INFLUENCE OF SUBSTRATE PHYSICOCHEMICALPROPERTIES AND ORGANIC MATTERINTERACTIONS ON PHOSPHORUS DYNAMICSIN REPLANTED MANGROVE FORESTS,SOUTHERN VIETNAM

Agent Orange spraying during the Second Indochina War caused massive mangrove destruction in the Saigon River Delta. This work describes the changes in nutrient cycling and availability caused by sediment acidification through destruction-induced pyrite oxidation. The aim was to determine the nutritional status, including phosphorus (P) and nitrogen dynamics, of reforested mangrove stands in order to assess the effect of sediment physicochemical properties on nutrient limitations. The low productivity of the so-called "sulphate acid soils" can be mainly attributed to P deficiency. The first chemical method for quantification of individual mineral and sorbed P species was developed (CONVEX). Subsequently, common sedimentary P fractionations of sediments sampled within the delta were interpreted using individual P species values of selected sediments analysed by the CONVEX method. The effect of physicochemical parameters on the distribution of P species, P uptake and plant growth is discussed using these analytical results. In summary, sediment pH was found to be most relevant for direct control of nutrient cycling and plant growth in reforested mangrove stands, rather than Eh.