Effects Of Spatial Expansion Between Phragmites Australis And Cyperus Malaccensis On Distribution And Stock Of Phosphorus In Plant-soil Systems Of Marshes In The Min River Estuary

As an important source,sink and converter of phosphorus,estuarine wetland plays an important role in maintaining the stability and health of wetland.The Min River is the largest river that flows into the sea alone in Fujian Province and the Min river estuarine marsh is a special ecological type formed by the interaction between the Min R iver and the East China Sea.It also effected by many factors such as estuarine hydrodynamics,ocean tides,and vegetation succession,which may directly or indirectly effects the occurrence and transfer of different forms of phosphorus in marsh soil.Spatial expansion of plants also called competition is the competition between different species for environmental resources actually such as light,nutrients,water,and living space.The expansion of marsh plants can not only affect the ecological characteristics and nutrient absorption and utilization of plants,but also change the structure and succession process of the marsh community and affecting the stability of the marsh system.Currently,phosphorus occurrence in the plant-soil system under the influence of the spatial expansion of wetland vegetation remains scarce.In this paper,to investigate the distributions and stocks of phosphorus（TP）in plant-soil systems of marshes in Shanyutan of the Min River estuary and explore its influencing factors,the Phragmites australis（PA）marsh and Cyperus malaccensis（CM）marsh before spatial expansion and the ecotonal marsh（PA-CM,the P.australis and C.malaccensis in EM were denoted by PA’and CM’,respectively）during spatial expansion were studied.Based on field sampling and laboratory experiments,the seasonal variation characteristics of phosphorus accumulation,the TP of marsh soil and the content of different forms of phosphorus and distribution of marsh plants under the background of spatial expansion are clarified.What’s more,a marsh plant-soil system phosphorus seasonal change model was constructed to comprehensively evaluated the occurrence of phosphorus in plants-soil system under the influences of the spatial expansion of PA and CM.The main results are as follows:i)During the spatial expansion of P.australis and C.malaccensis,the results showed that the order of phosphorus content of different wetland soil forms in four seasons are HCl-Pi>Na OH-Pi>Residual-P>Na HCO₃-Pi>Na OH-Po>Sonic-Pi>Resin-P>Na HCO₃-Po.The contents of easily decomposed phosphorus（Resin-Pi,Na HCO₃-Pi and Na HCO₃-Po）,moderately decomposed phosphorus（Na OH-Pi,Na OH-Po,Sonic-Pi and Sonic-Po）,and hardly decomposed phosphorus（HCl-P and Residual-P）forms in soils of PA-CM significantly increased by 26.6%,24.6%and 13.6%compared with those in soils of PA,while increased by 9.8%,8.1%and 0.6%compared with those in soils of CM,respectively.ii)The spatial expansion of PA and CM changed the TP content in wetland soil as a whole.Compared with PA and CM,the levels of TP in soils of PA-CM increased by 19.6%and 4.3%in four seasons,respectively.It shows that the wetland soil in the interlaced zone has a strong phosphorus supply capacity.The main factors affecting the changes of phosphorus forms in wetland soil under the influence of the spatial expansion of PA and CM changed vary greatly in different seasons.With the change of seasons,the main controlling factors affecting the content of different forms of phosphorus in marsh soil are determined by plants and soil in winter and spring.While by soil in summer and autumn.iii)With the spatial expansion,the average TP content of different organs of the plant has rapidly changed in for seasons.Compared with the pure P.australis community（PA）before the spatial expansion,the P.australis in P.australis-C.malaccensis community（PA’）increases the TP content of standing litters and sheaths by 23.9%and15.1%respectively.,but decreased the TP content of stems,leaves and roots by 46.7%,26.2%,18.6%and 15.1%,respectively.However,compared with the pure C.malaccensis community（CM）,the C.malaccensis in P.australis-C.malaccensis community（CM’）increased the TP content of all organs（the standing litters,roots,stems and sheaths）.Except for the leaves,the TP content of different organs of the PA’was lower than that in CM’.This indicates that the phosphorus nutrient supply of PA’may be restricted.iv)Under the influence of the spatial expansion of PA and CM,the TP content of plant organs in different marshes was highest in spring,followed by autumn and summer,and lower in winter.Under the influence of the spatial expansion,the TP content of PA’decreased in autumn and winter,and increased in spring and summer.While the TP content of CM’decreased in spring,summer,autumn and winter.v)The spatial expansion of PA and CM changed the distribution of phosphorus in the wetland plant-soil system greatly in the four seasons.As far as the plant-soil system is concerned,the phosphorus storage of different wetland soils has an absolute advantage in the plant-soil system.Under the influence of the spatial expansion,the proportion of phosphorus reserves in the plant system of the two plants are reduced.Except for winter,the percentages of the decline of CM’in spring,summer and autumn are higher than those in PA’,which further shows that CM’is negative in competition.vi)The spatial expansion of PA and CM also changed the phosphorus uptake and utilization status of the wetland plant-soil system.The absorption coefficients of the PA’and CM’compared with PA and CM were decreased by38.5%and 59.2%,indicating that the absorption of phosphorus by the two is restricted in the EM（ecotonal marsh）,and the limitation of CM’is even higher.In addition,their absorption coefficients increased by72.9%（PA’）and 95.8%（CM’）.In this case,both of them maintain their own growth by improving their own use efficiency of limited phosphorus nutrients.What’s more,the higher utilization coefficient of PA’indicate that it’s have advantage in competition.The study found that,the spatial expansion of P.australis and C.malaccensis community in the Min River estuarine marsh significantly increased the content of phosphorus（easily decomposable phosphorus,medium easily decomposable phosphorus,and hardly phosphorus）and TP in various parts of the soil in the PA-CM shows it has a high phosphorus supply potential,which can provide sufficient phosphorus nutrient support for the two plants and has important ecological significance.During spatial expansion,the two plants might adopt different adaptation strategies for P absorption and utilization to maintain their competitiveness.The PA’are likely to accelerate the absorption of phosphorus in the roots and increase the utilization efficiency of phosphorus by reducing the R/S,R/L and S/L of the peak growth season（summer and autumn）,increasing the plant density and root-to shoot ratio to maintain their own competitive advantage.However,The CM’are likely to increase the proportion of root biomass in the peak growth season（summer and autumn）,increase R/S,R/L and S/L and plant height to obtain light for CM’and promote phosphorus nutrient transform to stem organ to maintain its own growth.Although the phosphorus occurrence status of the marsh soil in the interlaced zone helps to alleviate the pressure of competition for phosphorus in the process of spatial expansion between PA and CM,the PA will continue to invade the wetlands of CM’into the CM due to their relative competitive advantages,and expanding the range of marsh in the interlaced zone.