Tree stems dominate methane but not nitrous oxide emissions in a riparian nature-based wastewater treatment system

Riparian zone soils are being applied as nature-based solutions for treating wastewater treatment plant effluents via intermittent horizontal subsurface flow systems. While their treatment efficiency is well documented, their role in greenhouse gas (GHG) dynamics, particularly emissions mediated through tree stems, remains largely unexplored. This study quantified tree stem and soil emissions of CO₂, CH₄, and N₂O within an innovative riparian-zone wastewater treatment system and evaluated the influence of hydrological conditions, soil properties, and tree species. From April to October 2023, treated wastewater was applied in alternating wet and dry cycles of one week each, with five sampling campaigns per condition. GHG fluxes were measured from soils (N = 15) and from tree stems at approximately 0.5 m height (N = 18). Concurrently, soil temperature, moisture, pH, and carbon and nitrogen content were assessed. The dominant tree species included Alnus glutinosa, Ulmus minor, Fraxinus excelsior, and the non-native Platanus × hispanica. Soil GHG emissions were primarily driven by environmental conditions. Soil CO₂ emissions were mainly controlled by temperature, whereas soil CH₄ and N₂O were ruled by groundwater table fluctuations. Soil N₂O emissions increased under shallower water tables and higher soil temperature and moisture. Soil CH₄ fluxes were spatially heterogeneous, with higher emissions in areas where groundwater table was shallower. Overall, the intermittent wet/dry management supported both soil GHG production and consumption without causing a substantial net increase in emissions.Tree stem emissions were strongly species-dependent and often exceeded soil CO₂ and CH₄ emissions, while N₂O emissions were almost negligible. Platanus × hispanica consistently showed the highest stem emissions across all gases, emitting approximately threefold more CO₂ and over two orders of magnitude more CH₄ than soils. Ulmus minor and Alnus glutinosa also exhibited elevated stem CH₄ emissions compared to soils (20 and 9 times higher, respectively), whereas stem N₂O emissions were generally about half of soil emissions for all species. Notably, Fraxinus excelsior frequently acted as a sink for N₂O. Stem CO₂ emissions increased with soil temperature and nitrogen content and peaked during the warmest months but were not influenced by hydrological conditions. In contrast, CH₄ emissions displayed a significant interaction between species and wet conditions, suggesting transport of CH₄ produced in deeper soil layers through stems or in situ microbial production. N₂O fluxes from stems were highly variable, with both emissions and uptake observed, indicating control by microscale and potentially internal stem processes.This study provides the first simultaneous assessment of soil and tree stem GHG emissions in a nature-based wastewater treatment system. The results demonstrate that tree species identity is a critical determinant of stem-mediated GHG fluxes and highlight the need to incorporate vegetation structure, particularly tree stems, into GHG budgets and the design of riparian wastewater treatment systems.