Dissolved Nutrients in Natural and Rewetted Peatlands: A Comparative Analysis

Intensive agriculture and artificial drainage have transformed natural peatlands into significant sources of greenhouse gas emissions as well as phosphorus and nitrogen pollution, the latter leading to an increased risk of eutrophication of adjacent water bodies. To address this issue and comply with the EU Water Framework Directive, restoring peatlands is a key strategy involving widespread rewetting to re-establish their roles as nutrient and carbon sinks. To achieve restoration objectives, it is essential to analyse and understand the temporal and spatial variability in porewater composition within natural and rewetted peatlands, with a specific focus on the influence of peatland type (bog or fen) and peat degradation status. Hence, an extensive international field survey of soil water and bulk soil was conducted from 1997 to 2017 on 60 natural and rewetted peatlands (both bogs and fens) in Germany, Poland, Estonia, Sweden, and the United Kingdom. The anoxic porewater samples were collected from water-saturated soil layers between 0 and 0.6 m depth. To quantify the concentrations of various chemical compounds in porewater at each location, at least three samples were taken using dialysis samplers within a spatial range of 5 to 10 meters. Selected sites were monitored for seasonal changes over a post-wetting period of 10 to 20 years. The results show significant differences in peat characteristics of upper soil layers from rewetted peatlands and natural peatlands, with the lowest values for nutrient contents, particularly in bogs. Notably, rewetted peatlands did not consistently display higher pore water concentrations of dissolved compounds compared to natural peatlands. However, in heavily drained and rewetted fens, anoxic pore waters exhibited concentrations of soluble reactive phosphorus (SRP), ammonium, and dissolved organic carbon one to two orders of magnitude higher than those in their natural counterparts. For example, SRP concentration in highly degraded peatlands ranged from 0.54 mg/L to 18.9 mg/L compared to 0.01 mg/L to and 3.6 mg/L in the natural peatlands. Weakly drained peatlands had, in some cases, slightly higher concentrations of dissolved substances compared to natural peatlands, but the differences were not statistically significant. Therefore, the research highlights the importance of porewater composition concerning the type of peatland, its degradation status, and spatial-temporal fluctuations in restored peatlands.