Nitrous oxide emissions in natural and managed wetlands across Europe

Wetlands play a complex role as both sources of greenhouse gases (GHGs) and carbon sinks, making it essential to understand their dynamics and effects on biodiversity. The increasing pressures from climate change and human activities can disrupt the natural balance of these ecosystems, potentially resulting in elevated GHG emissions. The intricate abiotic and biotic interactions that govern these processes remain poorly understood. Therefore, there is an urgent need to enhance our understanding of the factors influencing GHG production in wetlands and to improve our capacity to model these emissions on a larger scale. In this study, we investigated the emissions of N₂O, CO₂ and CH₄, with a particular focus on N₂O, which is primarily produced through the microbial process of denitrification, and for which a satisfactory large-scale model formulation is lacking. The objective of this study was to evaluate these GHG emissions under optimal conditions for denitrification and to identify unifying abiotic factors. To achieve this, we selected contrasting study sites that varied by wetland type and climate zone, thereby gathering extensive data essential for our modelling efforts.

The research was conducted across multiple wetland sites involved in the ALFAwetlands project (https://alfawetlands.eu/), a European initiative dedicated to the study and restoration of both natural and managed wetlands. A total of 21 sites were selected across five European countries, encompassing a range of climate zones from Mediterranean to arctic. These included floodplains, alluvial forests, drained forests, peatlands, and mountain peatlands (with four sites each in France and Spain, three in Belgium, and five each in Finland and Estonia). For each location, three core samples (10 cm depth and 10 cm diameter) were collected and stored in the dark at 4°C prior to conducting mesocosm experiments. The samples were then placed in a custom-designed “GHG-aquacosm”, which simulates the effect of flooding on wetlands soils. During the experiments, soil cores were submerged in heated water enriched with nitrate. GHG emissions, soil moisture, and soil temperature were continuously monitored until stabilization or end of emission.

While CO₂ and CH₄ emissions were recorded, they have not yet been analysed, as this study primarily focuses on N₂O emissions. The results indicated that N₂O emissions varied significantly based on wetland type and initial soil water content. Drained forests, located in cool sub-arctic regions in Finland, demonstrated the highest N₂O fluxes, ranging from 500 to 2000 µmol/m²·h. In contrast, floodplains and peatlands in Belgium and Estonia showed the lowest fluxes, between 5 and 150 µmol/m²·h. Significant variability was noted even among replicates, highlighting the considerable spatial heterogeneities of soils. Additionally, N2O emissions began immediately after nitrate addition, and for most sites ended 30 to 40 hours after, indicating the short temporal scale of N2O production and the challenges associated with in situ measurement. Ongoing data analysis and measurements are focused on further elucidating the spatial and temporal heterogeneities of denitrification processes, with the goal of effectively incorporating these factors into our modelling efforts.