Contrasting nutrient stocks and litter decomposition in stands of native and invasive species in a sub-tropical estuarine marsh

We compared the influence of invasion by an alien invasive species (Spartina alterniflora, smooth cordgrass) and a native aggressive species (Phragmites australis, common reed) as they have expanded into the native Cyperus malaccensis (shichito matgrass)-dominated wetland ecosystem in the Min River estuary of southeast China. S. alterniflora is a perennial grass native to North America, which has spread rapidly along the southeast coast of China since its introduction in 1979. Our study compared the above and belowground biomass, net primary production, litter decomposition, plant nutrient stocks and soil organic carbon storage of the grasses in three ecosystems: (1) the native ecosystem dominated by C. malaccensis; (2) ecosystems previously dominated by C. malaccensis but presently replaced by P. australis; and (3) ecosystems previously dominated by C. malaccensis but presently replaced by S. alterniflora. Our results demonstrate that the recent invasion (3 years) of the exotic invasive species S. alterniflora has already significantly increased live aboveground biomass and aboveground plant nutrient stocks. However, there was no significant difference in these variables between native aggressive species P. australis and native C. malaccensis. The majority of belowground root Carbon (C), Nitrogen (N) and phosphorus (P) stocks of the three plant species were all distributed in the upper surface layer and there was a decrease with soil depth. There was little difference in litter decomposition rates among the three grass species; they were ranked in the following order: C. malaccensis>S. alterniflora>P. australis. Litter element concentration showed similar patterns for the three species. However, important differences were found between N and P; the litter N concentrations in each of the three species were greater at the end of the 280 days decomposition than at the start, but P concentrations followed a fluctuating pattern during the decomposition period. Soil organic carbon stocks (0-50 cm) under S. alterniflora, P. australis and C. malaccensis stands were statistically indistinguishable, which may be due to the invasion of S. alterniflora having been a relatively recent phenomenon. Thus, recent invasion of the exotic species S. alterniflora has already altered the nutrient cycle of C. malaccensis in the ecosystem in the Min River estuary.