The “Iron Gate” Outcompetes the “Enzymic Latch” as the Dominant Soil Organic Carbon Stabilization Mechanism in Permafrost Peatlands of the Great Hing’an Mountains

Distinct paradigms, such as the "enzymic latch" and "iron gate" theories, have been proposed to elucidate SOC loss or accumulation, but their relative significance and whether they are mutually exclusive in permafrost peatlands remain unclear. To address this, we evaluated their relative importance and identified the dominant factors controlling SOC stability. Therefore, we employed a space-for-time substitution approach across a permafrost gradient (continuous, discontinuous, and isolated) by systematically quantifying extracellular enzyme activities, iron (Fe) phases, and iron-bound soil organic carbon (Fe-SOC) at various depths (0-10, 10-30, and 30-50 cm) in peatlands. Our results did not support the "enzymic latch" theory, with hydrolytic enzyme activities (β-glucosidase (BG), cellobiohydrolase (CBH), and β-<i>N</i>-acetylglucosaminidase (NAG)) showing positive correlations with phenolics but negative correlations with phenol oxidase (PHO) activity. However, ferrous iron (Fe(II)) was significantly positively correlated with PHO activity, and ferric iron (Fe(III)) stabilized SOC through co-precipitation with it to form Fe-SOC, supporting the "iron gate" theory. Moreover, Fe-SOC decreased from the continuous to the isolated permafrost zone, and with soil depth from 0-10 cm to 30-50 cm. Partial least squares path modeling (PLS-PM) analysis indicated that Fe(III) directly and indirectly (via Fe-SOC and phenolics) affected SOC. Our study demonstrated the primacy of the "iron gate" mechanism in controlling carbon stability in the Great Hing'an Mountains permafrost peatlands, providing new insights for projecting carbon-climate feedback.