Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon

Abstract. Reactive iron (Fe) oxides are vital for long-term soil/sediment organic carbon (SOC) storage. However, the patterns and drivers of Fe‐associated organic carbon (Fe-OC) over global geographic scales under various ecosystem types remain largely controversial. Here, we provided for the first time a systematic assessment of the distribution patterns and determinants of Fe-OC content and its contribution (ƒFe-OC) by assembling a global dataset comprising 862 observations from 325 sites in distinct ecosystems. We found that Fe-OC content across global ecosystems ranged from 0 to 83.3 g/kg (ƒFe-OC ranged from 0 to 82.4 %), reflecting the high variability of the Fe-OC pool. Fe-OC contents varied with ecosystem type, being greater in wetlands with a high molar ratio of Fe-OC/dithionite-extractable Fe (Fe_d) compared with marine and continental ecosystems. Furthermore, ƒFe-OC in wetlands was significantly lower than that in other ecosystems due to rich OC. In contrast with climate variables and soil pH, the random forest modelling and multivariate analysis showed that the Fe-OC:Fe_d and SOC were the predominant predictors of Fe-OC content and ƒFe-OC in wetlands and continents, whereas Fe_d content was a primary driver in marine ecosystems. Based on upper estimates of global SOC storage in various ecosystem types, we further estimated that 83.84 ± 3.86 Pg, 172.45 ± 8.74 Pg, and 24.48 ± 0.87 Pg of SOC were preserved by association with Fe oxides in wetlands, continental and marine ecosystems, respectively. Taken together, our findings highlighted the importance of reactive Fe oxides in global SOC preservation, and their controlling factors were ecosystem-specific.