Iron-associated organic carbon as a major carbon sink in permafrost-affected peatlands of Northeast China

Permafrost-affected peatlands are hotspots globally because of their large carbon storage and climate sensitivity. However, there have been limited studies on the abundance and controlling factors of iron-associated organic carbon (Fe-OC) in these important ecosystems. Here we conducted a large-scale comparison study of soils across major terrestrial ecosystems—including croplands, forest, grasslands, wetlands and peatlands—to understand differences in the distribution of Fe-OC abundance. Our results show that the Fe-OC abundance of peatlands (13.3±9.6 g kg^-1) are higher than that in non-peat forming wetlands (6.4±4.5 g kg^-1) and mineral soils, such as croplands (9.1±9.3 g kg^-1), forest (10.5±11.1 g kg^-1) and grasslands (2.0±2.2 g kg^-1), implying the efficient binding capacity of iron minerals with OC in peatlands. Further, our field and laboratory investigations focus on Northeast China, the major peatland-dominant region in China with permafrost-affected and non-permafrost peatlands—including Sphagnum-dominated bogs and sedge-dominated fens, to clarify the processes and mechanisms of Fe-OC accumulation. We find that permafrost-affected peatlands contain near 5-fold higher Fe-OC than non-permafrost peatlands. Our parallel factor analysis of fluorescence excitation-emission matrix results shows that microbial-derived carbon accounts for 25.5-90.7% of Fe-OC in permafrost peatlands, with an average contribution of 56.0%. Moreover, we observed a positive correlation between the Fe-OC abundance and the proportion of OC derived from microbes. Iron minerals in permafrost peatlands tend to bind a greater proportion of labile carbon—whether derived from plants or microbes—than in non-permafrost peatlands, suggesting that the presence of permafrost offers an important mechanism for climate change mitigation. Furthermore, nutrients (such as nitrate, phosphate and C:N ratio) are major controlling factors for Fe-OC in non-permafrost peatlands (with a total effect of up to 96.9%), while reactive Fe (with an effect of up to 96.9%) and other factors (including pH, climate, FeRB and microbial-derived OC) positively influence Fe-OC in permafrost peatlands. These findings demonstrate that iron minerals act as a crucial ‘OC protectors’ that greatly boost the rusty carbon sink in cryogenic ecosystems. Future climate warming and permafrost thaw will not only reduce low-temperature protection of previously frozen carbon—some of them labile—but also diminish the mineral-association protection of a large quantity of carbon.