Impacts Of Siltation By Spartina Alterniflora Invasion On Litter Decomposition And Nutrient Dynamic Of Itself And Cyperus Malaccensis In Tidal Marsh Of The Min River Estuary

Litter decomposition is a crucial link of nutrient cycling and energy flow in marsh ecosystems.It is one of the most important processes in maintaining ecological functions by connecting synthesis and decomposition of organisms.Decomposition rate of litter not only greatly affects the standing crop of residue and the returning of nutrients(e.g,carbon?C?,nitrogen?N?,phosphorus?P?and sulfur?S?and other substances to sediment,but also directly influences the material cycles in marsh ecosystem.The estuarine marsh is an very important ecosystem formed by the interaction between estuary and ocean.The decomposition of residue in estuarine marsh is generally affected by many factors(e.g,moisture,tide,salinity,microorganism and sedimentation?siltation?,and,among them,sedimentation plays a very important role in affecting litters decomposition.Particularly,the siltation caused by plant interception and the variation of nutrient conditions in decomposing environment might be more important during decomposition.Considerable efforts have been conducted in investigating the siltation rates and siltation mechanism of Spartina?especially Spartina alterniflora?,while information on the effects of siltation on litters decomposition and nutrient releasing was still very limited.In this paper,in situ filed decomposition experiment was conducted in S.alterniflora?SA?marsh,Cyperus malaccensis?CM?marsh and S.alterniflora-C.malaccensis?SA'-CM'?marsh in the Min River estuary by litters bag technique.Based on the monitoring data of siltation by S.alterniflora in the Min River estuary for two years(⁵cm·a^-1)and considering the related results of some previous studies(3.5⁶.5 cm·a^-1),the siltation intensity of SA in this experiment was set as 5 cm·a^-1.The in situ field siltation simulation experiment laid three one-off siltation treatments[no siltation treatment(S₀,0cm·a^-1),current siltation treatment(S₅,5 cm·a^-1)and strong siltation treatment(S₁₀,10cm·a^-1)]to investigate the effects of siltation alteration of SA on decomposition and nutrient releasing of SA and CM litter.The in situ field nutrient alteration experiment also laid three one-off siltation treatments[no siltation treatment(S₀,0 cm·a^-1),current siltation treatment(S₅,5 cm·a^-1),current siltation treatment+nitrogen treatment(S_5N,5N cm·a^-1)]to explore the effects of nutrient variation caused by SA siltation on decomposition and nutrient releasing of SA and CM litter.The main results were as follows:i)The decomposition rates of SA and CM litter significantly decreased with increasing siltation depth and the decrease amplitude was more evident during the decomposition of CM litter.The reason was that the application of sediment in different siltation treatments inhibited litters decomposition rates via many direct and indirect mechanisms,such as,alteration of ambient water chemistry,compaction of detritus,reduction of gas and nutrient exchange between litters and surroundings.At same siltation depth,the decomposition rate of SA litter was significantly lower than that of CM litters?P<0.05?,which could be ascribed to their initial substrate quality?e.g,C/N?C/S and C/P?and the effects of nutrient alteration on substrate quality during decomposition.ii)With increasing siltation depth,the concentrations of TC,TN,TP and TS in SA and CM litter generally increased during decomposition and the effect of siltation on nutrients alteration in CM litter was more obvious.In different siltation treatments,the net releasing of C,N,P and S in SA and CM litter were observed during decomposition.The nutrient releasing amounts from different litters generally declined with increasing siltation depth.At the same siltation depth,the nutrient releasing amounts from CM litter were greater than those from SA litter,which,not only rested with the difficult degree of nutrients obtained from decomposition surroundings and the influence of microbes on nutrient releasing during decomposition,but also depended on the substrate qualities?e.g,C/N,C/S and C/P?of different litters and the initial concentrations of four elements in decomposing detritus.iii)Although the nutrient variation caused by siltation decreased the decomposition rates of CM and SA litter,no significant difference was observed between them?P>0.05?.In addition,at same nutrient condition,the decomposition rate of SA litter was significantly lower than that of CM?P<0.05?.Under the conditions of siltation or nutrient alteration,the contents of TC in different litters generally increased during decomposition,which was mainly related to litters mass remaining and the loss of non-carbon substances.By comparison,the contents of TN,TP and TS in decomposing litters generally decreased.The alteration of nutrient caused by siltation showed no significant effect on nutrient variations in decomposing litters?P>0.05?and there were two possible reasons.For one thins,it might be related to the significant influence of siltation intensity on variations of nutrients in decomposing litters,which,to a great extent,covered the effect of nutrient change.For another,it might be due to the physical leaching of nutrients at the early stage of litters decomposition.At this stage,the nutrients in residues were more likely to be leached.iv)Under the conditions of nutrient alteration caused by siltation,although the C in decomposing litters of SA and CM showed releasing,it inhibited C release as a whole which was closely related to the litters mass remaining at this stage.In contrast,the N,P and S in the two residues?except for the stem of SA?also showed releasing,but the alteration of nutrient caused by siltation promoted the releasing of three nutrients and this was related to the releasing of nutrients from the residue by leaching at the early stage of decomposition.The N in stems of SA showed accumulation during decomposition and the degree of accumulation generally decreased as the nutrient condition was greatly altered,which was closely correlated with its lower initial N content.The effect of nutrient alteration on accumulation or releasing of elements in same residue was not significant?P>0.05?.Under same nutrient condition,the releasing amounts of elements from CM litters were significantly higher than those from of SA litters?P<0.05?.This paper found that,in the perspective of decomposition and nutrient returning,the decomposition rate and nutrient releasing of C.malaccensis were significantly higher than those of S.alterniflora under the conditions of same siltation or/and nutrient alteration,indicating that S.alterniflora was very likely to enhance its invasion advantage by absorbing and utilizing the S nutrient releasing from C.malaccensis at the initial invasion stage.