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

## TL;DR

This study finds that the 'iron gate' mechanism, not the 'enzymic latch', is the main way carbon is stabilized in permafrost peatlands of the Great Hing’an Mountains.

## Contribution

The study identifies the 'iron gate' as the dominant carbon stabilization mechanism in permafrost peatlands, challenging the 'enzymic latch' theory.

## Key findings

- Hydrolytic enzyme activities were not correlated with phenolics as expected by the 'enzymic latch' theory.
- Ferrous iron positively correlates with phenol oxidase activity, and ferric iron stabilizes soil organic carbon through co-precipitation.
- Fe-SOC decreases with permafrost degradation and soil depth, indicating vulnerability to climate change.

## Abstract

The “enzymic latch” and “iron gate” theories represent two prevailing and contrasting mechanisms governing ecosystem carbon stability: the former via a phenolics accumulation mediated biochemical cascade that suppresses hydrolytic enzyme activity, and the latter via an abiotic pathway where ferrous iron oxidation suppresses phenol oxidase activity and promotes iron-bound soil organic carbon formation. Therefore, deciphering the stabilization mechanisms for the vast carbon stocks in permafrost peatlands represents a central challenge for climate change projections. In this study, we assessed the spatial distribution and interrelationships of peatland soil extracellular enzyme activities, iron phases, and iron-bound soil organic carbon across three permafrost zones in the Great Hing’an Mountains. Contrary to the “enzymic latch” mechanism, our data revealed that hydrolytic enzyme activities (β-glucosidase, cellobiohydrolase, and β-N-acetylglucosaminidase) were neither negatively correlated with phenolics nor positively correlated with phenol oxidase activity. Instead, iron emerged as the central regulator, with a positive correlation between ferrous iron and phenol oxidase activity and with ferric iron stabilizing soil organic carbon through co-precipitation. Our results highlighted that permafrost degradation could poses a threat to the dominant “iron gate” carbon sequestration mechanism in peatlands, potentially triggering a positive climate feedback.

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-N-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.