Identified hot spots of denitrification contribute to 80% of the total retention

Floodplains are known to act as nutrient sinks when inundated because retention processes take place and contribute to the Ecosystem Service water purification. The importance of this Ecosystem Service is reported in science and political discussions differently depending on the considered data base, its spatial extent and definitions. Its quantification is a challenging task due to the complex, dynamic, and context-dependent nature of floodplain ecosystems resulting in a high spatial, temporal, and ecological variability of so-called functional process zones. In this study, we assessed denitrification at the floodplain scale in a 99 ha section of the Gülpe floodplain along the Lower Havel River (Germany) and mapped functional process zones of nitrogen retention. We found that up to 10,000 kg N yr?¹ are retained, with an average of 91 kg N ha?¹ yr?¹, and that 12% of the area accounts for 80% of total denitrification, exceeding 700 kg N ha?¹ yr?¹ in hotspots. These areas were associated with specific combinations of plant community composition, soil microbial structure (including denitrifying bacteria), and soil texture, particularly in low-lying zones dominated by Flutrasen biotope types. Our findings underscore the importance of integrating soil–water–vegetation dynamics in spatially explicit assessments to support floodplain restoration and the enhancement of water purification services. These benefits should be explicitly considered when accounting for ecosystem services of floodplains.