Sulfur Biological Cycling Of Phragmites Australis And Cyperus Malaccensis Marshes In The Process Of Their Spatial Expansion In The Min River Estuary

Estuarine marsh located in the special zone of interaction between river and ocean,which is one of the most sensitive ecosystems to global climate change and human activities.Sulfur（S）is the fourth important nutrient ranking nitrogen,phosphorus and kalium.Due to its multiple valence states,the biogeochemical cycle of S in estuarine marsh is more complex and plays an important role in affecting global environmental changes.Spatial expansion refers to the competition between two or more organisms/species due to insufficient environmental resources or space,which has a profound impact on the composition,structure and function of plant communities.At present,studies on marsh plant competition mainly focused on the factors affecting the competitiveness and the relationships between soil nutrient levels and competitiveness.However,few studies were conducted to investigate the spatial and temporal distributions of S in marsh plant-soil（water）system,the accumulation and distributions of S in plants,and the releasing of S nutrient from decomposing litters during the spatial expansion of different plant communities.In this paper,the Phragmites australis marsh（PA）,Cyperus malaccensis marsh（CM）and ecotonal marsh（P.australis C.malaccensis marsh（PA′CM′）,formed by the spatial expansion between P.australis and C.malaccensis）in Shanyutan of the Min River estuary were studied.The temporal and spatial distributions of total sulfur（TS）and inorganic sulfur fractions（H₂O-S,Adsorbed-S,HCl-Soluble-S and HCl-Volatile-S）in marsh soils during the spatial expansion of P.australis and C.malaccensis were investigated.Meanwhile,the ecological traits of marsh plants,the characteristics of S accumulation and allocation in plants,and the releasing characteristics of S nutrient from decomposing litters were discussed.Finally,the S biological cycling compartment model of marsh plant-soil（water）system during the spatial expansion of P.australis and C.malaccensis was established.Based on the evaluation of S cycling status,the mechanism of S competition of P.australis and C.malaccensis was illustrated during their spatial expansion.Results were as follows:ⅰ)The contents of TS in soils of ecotonal marsh and C.malaccensis marsh were significantly higher than those of P.australis marsh.The great variations of soil grain composition,pH,organic matter,soluble salts and soil moisture during the spatial expansion of P.australis and C.malaccensis could better explain the increased S stocks in soils of ecotonal marsh.ⅱ)The spatial expansion of P.australis and C.malaccensis significantly increased the contents of total inorganic sulfur（TIS）in soils,and the values in soils（0-60 cm）of the three marsh were characterized by spring>winter>autumn>summer.The contents of H₂O-S and Adsorbed-S in ecotonal marsh soils were much higher than those of pure community marsh,and the values were generally higher in winter and spring compared to summer and autumn.The contents of HCl-Soluble-S in soils of different marshes in winter,summer and autumn generally followed the sequence of PA′-CM′>PA>CM,but,in spring,the levels were characterized by PA>PA′-CM′>CM.Although the contents of HCl-Volatile-S in soils of different marshes were in the order of CM>PA>PA′-CM′,the values were generally higher in autumn and winter compared with spring and summer.ⅲ)The spatial expansion between P.australis and C.malaccensis significantly influenced the distributions of SO₄^2-concentration in porewater of marsh soils.The SO₄^2-concentration in soils of C.malaccensis marsh was much higher than those of P.australis marsh and ecotonal marsh.Due to the effects of temperature,precipitation as well as plant absorption and utilization,the concentrations of SO₄^2-in porewater of different marsh soils were the highest in winter,followed by autumn,and lower in summer and spring.ⅳ)The ecological traits of P.australis and C.malaccensis were obviously altered due to the competition of the two plants.The contents of TS,the accumulation amounts of S and the accumulation coefficient of S in aboveground organs of P.australis and C.malaccensis were generally higher than those in ecotonal marsh.As competition was produced in the process of spatial extension between P.australis and C.malaccensis,P.australis could maintain its competitiveness by enhancing the ability of S absorption and utilization through root to expand belowground space.In contrast,C.malaccensis could expand its aboveground space by increasing the contents of S and the accumulation of S in aboveground organs involved in chlorophyll synthesis to resist the competition of P.australis.ⅴ)The decomposition of C.malaccensis litter was faster than those of P.australis litter whether at the stage of before expansion（BE）or at the stage of during expansion（DE）.Compared with BE stage,the decomposition rate of P.australis significantly increased at DE stage,whereas that of C.malaccensis decreased greatly.Salinity and substrate quality were the key factors inducing the differences of decomposition rate between P.australis and C.malaccensis.Although the contents of TS in different litters fluctuated greatly,the values generally increased during decomposition.Except for the litters of P.australis in pure community,the other litters in pure community or ecotonal community showed S releasing with different degrees.C/S ratio might be the key factor regulating the variations of TS content in litters during decomposition.C.malaccensis might maintain its competitive advantage in the process of spatial extension through faster residue decomposition and S nutrient returning.This paper found that the stock of S in marsh soil（water）was significantly increased in the process of spatial expansion between P.australis and C.malaccensis,and P.australis could keep its competitive advantage by enhancing the ability of absorption and utilization for S nutrients.In addition,P.australis could also maintain the S supply capacity of root by reducing its S cycle coefficient.This paper also found that the spatial expansion of the two plants increased the contents of H₂O-S、Adsorbed-S、HCl-Soluble-S and TIS in soils but decreased the levels of HCl-Volatile-S,indicating that the spatial expansion of the two plants not only improved the supply capacity of available S in soils but also reduced the adverse effects of volatile sulfide on the growth of the two plants.Although the S nutrient supply capacity of P.australis was obviously increased during spatial expansion,but the circulation coefficient of C.malaccensis was much higher,which could ensure its competitive advantage in the process of spatial expansion through faster residue decomposition and nutrient returning.Therefore,although the two plants had different competition and ecological adaptation strategies during spatial extension,in the long run,P.australis showed relative competitive advantage,which would continue to invade C.malaccensis marsh.