Tidal marshes have been shown to be important recycling surfaces for biogenic silica (BSi) and macro- and microelements at the land-sea interface, and also as key places to probe into the decomposition process of wetland plant litters. In this study, in situ decomposition experiments were conducted with Phragmites australis, Cyperus malaccensis, and Spartina alterniflora in the Min River estuary wetland. Litterbags of 0.2 mm mesh size were used to evaluate the litter decomposition process and litter residual amounts of BSi and macro- and microelements, including carbon (C), nitrogen (N), Chromium (Cr), Cuprum (Cu), Cadmium (Cd), Zinc (Zn), Plumbum (Pb), Aluminum (Al), Manganese (Mn) and Ferrum (Fe), for a period of 520 days. The litter decomposition rate significantly differed among species in the following order: C. malaccensis (0.005 d-1) > S. alterniflora (0.004 d-1) > P. australis (0.003 d-1) with a BSi release rate of 98.64 %, 97.23 %, and 96.75 %, respectively. Although there were net release of BSi, C, and N from the three litters, continuous decrease of BSi/(C, N) ratio indicated that BSi removed from the litters much faster than C and N. Accumulation index results showed that Cu, Pb, Al, and Fe were net accumulated of in the litters while Cd, Mn, Cr, and Zn were predominantly released during the litter decay. Pearson correlation analysis results showed that the amounts of N, Cu, Cd, Pb, Al, and Fe in the litter presumably restrained the release of BSi from the litter with significant adverse correlation. These findings in the Min River estuary have important implications for geochemical cycles within the wetland system and for transport processes of potential nutrients out of the system.
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Litter decomposition is one of the important ecosystem processes that modulated elemental biogeochemistry in estuarine wetland. Here we conducted in situ decomposition experiments with Phragmites australis, Cyperus malaccensis and Spartina alterniflora in the Min River estuary in southeast China. We used litter bags technique to evaluate the litter decomposition process and litters residue quality of BSi and macro- and microelements, including C, N, Cr, Cu, Cd, Zn, Pb, Al, Mn and Fe, during 520 days. To extract BSi, 30 mg of litter powder was digested for 5 h with Na2CO3 (0.1 mol/L) at 85 °C (DeMaster, 1981; Struyf et al., 2005). Dissolved BSi in the extractions was measured by the Molybdate blue spectrophotometric method (Saccone et al., 2006). Contents of total organic carbon and total nitrogen in the litters were measured using a Vario EL Ⅲ Element Analyzer (Elementar Scientific Instruments, Langenselbold, Germany). 0.04 g ground sample was digested with 2 ml HNO3 (70%) and 2 ml H2O2 (30%) at 180 ℃ for 10 h in closed 60-mL Teflon bottles (Savillex, Minnetonka, MN, USA). Digested samples were diluted with 1 %HNO3 and finally, concentrations of heavy metals (Cr, Cu, Cd, Zn, Pb, Al, Mn, and Fe) in all samples were determined by inductively coupled plasma mass spectrometry (ICP-MS, XSeriesII, Thermo Company, USA) analysis. Quality assurance and quality control were assessed using duplicates (three replications), method blanks and certified reference materials (GBW10020) from the National Research Center for Standards in China with each batch of samples (two blank and one standard for each 30 samples). Temperature and conductivity of the topsoil (0-10 cm in depth) was measured by a portable instrument (Spectrum Technologies Inc., Chicago, IL, USA) while pH of the topsoil was determined by a portable pH meter (HACH-sensION3, Loveland, CO, USA).