The role of land-use change and restoration on nitrogen processing in tropical coastal wetlands of Vietnam

Coastal wetlands, including mangrove and melaleuca forests, are globally important due to their capacity in sequestering carbon and nitrogen, and intercepting nutrient pollution from vast, nutrient-rich, tropical riverine networks. Despite this, the environmental drivers controlling soil biogeochemistry in these ecosystems remain poorly understood. Here we conducted a study across gradients of restoration and land-use in the mangrove forest of Xuan Thuy National Park in the Red River Delta, northern Vietnam and the melaleuca forest of U Minh Thuong National Park in the Mekong River Delta, southern Vietnam. We investigated nitrogen transformation processes and greenhouse gas production in mangrove and melaleuca forest soils using a ¹⁵N-Gas flux method to determine rates of denitrification, and its relative contribution to soil N₂O emissions. We found that denitrification was a more dominant source of N₂O in the melaleuca soils, despite higher rates of denitrification in the mangrove soils resulting from more complete denitrification in the mangroves. N₂O and CO₂ emissions were significantly higher from the melaleuca soils. Disturbance and subsequent recovery or restoration of these forests did not have a significant effect on soil biogeochemistry. The mangrove system, therefore, may remove excess nitrogen and improve water quality while maintaining low emissions of greenhouse gases whereas melaleucas process nutrients at a cost of N₂O and CO₂ emissions. Melaleucas, however, may act as a significant CH₄ sink at least partially balancing these emissions.