Abstract: Nitrogen is one of key regulators of wetland methane (CH₄) emissions. However, how increased input of different nitrogen forms in winter affects CH₄ production and oxidation in high and low tidal zones of estuarine wetlands has been poorly quantified. In this study, soil samples were collected from three soil layers (0-10, 10-20, and 20-30 cm) of Cyperus malaccensis marshes in the high and low tidal zones of Shanyutan in the Minjiang Estuary in winter. Soil microcosm incubation experiments were conducted under anaerobic and aerobic conditions. Based on the background nitrogen input of 21 g·m^-2·a^-1 in the Minjiang Estuary wetlands, three treatments of control (no nitrogen addition), low level addition (50 mg N·kg^-1 dry soil; 2.5 times of the background input rate), and high level addition (100 mg N·kg^-1 dry soil; five times of the background input rate) of nitrate (NO₃^--N) and ammonium (NH₄⁺-N) were set up. The responses of CH₄ production and oxidation potentials in marsh soils of high and low tidal zones to the addition of different forms of nitrogen in winter were different. After adding high-level nitrate, CH₄ production potential decreased by 95.4% in the 0-10 cm soil layer of the high tidal zone, and decreased by 83.3% in 20-30 cm soil layer of the low tidal zone. However, responses of CH₄ production potentials in the different soil layers of high and low tidal zones to ammonium addition were not significant. Addition of ammonium and nitrate did not affect CH₄ oxidation potential of the 0-10 cm soil layer in high and low tidal zones. Soil total carbon content, total nitrogen content, and C∶N ratio was the main soil factors affecting CH₄ production potential.

Key words: methane production, methane oxidation, nitrogen enrichment, tidal marsh, Minjiang River estuary