NITROUS OXIDE EMISSION FROM SEDIMENT OF A TROPICAL MANGROVE FOREST IN CENTRAL VIETNAM-IMPLICATIONS OF SHORT-TERM LABORATORY EXPERIMENTS

This study investigated nitrous oxide (N2O) emission and the environmental parameters responsible for its generation in Ru Cha mangrove sediment (Thua Thien Hue, Vietnam). To determine cumulative amounts of N2O emission from different sediment layers, closed incubation experiments were conducted at 25ºC for 7 days in 100 mL glass bottles that contained 5 g of sectioned sediments (0‒5, 5–10 and 10‒15 cm depths) and 25 mL of the ambient water. Cumulative amounts of N2O emission decreased with sediment depth, which coincided with decreases in the density of denitrifiers, and contents of total organic carbon, total nitrogen and ammonium from upper to lower sediment layers. A similar incubation experiment was conducted using the 0–5 cm sediment layer to investigate the effect of different nitrate (NO3-) concentrations (0, 1, 3 and 5 mg N L-1) on potential N2O emission. Nitrous oxide emission increased with NO3- addition, indicating that N2O emission was limited by NO3-. Nitrate was completely removed from the solution in all NO3- treatments by denitrification. Nitrous oxide emission accounted for 20%, 55% and 100% of the amounts of NO3- removed in 1, 3 and 5 mg N L-1 treatments, respectively. Our laboratory experiments suggest that the Ru Cha mangrove forest could be a hotspot of N2O emissions when sediment receives high NO3- inputs.