Nutritional state and organic matter diagenesis in an ecotone of halophyte and planted mangrove in Bac Lieu province, Vietnam

Abstract

Mangroves play many significant roles in human life and socio-economical activities. However, their coverage has seriously declined worldwide due to various reasons while understanding of the roles and functions of this ecosystem is still insufficient. With a vast area of mangroves along the coast line, several studies on this ecosystem have been done in Viet Nam. Notwithstanding, the knowledge of nutrient and organic matter dynamics in the mangroves of Viet Nam, in general, and in the Mekong Delta, in particular, is still a gap. This study was conducted to understand the nutrient dynamics in a mangrove replanted in an abandoned salt-pan in Ganh Hao, Bac Lieu province a coastal area in the South of Viet Nam. Sediments and fresh leaves were collected from 8 different landscapes along a transect which was ca. 700 m in length. Sampling was conducted in the dry and rainy season in 2009. The nutrient contents (NH4 , NO2-, NO3- and available P for plant uptake), total organic carbon and total nitrogen were determined to assess the nutritional state in the study area. The composition and concentration of the amino acids in the sediments were quantified to understand the organic matter diagenesis in the area. Glucosamine, galactosamine and amino acids in the fresh leaves were analyzed to find the source of the organic matter. The chitin content in the sediments was determined by 2 methods to calculate the contribution of chitin to the N pool. Chitin was directly quantified through the binding of N-acetylglucosamine and WGA-FITC. On the other hand, chitin was calculated from the concentration of glucosamine in the sediments. The study area was subject to a serious deficiency of nutrients, especially nitrogen. The deficiency of nitrogen resulted in the deficiency of phosphorus in the sediments. The nitrogen and phosphorus contents in the sediments were controlled by the physico-chemical properties of the sediments and the vegetation. The carbon-to-nitrogen ratios showed that the organic matter in the deep sediments (30-35 cm) mostly derived from marine sources. The organic matter derived from terrestrial plants was found mostly in the surface sediments. The forestation and the invasion of Sesuvium portulacastrum, therefore, have recently contributed to the pool of organic matter in the sediments. The organic matter content in the study area was lower compared to other coastal areas, probably due to the higher turnover rate in the sediments, which resulted from the high temperature and the aeration in the sediment. The composition and contents of the sedimentary amino acids were affected by the composition and contents of the amino acids in leaves. In general, the contents of sedimentary amino acids decreased with depth. However, the soil preparation for mangrove plantation resulted in a disturbance in the variation trend with depth in the amino acid contents under the planted mangrove. The chitin content directly quantified through the binding between N-acetylglucosamine and WGA-FITC exceeded the organic carbon content in the sediments. However, these chitin data revealed an ecological relationship between chitin and the diatom frustules. The content of chitin calculated from glucosamine concentration showed that chitin contributes less than 2% to the OC pool and less than 3% to the N pool. In the deep sediments (30-35 cm), the crustacean sheaths was the major source of chitin while in the shallow sediments, chitin mostly derived from diatoms. In conclusion, this study showed that the nutritional state and the organic matter dynamics in a mangrove planed in extreme conditions was driven by tides, vegetation and the physico-chemical properties of the sediments. These effects might relate to the activities of sediment bacteria functioning in the nitrogen and phosphorus mineralization and solubilization. Improving the sediment conditions by irrigational solutions will help to maximize the bacterial activities and enhance the efficiency of mangrove plantation in abandoned salt-pans.