To investigate the effects of spatial expansion between native invasive species (Phragmites australis) and commom native species (Cyperus malaccensis) on variations of micro-elements (Pb, Cr, Cu, Zn, Ni, Cd and As) in decomposing litters in the Min River estuary, in situ filed decomposition experiment was conducted in P. australis (PA) community (before expansion, BE), C. malaccensis (CM) community (before expansion, BE) and P. australis-C. malaccensis (PA'-CM') community (during expansion, DE) from February 2016 to February 2017 by space-for- time substitution method. Results showed that the decomposition of C. malaccensis were faster than those of P. australis whether at BE stage or at DE stage. The decomposition rate of PA' increased by 24.40% compared to PA whereas the value of CM' decreased by 15.67% compared with CM. The concentrations of Pb, Cu, Zn, Ni, Cd and As in decomposing litters of P. australis (PA and PA') and C. malaccensis (CM and CM') generally showed increasing tendency and the values in the former were significantly lower than those in the latter (p < 0.05). The physicochemical sorption onto recalcitrant organic fractions and the substrate quality (C/N and M/C ratios) of decomposing litters were two important factors affecting the differences in As/metals variations between species. The levels of Cr in decaying litters increased initially and decreased afterward, and the values in P. australis were significantly higher than those in C. malaccensis (p < 0.05). Whether at BE stage or at DE stage, stocks of As/metals in decomposing litters of P. australis (PA and PA') were generally higher than those of C. malaccensis (CM and CM'). The lower stocks of As/metals in CM or CM' might be more dependent on its lower mass remaining. Compared with PA at BE stage, the accumulation of As/metals in decomposing litters of PA' at DE stage decreased greatly, which might be ascribed to the lower precipitation of metal sulfides in PA'. Stocks of Zn, Ni, Cd and Cr in CM' and stocks of Cr in PA' generally evidenced the export of metals from decomposing litter to environment, indicating that the potential exposure risk of Zn, Ni, Cd and Cr might be increased as CM was invading by PA. This study found that the spatial expansion between P. australis and C. malaccensis not only altered the stocks of As/metals in decomposing litters but also increased the exposure risk of Zn, Ni, Cd and Cr in ecotone. In future, as the ecological functions of ecotone was precisely evaluated during the expansion of the two plants in the Min River estuary, the alterations of litter decomposition rates and the exposure risks of Zn, Ni and Cd caused by CM' should be emphasized.
Keywords: Arsenic; Decomposition; Heavy metals; Min river estuary; Spatial expansion.
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