Effect of Vegetation Restoration on Soil Iron-Associated Carbon Dynamics: Insights From Different Soil Textures

Soil iron (Fe)-associated carbon (C) (Fe-OC) plays a vital role in the soil C cycle due to its high stability, but vegetation restoration might alter the composition and quantity of Fe-OC by introducing a large amount of plant-derived C and affecting soil properties. However, how vegetation restoration affects soil Fe-OC remains unclear. Herein, plant and topsoil samples from grasslands, shrublands, and forestlands across three soil types (loam, loess, and sandy soil) since cropland conversions were collected to address this issue. The results showed soil Fe-OC content decreased in loam soil but increased in loess and sandy soil following vegetation restoration. Additionally, the Fe-OC accumulation efficiency induced by vegetation restoration increased with the coarser soil texture. Vegetation restoration promoted the accumulation of Fe-OC by increasing soil microbial biomass C, dissolved organic C, aromatic-C, and citric acid, but also disrupted the combination of Fe oxides and C by introducing oxalic acid, reducing Fe oxide content and iron trivalent (Fe(III)). There were two-sided effects of vegetation restoration on Fe-OC, but the overall effect depends on the soil types. Moreover, isotopic evidence indicated that microbial source C is the main source of Fe-OC, but Fe oxides preferentially adsorbed dissolved organic matter (DOM) and root deposits from plants rather than microbial residues and metabolites following vegetation restoration. In addition, Fe oxides preferentially adsorbed aromatic-C compared to other functional group components. These findings indicated that vegetation restoration in coarser-texture soils, coupled with selecting species that increase soil microbial biomass, produce more root deposits, and enhance DOM, contribute to the accumulation of soil Fe-OC.