Abstract. Iron oxide minerals play an important role in stabilizing organic carbon (OC) and regulating the biogeochemical cycles of OC on the earth surface. To predict the fate of OC, it is essential to understand the amount, spatial variability, and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concentrations and characteristics of Fe-bound OC in soils collected from 14 forests in the United States and determined the impact of ecogeographical variables and soil physicochemical properties on the association of OC and Fe minerals. On average, Fe-bound OC contributed 37.8 % of total OC (TOC) in forest soils. Atomic ratios of OC : Fe ranged from 0.56 to 17.7, with values of 1–10 for most samples, and the ratios indicate the importance of both sorptive and incorporative interactions. The fraction of Fe-bound OC in TOC (fFe-OC) was not related to the concentration of reactive Fe, which suggests that the importance of association with Fe in OC accumulation was not governed by the concentration of reactive Fe. Concentrations of Fe-bound OC and fFe-OC increased with latitude and reached peak values at a site with a mean annual temperature of 6.6 °C. Attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliphatic than non-Fe-bound OC. Fe-bound OC also was more enriched in 13C compared to the non-Fe-bound OC, but C ∕ N ratios did not differ substantially. In summary, 13C-enriched OC with less aliphatic carbon and more carboxylic carbon was associated with Fe minerals in the soils, with values of fFe-OC being controlled by both sorptive and incorporative associations between Fe and OC. Overall, this study demonstrates that Fe oxides play an important role in regulating the biogeochemical cycles of C in forest soils and uncovers the governing factors for the spatial variability and characteristics of Fe-bound OC.

<p><strong>Abstract.</strong> Iron oxide minerals play an important role in stabilizing organic carbon (OC) and regulating the biogeochemical cycles of OC on the earth surface. To predict the fate of OC, it is essential to completely understand the amount, spatial variability and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concentrations and characteristics of Fe-bound OC in soils collected from 14 forests in the United States, and determined the impact of ecogeographical variables and soil physicochemical properties o n the association of OC and Fe minerals. We found that Fe-bound OC contributed up to 57.8 % of total OC (TOC) in forest soils. Atomic ratios of OC:Fe ranged from 0.56 to 17.7 with values of 1–10 for most samples, and these ratios indicate an importance of both sorptive and incorporativ e interactions. The fraction of Fe-bound OC in TOC (<i>f</i><sub>Fe-OC</sub>) was not related to the concentration of reactive Fe, which suggests that the importance of association with Fe in OC accumulation was not governed by the concentration of reactive Fe. Concentrations of Fe-bound OC and <i>f</i><sub>Fe-OC</sub> increased with the latitude and reached peak values at a site with a mean annual temperature of 6.6 °C Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliphatic than non-Fe-bound OC. Fe-bound OC also was more enriched in <sup>13</sup>C compared to the non-Fe-bound OC, but C/N ratios did not differ substantially. In summary, <sup>13</sup>C-enriched OC with less aliphatic carbon and more carboxylic carbon was associated with Fe minerals in the soils, with values of <i>f</i><sub>Fe-OC</sub> being controlled by both sorptive and incorporative associations between Fe and OC. Overall, this study demonstrates that Fe oxides play an important role in regulating the biogeochemical cycles of C in forest soils, and uncovers the governing factors for the spa tial variability and characteristics of Fe-bound OC.</p>

Solid-phase iron (Fe) plays an important role in the accumulation and stabilization of soil organic matter (SOM). However, ferric minerals are subject to redox reactions, which can compromise the stability of Fe-bound SOM. To date, there is limited information available concerning the fate of Fe-bound SOM during redox reactions at Fe mineral surfaces. In this study, we investigated the release kinetics of hematite-bound organic carbon (OC) during the abiotic reduction of hematite-humic acid (HA) complexes by dithionite, to elucidate important processes governing the stability and fate of organic matter during the redox processes. Our results indicate that the reductive release of Fe obeyed first-order kinetics with release rate constants of 6.67-13.0 × 10 −3 min −1 . The Febound OC was released rapidly during the initial stage with release rate constants of 0.011-1.49 min −1 , and then became stable with residual fractions of 4.6-58.2% between 120 and 240 min. The release rate of aromatic OC was much faster than for the non-aromatic fraction of HA, and 90% of aromatic OC was released within the first hour for most samples. Our findings show that in the reductive reaction the mobilization of Fe-bound OC was asynchronous with the reduction of Fe, and aromatic OC was released more readily than other components of SOM. This study highlights the importance of evaluating the release of SOM bound with Fe during the redox reactions, especially the influence of the physicochemical properties of SOM.