Nitrogen loss through denitrification, anammox and Feammox in a paddy soil

Highlights: • The widespread distribution of Feammox was observed in soil profiles. • The contribution of Feammox to N-loss was the highest in 20–30 cm soil depth. • The order of contribution to nitrogen loss: denitrification > anammox > Feammox. • The main NH₄⁺ and NO₂⁻ consumer were anammox and denitrification, respectively. Since the process of anaerobic ammonium oxidation (anammox) coupled with ferric iron reduction (termed Feammox) was discovered, it has been observed in various natural environments. However, besides the vertical distribution of Feammox in paddy soils, its differences and relationships with traditional nitrogen loss processes, including denitrification and anammox, remain unclear. Here, we studied the distribution of nitrogen loss pathways in different layers (0–50 cm) of paddy soil in southeastern China using ¹⁵N isotope tracer technology and molecular analysis. Our study showed that denitrification had a rate of 2.19 ± 0.39 mg N·kg⁻¹·d⁻¹, which was the highest activity in the surface layer (0–10 cm). The activities of anammox and Feammox reached peak values in the 10–20 cm (1.13 ± 0.16 mg N·kg⁻¹·d⁻¹) and 20–30 cm (0.23 ± 0.02 mg N·kg⁻¹·d⁻¹) soil layer, respectively. The nitrogen loss in the surface layer was more serious than that in the deep layer under paddy cultivation. In this study, denitrification was the main nitrogen loss pathway in the surface soil, but Feammox became an important nitrogen loss pathway (up to 26.1%) in the 20–40 cm depth. Overall, our research could improve and perfect the nitrogen cycle pathways in paddy soil.