Shift from Iron- to Enzyme-Dominated Controls on Soil Carbon Preservation in a Fen-Bog Comparison

"Enzyme latch" and "iron gate" are two essential mechanisms that govern the cycling of soil organic carbon (SOC) in wetlands. However, the specific contributions of these mechanisms to the response of different peatland SOC to water table decline under global warming remain poorly understood. We addressed this knowledge gap by simulating oxygen exposure following water table decline and warming in fen and bog soils and quantifying CO₂ emissions. In fen soils, aerobic conditions alone did not significantly increase CO₂ emissions in all peat layers, primarily because Fe(II) oxidation inhibited phenol oxidase activity, counterbalancing the stimulating effect of oxygen. Furthermore, Fe-bound OC slowed carbon mineralization. While simultaneous aeration and warming did not affect Fe-bound OC levels, they significantly enhanced phenol oxidase activity throughout the profiles, thereby increasing CO₂ emissions. Therefore, both iron protection and enzyme stimulation mediated the changes in CO₂ emissions from fen soils under aerobic conditions, with the former dominating without warming and the latter in its presence. In bog soils, aerobic conditions enhanced phenol oxidase activity and CO₂ emissions across all layers, regardless of temperature, demonstrating that enzyme activities dominate OC mineralization. In a warming environment, the "enzyme latch" will dominate the fate of peatland soil carbon.