Effects Of Land Use Change On Distribution Of Organic Carbon Fractions And Heavy Metals In Mangrove Sediments In Zhangjiangkou

The study aimed to determine the effects of land use change on distribution of organic carbon fractions and heavy metals in mangroves sediment.The surface soil from five land use types in Zhangjiangkou Estuary were selected as the research objects,include cropland,culture pond,Spartina alterniflora,mudflat and mangroves.The distribution of organic carbon and heavy metals in different size water stable aggregates were measured to privde theoretical guidance for increasing soil carbon burial and heavy metals absorption.The main conclusions are as follows:1.The physical and chemical properties of sediments changed significantly after land use change occurs in mangroves.After mangroves were reclaimed as cropland,environmental factors changed significantly except TOC,TN increased 38.61%,C/N value decreased 18.84%,salinity decreased 99.91%,moisture decreased 64.59%,and pH decreased 14.63%;mangroves sediment's physicochemical properties are similar to culture ponds,no significant differences in pH?TOC?TN?C/N?salinity?moisture,only C/N value increased 40.62%;Significant difference in physicochemical properties between Spartina alterniflora and mudflat,compared with mudflat,the TOC of Spartina alterniflora increased 107.07%,TN increased 170.69%,C/N value decreased 22.24%,salinity increased 159.43%,moisture increased 34.46%and the pH decreased 20.74%.2.The proportion of water stable macro-aggregates(WSAmacro)in five land types is:cropland>culture ponds>Spartina alterniflora>Mangroves>mudflat(there is no WSAmacro in mudflat),which is exactly opposite of the proportion of clay and silt(<53 ?m)in soil.No differences were found in proportion of water stable micro?aggregates(WSAmicro)between land uses.Therefore,when the mangroves were convert to cropland and culture ponds by human disturbance,the proportion of WSAmacro in the soil increased remarkably,the proportion of<53 ?m is significantly reduced,and the proportion of WSAmicro was relatively stable.Correlation analysis showed that the proportion of WSAmacro significantly negatively correlated with the proportion of<53 ?m,and the pH was significantly negatively correlated with the proportion of WS Amacro.Principal Component Analysis(PCA)showed that pH,moisture,TOC and TN were important environmental variables affecting the proportion of aggregates in soil.Compared with other land use types,when the mangroves were cultivated to cropland,the soil structure and the influx of the tides are more varied.3.There was no significant difference in organic carbon concentration between land uses in WSAmacro and<53 ?m,but in WSAmicro,organic carbon concentration In mangroves was significantly higher than other land use types.When mangroves were converted to cropland,the C/N values of WSAmacro,WSAmicro,<53 ?m were significantly decreased,indicating that the mineralization rate of organic carbon in the aggregates increased.Nearly 70%of the organic carbon in the topsoil of cropland is present in the aggregates.About 40%of the organic carbon in ponds and Spartina alterniflora is present in the aggregates,which is significantly higher than that of mangroves.In cropland surface soil,nearly 70%of the organic carbon in the aggregates.And in culture ponds or Spartina alterniflora soil about 40%of the organic carbon in the aggregates,which is significantly higher than that in mangroves.4.After mangroves were reclaimed to cropland,the concentration of humic(HA)and fulvic(FA)acids in different size aggregates changed significantly.There was no significant difference between land uses in WSAmacro;The order of HA concentration in WSAmicro in five land use types:cropland>mangroves>culture ponds>Spartina alterniflora>mudflat and the HA concentration in WSAmicro in cropland was significantly higher than that in other four land types;The relationship between HA concentration in<53 ?m of five land types was:cropland>mangroves>culture ponds>mudflat>Spartina alterniflora,there are Significant difference between cropland and others.Therefore,after mangroves were converted to cropland,the HA concentration in WSAmacro,<53 ?m will increase significantly.The FA concentration in WSAmacro,WSAmicro did not change significantly when the mangroves were reclaimed as cropland.However,the FA concentration in<53 ?m increased significantly.In WSAmicro,the HA/FA value of mudflat was significantly higher than that of other land use types,indicating that the humification degree of humic substances in aggregates did not change significantly after the mangroves were converted to cropland and culture ponds.However,after mudflat was invaded by Spartina alterniflora,humification degree of humic substances in WSAmicro was significantly decreased.Correlation analysis showed that there was a strong correlation between the concentration of HA,FA in WSAmicro and the proportion of aggregates,indicating that the concentration of HA and FA in WSAmicro is an important factor affecting aggregates distribution.5.When mangroves were converted to cropland or culture ponds,soil heavy metals will leaching out.Compare with mangroves,concentration of Ni?Cu?Zn?Pb in cropland were significantly different among different aggregates and showed a trend of enrichment in large aggregate.It's indicated agricultural activities affect distribution of heavy metals in aggregates.6.Redundancy analysis(RDA)showed that the proportion of<53 ?m,WSAmacro,TN,moisture were significant factors affecting the total amount of heavy metals.Among them,the proportion of<53 ?m has the greatest explanation for the variance of heavy metals(38.8%).PCA analysis shows that the sources of heavy metals in Zhangjiangkou mangroves area can be roughly divided into two categories:Cu,Zn,Cr,Ni and Pb were mainly derived from industrial activities,and Cd is mainly derived from agricultural activities.According to the linear fitting results,PC2 score was significantly positively correlated with the proportion of WSAmacro,and was significantly negatively correlated with the proportion of WSAmicro,<53 ?m indicating Cd input in agricultural pollution could significantly affect the distribution of different size aggregates in the soil.