Rewetting drained peatlands through subsoil infiltration stabilises redox-dependent soil carbon and nutrient dynamics

Centuries of drainage have stimulated peat decomposition. To counteract the resulting increase in greenhouse gas emission and land subsidence in Dutch agricultural peatlands, passive and active subsurface infiltration (SSI) systems have been developed for peatland rewetting. Here, we studied the effects of SSI systems on groundwater levels, porewater composition and redox potential in four drained peatlands in the Netherlands to determine how soil processes are affected, especially carbon and nutrient dynamics. For three years, groundwater levels were measured continuously, and porewater samples were collected 8-10 times per year in paired SSI (active and passive) and control plots. SSI plots had higher summer groundwater levels and less seasonal fluctuation in groundwater levels than control plots. Redox potential and porewater composition in control plots reflected dominance of oxidation processes during dry periods in the upper soil layers, whereas SSI plots showed dominance of reduction processes in these layers throughout the year. These differences between control and SSI plots were strongest at locations with active SSI systems. Our results show that SSI systems can be effective measures to raise and stabilise groundwater levels in drained peatlands, which results in more stable redox zonation and dominance of anaerobic soil processes, especially during dry periods and when active SSI systems are applied. On the short term, a switch from oxic to anoxic conditions can cause mobilisation of phosphorus and ammonium, while on the longer term the application of SSI can lead to substantial change in carbon and nutrient dynamics. Understanding the full effects of implementing SSI systems and other mitigation measures in drained peatlands is important before they are applied on a large scale.