and fluvo-aquic soil was significantly increased by 46.1% and 13.3%, respectively. The content of Fe_PP in the two soils were lower than those under Fe_HH and Fe_DH, but the content of complexed iron-bound organic carbon (OC_PP) were significantly higher than those of amorphous iron oxide-bound organic carbon (OC_HH) and crystalline iron oxide-bound organic carbon (OC_DH), accounting for 71.4%-93.2% of the total iron oxide-bound organic carbon content, indicating that OC_PP was the main form of iron oxide-bound organic carbon. Compared with NPK treatment, the Fe_PP content in black soil was significantly reduced by 65.6%. Fe_HH content was significantly reduced by 17.8%, and Fe_DH content was not significantly changed under NPKS treatment, while the Fe_PP content in fluvo-aquic soil was significantly increased by 15.9%. Fe_HH content was not significantly changed, and Fe_DH content was significantly increased by 24.6%. Compared with NPK treatment, NPKS treatment significantly reduced the content of total iron oxide bound organic carbon in black soil, among which OC_PP decreased by 11.3%, while OC_DH increased by 53.7%, and OC_HH did not change significantly, but had no significant effect on the content of total iron oxide bound organic carbon in fluvo-aquic soil. Further analysis of the binding organic carbon content per gram of iron oxides showed that compared with NPK treatment, NPKS treatment significantly increased the binding capacity of three iron oxides in black soil, but had no significant effect in the fluvo-aquic soil. Fe_PP was mainly combined with macromolecular organic carbon with strong hydrophobicity and weak aromaticity while Fe_HH was mainly combined with small molecule organic carbon with strong hydrophobicity and aromaticity, and Fe_DH was mainly combined with some macromolecular compounds with poor hydrophobicity and aromaticity.

Comparing with the single application of chemical fertilizer, the content of crystalline and amorphous iron oxide bound organic carbon in black soil and fluvo-aquic soil increased significantly after long-term straw returning to the field, while the content of complexed iron bound organic carbon in black soil decreased significantly and did not change in fluvo-aquic soil. This was one of the important mechanisms for the different increases of soil organic carbon levels in different soil types after straw returning.