Studies On Dynamics Of Heavy Metal With Decomposition Of Litter Fall In Mangrove Wetland At Jiulongjiang River Estuary

This paper mainly studied the dynamics of decomposition and heavy metals in the litter leaves of Kandelia candel and Avicennia marina mangrove when placed on the wetland surface and buried in the sediments.The study sites were located in southwestern mangrove wetlands of Jiulong River Estuary Mangrove Nature Reserve on Haimen Island,Fujian.The litter leaves' decomposition experiments that had been divided into two seasonal periods of winter and summer were conducted from December 2007 to July 2008.The results indicated that:1.Decomposition rate and half-decomposition period.The decomposition of mangrove litter leaves in Jiulong River Estuary appeared as: Kandelia candel had a faster decomposition rate than Avicennia marina," The decomposition of Kandelia candel went faster in summer than in winter.The half-decomposition period of Kandelia candel litter leaves in winter was 53 days when on the surface and 30 days when buried in the sediments.And respectively the period was 28 days and 26 days in summer.When it came to Avicennia marina litter leaves in winter,the half-decomposition period was found as 63 days and 134 days respectively.In winter time both Kandelia candel and Avicennia marina had a comparatively fast stage and a slow stage as well.The turning point of the fast and slow decomposition of Kandelia candel mangrove both on the surface and in the sediments came around on the 28th day,while the turning point of Avicennia marina litter leaves decomposition on the wetland surface was around 42 days and about 21 days in the sediments.All the turning points of those decomposition appeared as the weight loss rate reached 50%or so,with the exception that the weight ratio was about 35.5%at the two decomposition turning points in the Avicennia marina mangrove sediments.2.The contents and release of organic carbon.The contents of organic carbon during the decomposition of Kandelia candel and Avicennia marina mangrove litter leaves at Jiulong River Estuary in winter decreased remarkably along with the decomposition time,among which the decrease rate of the surface group samples was significantly faster than that of the sediment group(Kandelia candel <0.01%,Avicennia marina >0.05%).The variation of organic carbon,however,was not distinct in the decomposing process of Kandelia candel mangrove litter leaves in summer.The decomposition of mangrove litter fall exerted a positive effect on the contents of organic carbon.But in general the contents of surface organic carbon were relatively stable.3.The contents and release of soluble sugar.The contents of soluble sugar decreased fast in the early stage of the decomposition of Kandelia candel and Avicennia marina mangrove litter leaves both in winter and summer.Seven days after the decomposition of Kandelia candel mangrove litter leaves in winter,the soluble sugar contents dropped by 49.09%on the wetland surface and 41.04%in the sediments,while the corresponding falling ranges of the soluble sugar in summer were 67.26%and 54.45%.Additionally the ranges of Avicennia marina mangrove in winter were respectively 71.33%and 61.51%.The release rate of soluble sugar was in order of "Avicennia marina group in winter">" Kandelia candel in summer">" Kandelia candel in winter".The decrease rate of soluble sugar contents in the decomposition of mangrove litter fall on the wetland surface was faster than that in the sediments,and this was considered due to the phenomenon that the leaching and loss of soluble sugar was possibly related much to the effect of repeated tide flushing.4.pH of mangrove litter fall and sediments.Kandelia candel and Avicennia marina mangrove litter leaves in Jiulong River Estuary were acid.The Kandelia candel mangrove litter leaves had a stronger acidity than Avicennia marina mangrove litter leaves,and a slightly stronger acidity in winter than in summer.The pH values during the decomposition of the two kinds of mangrove litter leaves increased distinctly in winter both on the surface and in the sediments,moreover these trends were more obvious on the surface litter fall sampling group with a big difference(P<0.05).In general,the dynamic variation of pH during the decomposition of Kandelia candel mangrove litter fall in winter could be divided into three stages:slow-fast-slow;simultaneously four stages for Avicennia marina mangrove litter fall in winter:fast- slow-fast-slow.Litter fall had some certain effects on the surface sediment below.5.Decomposition of litter fall,adsorption and transportation of heavy metal elements.The two mangrove litter leaves could have adsorption and transportation of different degree on 9 heavy metals,except for the effect of Kandelia candel mangrove litter fall on Mn,among which the degrees were Pb >Fe> Cd > Cr> Ag> Ni >Cu>Zn on the sediment surface and Pb > Cd > Cr >Fe > Ag> Ni >Cu in the sediments. Simultaneously,the orders of the degrees of Kandelia candel in summer were Pb >Fe> Ni >Cd > Cr> Ag> Zn >Cu and Pb >Fe> Ni > Cr> Ag> Zn >Cu,while the orders of the degrees of Avicennia marina in winter were Pb >Fe> Mn> Ni > Cr> Ag> Cd > Cu>Zn and Pb >Fe> Cr> Ni >Mn>Cu.The maximum accumulation multiple of Avicennia marina litter fall on Ni,Cu,Fe and Mn was higher than that of Kandelia candel mangrove litter fall.On the other hand,the maximum accumulation multiple of Kandelia candel litter fall on Cr,Ag,Cd and Pb was higher than that of Avicennia marina mangrove litter fall.Moreover,the accumulation multiple of these two mangrove species to Zn was almost the same.6.Studies on the relativity between the variation of heavy metal contents and the changes of organic carbon and pH during the decomposition of litter leaves showed the following three combinations were significantly related to each other:the variation of contents of Ni,Cu,Fe,Pb and organic carbon during the decomposition of litter fall,the changes ofpH and the content variation ofMn,Ni,Cu,Fe,Pb,the content variation of Fe,organic carbon,pH and Ni,Cu,Cd,Pb.From above it had been indicated that the changes of organic carbon and pH were positively correlated to the adsorption and accumulation of heavy metal contents during the decomposition of litter leaves.Besides,it had been demonstrated that there might been coupling and co-deposition effects between Fe and Ni,Cu,Pb.7.The storage of residues and its organic carbon in mangrove wetland. The present storage of residues in mangrove wetland at Jiulong River Estuary in winter and summer were 55.42 and 61.36 g/m~2 respectively,among which the content of litter leaves in winter was 34.03 g/m~2 accounting for 61.40%of all the mangrove litter fall,and the content of litter leaves in summer was 38.71 g/m~2,accounting for 63.09%.The storage of organic carbon in winter and summer was 24.63g/m~2 and 29.50 g/m~2,out of which the remained leaves took the greatest proportion accounting for 63.05%and 66.14%respectively in winter and summer.8.The storage of metal elements in mangrove residues.The order of the six certain heavy metal in mangrove residues at Jiulong River Estuary was as follows:Mn >Fe> Zn> Ni >Cu > Cr in winter;Mn >Fe> Zn> Cr >Cu >Ni in summer.And the storage of Cr,Fe,Cu and Zn in summer is lager than that in winter,while for Mn and Ni the situation is reverse.