Vegetation influence on nutrients distribution in pore water of salt marsh sediment

The variations of nutrients in pore water of salt marsh sediment were surveyed in the middle intertidal zone of Chongming Dongtan during August 2007 to May 2008 to identify plant impact on nutrients distribution. The results show that NH₄⁺-N and PO₄^3--P concentrations are lower in pore water of Spartina alterniflora and Phragmites australis zones than in bare flat, and specially, NH₄⁺-N concentrations in summer and autumn decrease by one more orders of magnitude. Compared to winter, nutrients concentrations are obviously higher during the period of plant growth, and plant biomass is clearly correlative to nitrogen and phosphorus. Vegetation growth influences nitrogen content intensively. NH₄⁺-N concentrations in Spartina alterniflora and Phragmites australis zones are 44.21 and 74.38μmol·L^-1 respectively, distinctly lower than that in bare flat and Scirpus mariquete zone (340.14 and 291.87μmol·L^-1 respectively). Moreover, NO_x^--N concentration is one to two order (s) of magnitude lower than NH₄⁺-N, and its highest value exists in Phragmites australis zone (5.94μmol·L^-1). The results of molecule diffusive flux of nutrients in the surface sediment-overlying water interface indicate that marsh sediment is the source for SiO₃^2--Si, NH₄⁺-N and PO₄^3--P, and the rank for NO_x^--N (NO₃^--N + NO₂^--N), and NO_x^--N flux from overlying water to sediment [16.23μmol· (m²·h)^-1] is higher than NH₄⁺-N flux from sediment to overlying water [15.53μmol·(m²·h)^-1]. Vegetation growth accommodates nutrient structure of the estuarine ecosystem by affecting sediment-water interface mass flux and nutrient ratios in pore water and overlying water.