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

Abstract

<p>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 <sup>15</sup>N-Gas flux method to determine rates of denitrification, and its relative contribution to soil N<sub>2</sub>O emissions. We found that denitrification was a more dominant source of N<sub>2</sub>O in the melaleuca soils, despite higher rates of denitrification in the mangrove soils resulting from more complete denitrification in the mangroves. N<sub>2</sub>O and CO<sub>2</sub> 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 N<sub>2</sub>O and CO<sub>2</sub> emissions. Melaleucas, however, may act as a significant CH<sub>4</sub> sink at least partially balancing these emissions.</p>