Climate regulates the accumulation of iron-bound organic carbon in a mountain ecosystem

Abstract Iron-bound organic carbon (OC–Fe) is a critical component of persistent soil carbon pools. However, the environmental controls on its accumulation in drylands remain uncertain. Here, we investigated OC–Fe stabilization along an approximately 1200-km climatic transect on the northern slope of the Kunlun Mountains, spanning an elevation range of approximately 1300–4200 m from warm-dry to cold-wet zones. We found that although absolute OC–Fe concentrations increased threefold from warm-dry to cold-wet ecosystems, OC–Fe consistently accounted for 17.3 ± 0.5% of total soil organic carbon (SOC) across the climatic gradient, with contrasting stabilization mechanisms across climatic conditions. Our results showed that OC–Fe formation in warm-dry lowlands was primarily associated with vegetation-related carbon inputs, as indicated by the dominant effects of Normalized Difference Vegetation Index (NDVI) and dissolved organic carbon (DOC), whereas iron-related variables contributed little within this climatic group. In contrast, stabilization in moderate zones was more strongly associated with soil texture and Fe oxide pools, while in cold-wet highlands biotic factors explained the greatest share of variation, with FeOB abundance emerging as the strongest predictor despite the large Fe oxide pools. These findings demonstrate that iron oxides do not function as a uniform carbon sink. Instead, the climate rearranges the hierarchy of constraints, from carbon supply to mineral capacity, to determine the persistence of mineral-associated carbon in dryland soils.