Effect of long-term fertilization on the stabilization of soil organic carbon by iron oxides in red soil

The preservation and decomposition of soil organic carbon (SOC) has been the subject of scientific inquiry for decades, owing to its critical role in regulating atmospheric CO₂ concentrations. Iron (Fe) oxides are widely recognized as a rusty sink for carbon (C) because of their large surface area and high adsorption affinity. Fe oxides such as amorphous Fe hydroxides, crystalline Fe hydroxides, and organo-Fe complexes coexist in soils and can be converted to one another. An in-depth understanding of the stabilization of SOC by different types of Fe oxides will strengthen our understanding of soil C cycling. Based on a long-term (25 years) fertilization field experiment in Hengyang, Hunan Province, China, investigations were performed to clarify the stabilization of SOC using different Fe oxides, and its responses to long-term fertilization were discussed. A selective extraction was conducted sequentially to determine the distribution of organic carbon (OC) among different Fe oxides: Na-pyrophosphate (organo-Fe complexes) followed by HCl-hydroxylamine (amorphous Fe hydroxides) and dithionite-HCl (crystalline Fe hydroxides). Ultraviolet and visible light adsorption measurements were used to analyze the composition of Fe oxide-bound OC. The OC contents differed among different Fe oxides in the red soil in the following order: organo-Fe complex-bound OC (2.45–3.59 g∙kg^–1, OC_PP) > crystalline Fe hydroxide-bound OC (1.46–1.51 g∙kg^–1, OC_DH) > amorphous Fe hydroxide-bound OC (0.39–0.70 g∙kg^–1, OC_HH). OC_PP was formed by the coprecipitation/chelation of organo-Fe complexes with low aromaticity, high molecular weight, and high hydrophobicity compounds. OC_HH and OC_DH were primarily formed by Fe hydroxide-adsorbed aromatic compounds. OC_HH had greater average molecular weights and higher aromaticity than OC_DH. Long-term application of chemical fertilizers (NPK) facilitated (P<0.05) the binding of OC with organo-Fe complexes and amorphous Fe hydroxides. However, organic fertilizer (M) addition solely increased (P<0.05) the association of OC with amorphous Fe hydroxides. In addition, NPK treatments increased (P<0.05) the average molecular weights of OC_DH and the hydrophobicity and aromaticity of OC_PP. However, M treatments decreased (P<0.05) the average molecular weights of OC_PP and the hydrophobicity and aromaticity of OC_PP and OC_HH. These findings suggest that long-term fertilization may increase the stabilization of SOC by Fe oxides in red soil; however, the response of SOC stabilization by Fe oxides with varying crystallinity to long-term fertilization is different. In addition, long-term fertilizer addition may change the composition of Fe oxide-bound OC.