Iron and sulphur regulate carbon dioxide emissions in drained coastal peatlands of The Netherlands

Fluctuating groundwater levels in drained peatlands create a transition zone with seasonally changing oxygen availability. This zone drives dynamic iron (Fe) and sulphur (S) cycling under alternating anoxic and oxic conditions, influencing decomposition rates. This study investigated how Fe and S affect decomposition rates and resulting carbon dioxide (CO₂) emissions under fluctuating redox conditions in transition zone. In a controlled laboratory experiment, peat samples from two drained Dutch coastal peatlands were amended with ferric iron (Fe³⁺) and sulphate (SO₄²⁻) and incubated anoxically to mimic high groundwater tables. This was followed by an oxic phase simulating groundwater table drops. The cycle was repeated with lactate addition to replenish labile carbon. Carbon dioxide emission rates were monitored continuously throughout the anoxic–oxic cycles. Water soluble Fe and S concentrations, exoenzyme activities, and pH were measured before and after the experiment. Carbon dioxide emission rates increased under anoxic conditions with Fe³⁺ and SO₄²⁻ amendments potentially due to stimulation of microbial activity using these compounds as alternative electron acceptors. Short-term oxygenation suppressed emissions compared to controls without amendments. Water-soluble Fe remained stable across treatments, while water-soluble S concentrations changed significantly from initial levels. Exoenzyme activities were primarily influenced by pH, with minimal effects from amendments. The findings show that transition zone is an active redox zone where decomposition dynamics are determined by available electron acceptors in the system, influencing greenhouse gas (GHG) emissions from managed peatlands. This zone should be integrated into future models to improve the accuracy of reporting national GHG emissions.