Iron-bound organic carbon and their determinants in peatlands of China

There is a strong correlation between iron oxide mineral and organic carbon (OC) concentrations. However, the role of iron oxide minerals in the preservation of OC in peatlands is poorly understood. In this study, we collected soil samples from 10 peatlands in central and west China to comprehensively investigate the concentration, characteristics, and spatial variability of iron oxide-bound OC (Fe-bound OC) concentrations and determine the impact of climatic factors and soil physicochemical properties on the correlation between OC and iron oxides. Our results showed that the concentration of Fe-bound OC ranged from 3.91 to 18.79 g kg−1. On average, Fe-bound OC accounted for 3.42 ± 1.32% of soil OC. The molar ratio of OC: Fe ranged from 2.17 to 12.18, indicating that coprecipitation was the dominant mechanism affecting the binding of OC to reactive iron oxides in peatlands. δ13C isotope analyses revealed that Fe-bound OC was relatively enriched in 13C in peatlands. Iron mineral sources, local vegetation, and the water table were responsible for differences in the concentration of Fe-bound OC identified among the 10 sites. Correlation analysis results indicated that total organic carbon (TOC) was the main determinant factor for the concentration of Fe-bound OC (r = 0.549, p < 0.01), and the binding mechanism was jointly regulated by TOC and reactive Fe content. The proportion of Fe-bound OC to TOC (fFe-bound OC) was positively correlated with the reactive Fe (r = 0.457, p < 0.05), indicating that the importance of binding with iron minerals in OC accumulation is regulated by the concentration of reactive Fe in peatlands. This study highlights the quantitative characteristics and spatial variability of Fe-bound OC in peatlands, providing important information for assessing and managing the carbon cycle in peatland ecosystems.