Nutrient dynamics of the litters during standing and sediment surface decay in the Min River estuarine marsh

Abstract

The above-ground litter component in wetland ecosystems is an important carbon and nutrient store, and the rate of its decomposition influences both wetland productivity and those processes that contribute to biogeochemical cycling. This paper examined the decomposition and nutrient dynamics of the leaf and flower litter produced by two contrasting emergent macrophytes in the Min River estuary of southeast China; Spartina alterniflora is an invasive species and Phragmites australis a native one. The results demonstrated that: (1) Decomposition of standing litter within the vegetation was an important stage in the decomposition process with a loss of dry mass between 13%-15% in P. australis and 21%-32% in S. alterniflora. At day 210, the loss rate of dry mass in P. australis and S. alterniflora was accounting about 64%-67% and 59%-66%, respectively. There was no significant difference in litter mass loss between S. alterniflora and P. australis. (2) In the first 90 days of decomposition, carbon (C) concentration of P. australis litter reduced in a fluctuating pattern whereas S. alterniflora was relatively stable. However, in the later decay at the sediment surface (after 180 days), the C concentration in both species increased. At day 210, the C concentration of the flower litter in S. alterniflora and P. australis had regained their initial values and leaf litter concentrations were 107% and 106% respectively. The variation in litter nitrogen (N) concentrations of the two species showed the same pattern with a slow decrease during the standing stage, and an increase during the days of decay at the sediment surface. At day 210, the N concentration of the flower litter in S. alterniflora and P. australis was 125% and 254% of their initial values and leaf litter was 191% and 185%, respectively. However, average phosphorus (P) concentration varied distinctly between the two species: there was a quick decrease in P concentration for P. australis in the first 15 days of decomposition to 14% and 12% for flowers and leaves, whereas S. alterniflora was relatively stable in the first 90 days of decomposition. The P concentration in the two plants declined first and then rose in the sediment surface decay. The P concentration of leaf litter in S. alterniflora was significantly greater than that of P. australis during decomposition, on the contrary, the flower litter of S. alterniflora was significantly lower than that of P. australis. (3) The element accumulation index (NAI) values of C and N of the two plants were less than 100%, which indicated a release of C and N in both species. NAI values of P in P. australis decreased first, and then increased both in standing and sediment surface decay. There was no distinct fluctuation of S. alterniflora during standing decomposition, whereas there was a drop in the days of decay at the sediment surface. (4) The litter C concentration of S. alterniflora was significantly greater than that of P. australis, but the N concentration of flower litter in S. alterniflora was significantly lower than that of P. australis. Where the invasive S. alterniflora was present in the Min River estuary the carbon pool of leaf and flower litter was greater compared to where P. australis was present but the nitrogen pool was lower, and the phosphorus pool showed no obvious difference.