Mangrove reforestation in Vietnam: the effect of sediment physicochemical properties on nutrient cycling

Background

Mangrove forests depend on unique physicochemical properties found in tidal coastal sites. One of the main reasons why mangrove reforestation fails is due to changes in the sediment properties that occurred under deforestation conditions. The iron sulphides common in mangrove sediments are oxidated to sulphuric acid, drastically lowering the sediment pH. Other changes also take place as nutrients leach. This study assesses the effects of pH and redox potential on phosphorus and nitrogen cycling in reforested mangrove stands in southern Vietnam.

Research goals & methods

Mangrove forests in the Saigon River Delta had been destroyed by aircraft-sprayed herbicides during the Vietnam War and were partially regenerated by plantations between 1978-1986. Sampling was conducted along five transects in replanted stands in 2005.

Conclusions & takeaways

Spatial gradients of sediment Eh and pH were affected by the tidal regime and pyrite oxidation. Sediment Al/Fe-P correlated with Eh or pH, depending on the sediment layer, whereas Ca-P, available P and leaf P were influenced by the pH. The highest concentrations of Al/Fe-P were recorded at pH 6.5 probably due to adsorption effects. Sediment Ca-P increased strongly at pH below 4 and above 6. Since foliar and sediment N:P ratios were influenced mainly by pH, shifts in pH likely cause limitation transitions. Overall, sediment pH rather than Eh was found to control the nutrient status.