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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.

Peat decomposition driven by soil metabolic processes is responsible for approximately 2 % of global annual anthropogenic greenhouse gas emissions. A peat soil's redox potential (Eh) and pH reflect its biogeochemical state and are therefore linked to the rate of peat decomposition and greenhouse gas production. In this study, we aim to establish if continuous Eh measurements are an effective tool to monitor metabolic peat decomposition processes and thus to quantify the effects of peat wetting efforts. We applied continuous in-situ Eh measurements (>150 sensors 2020–2022) as a proxy for metabolic peat decomposition processes, which we validated under field conditions with extensive sampling of porewater chemistry (pH, NO3–, SO42−, Mn(II), Fe(II), S2− and CH4, >2000 samples) for five agricultural, drained, minerotrophic peatland sites in the Netherlands. These 5 sites consisted of plots with and without subsoil irrigation (SSI), where SSI aims to raise groundwater levels and thus wet the peat soil. We found that in-situ continuous Eh measurements closely reflected spatial and temporal heterogeneity in pore water chemistry. Therefore, we concluded that Eh is a robust proxy for peat decomposition processes. Building on this result, we used continuous Eh measurements to study the prevalence of specific metabolic processes from site-to-site in relation with groundwater level changes. We found that, while groundwater levels are an important driver for (an)aerobic conditions, groundwater levels do not explain the full dynamics and extent of (an)aerobic conditions. O2 intrusion was mostly limited to approximately 0.5 m depth at deep (>0.8 m) groundwater levels, likely due to air diffusion limitation. Higher and more constant groundwater levels year-round at SSI plots decreased oxygen intrusion and tended to deplete porewater Fe(II) and SO42−, which led to more reducing Eh and higher porewater CH4 concentrations. The depletion of electron acceptors and occurrence of methanogenesis differed from site to site. In summary, high-frequent Eh monitoring is found to be an effective tool to monitor metabolic peat decomposition processes and quantify the effects of peatlands wetting efforts. Therefore, this methodology is suitable to evaluate and further optimize peatland monitoring and preservation.

Climate change is increasing the frequency and severity of short-term (~1 y) drought events-the most common duration of drought-globally. Yet the impact of this intensification of drought on ecosystem functioning remains poorly resolved. This is due in part to the widely disparate approaches ecologists have employed to study drought, variation in the severity and duration of drought studied, and differences among ecosystems in vegetation, edaphic and climatic attributes that can mediate drought impacts. To overcome these problems and better identify the factors that modulate drought responses, we used a coordinated distributed experiment to quantify the impact of short-term drought on grassland and shrubland ecosystems. With a standardized approach, we imposed ~a single year of drought at 100 sites on six continents. Here we show that loss of a foundational ecosystem function-aboveground net primary production (ANPP)-was 60% greater at sites that experienced statistically extreme drought (1-in-100-y event) vs. those sites where drought was nominal (historically more common) in magnitude (35% vs. 21%, respectively). This reduction in a key carbon cycle process with a single year of extreme drought greatly exceeds previously reported losses for grasslands and shrublands. Our global experiment also revealed high variability in drought response but that relative reductions in ANPP were greater in drier ecosystems and those with fewer plant species. Overall, our results demonstrate with unprecedented rigor that the global impacts of projected increases in drought severity have been significantly underestimated and that drier and less diverse sites are likely to be most vulnerable to extreme drought., Competing Interests: Competing interests statement:The authors declare no competing interest.