| As the largest container of organic carbon and heavy metals,coastal soil has a special role in global climate and environment change mitigation.However,a systematic knowledge is limited on coastal soil organic carbon and heavy metal accumulation characteristics,dynamics and influencing factors under the impacts of global climate change and intensive anthropogenic activities.951 coastal top-soil samples(0-20 cm)including agricultural(farmland and orchard soils),coastal protection forest,mangrove,salt marsh,seagrass,sand flat and mud flat soils as well as 6 soil cores including mangrove,salt marsh,seagrass were collected along the coastal zone of China.These samples were totally or partially measured for soil organic carbon content and fractions,heavy metal content and sequential extracted fractions,radioactive lead isotope,stable carbon isotope and basal soil properties.The measured results of soil samples were employed to analyze the accumulation and partitioning characteristics of coastal soil organic carbon and heavy metal,to investigate the dynamics of coastal soil organic carbon and heavy metal accumulation in the last century and to reveal the impact of soil organic carbon on heavy metal accumulation at national scale.Moreover,the accumulation,dynamics and spatial distribution of soil organic carbon and heavy metals were analyzed at regional scale based on coastal orchard which was strongly affected by agricultural activities.These results are expected to provide better understanding of biogeochemical cycles of soil organic carbon and heavy metal in coastal soils and benefit the sustainable development of the coastal zone.The main results are summarized as follows:(1)The coastal soils of China contained a total of 233.04±20.98 Tg organic carbon in the top 20 cm,of which was dominated by"green carbon"stored in agricultural soils.Mangrove,salt marsh and seagrass beds soils only account for 1.4%of the total coastal soil organic carbon stock,whereas the coastal protection forest,sand flat and mud flat soils contribute importantly to the coastal soil organic carbon stock though with low soil organic carbon densities.Coastal soils in China have great carbon sequestration potential,especially agricultural soils.Mangroves,salt marshes,seagrass beds and coastal protection forests are small in areas,which leading to low carbon sequestration potentials.The distribution of coastal soil organic carbon stock is influenced by climate and geomorphology.Mangroves,salt marshes and seagrass soil carbon stocks are more sensitive to climate.The input of terrestrial organic carbon and fine particulate matter promote the accumulation and preservation of organic carbon in salt marsh,seagrass and mud flat soils,but diluted the soil organic carbon in mangrove soils.Dissolved organic carbon(DOC),readily oxidized organic carbon(POC)and particulate organic carbon(POC)in coastal soils are mainly derived from the decomposition of plant litter and secretions in the soils.The accumulation of organic carbon in coastal soils is largely depend on oxidizable organic carbon(16?53%)and particulate organic carbon(17?97%),resulting in a decrease in the stability of carbon pools.(2)The sediment accretion rates of typical mangroves(Shenzhen,Guangdong;Beihai,Guangxi),salt marshes(Yancheng,Jiangsu;Ningbo,Zhejiang)and seagrasses(Dongying,Shandong;Weihai,Shandong)in China are relatively high(1.36?2.14,0.81?1.63 and 0.88?4.00 cm yr-1,respectively)due to the abundant land-source sediment input.Soil organic carbon density(202?210 Mg C ha-1)and burial rate(284.9±92.3?433.1±179.7 g m-2 yr-1)are relatively high for mangrove soils,but relatively low for salt marsh soils(soil organic carbon density,58?67 Mg C ha-1;soil organic carbon burial rate,47.3±44.3?91.5±21.1 g m-2 yr-1)whereas varies greatly for seagrass soils(soil organic carbon density,45?288 Mg C ha-1;soil organic carbon burial rate,39.3±17.6?1152.4±487.1 g m-2 yr-1),reflecting different of carbon sequestration capacities.Organic carbon density was 2.81±0.82 kg m-2 for orchard soils in Jiaodong peninsula,which was higher than the organic carbon density for coastal soils,exhibiting high organic carbon accumulation capacity.The organic carbon content and density show moderate spatial variability.However,in the long-term intensive management process,the orchards soil organic carbon content in orchards increased first and then decreased,accompanied by soil acidification and other issues,which led to the decline of soil organic carbon pool.(3)The coastal soils in China showed different degree of heavy metals accumulation,but the overall accumulation is low.The average contents of Cu,Zn,Pb,Cr and Ni in salt marsh soils,mangrove soils,mud flat soils,and agricultural soils exceeded the Chinese soil elements background values.The average contents of heavy metals in coastal defense forests,seagrass beds and sandy beaches were significantly lower than the background values.The coastal soils at the sampling sites are not polluted,but with moderate to mild pollution in some area.Heavy metals accumulation was linked with regional economical and the degree of anthropogenic activities.The developed region with intensive anthropogenic activities exhibited remarkable heavy metal accumulation(For example,the mangrove soil core in Shenzhen).Although heavy metals in coastal soils are slightly polluted or not polluted at present,the accumulation trend of heavy metals indicates the degradation of coastal soil environment.The frequent use of copper-containing fungicides leads to copper accumulation in soil.The accumulation of total copper content is 3.30 mg yr-1,of which 36.2%is in the available form.The proportion of potential mobile copper(weak acid soluble,Fe/Mn oxides bound and organic bound)also shows a trend of accumulation.Box-Cox,Johnson,Rank order and Normal score normal transformation methods can reduce the skewness and kurtosis of soil available copper and the corresponding residual.The spatial interpolation accuracy of soil available copper was improved by the interpolation method combined with planting duration information,especially by combining Normal score normal transformation.(4)The content of organic carbon in coastal soils has a significant positive correlation with most heavy metals.The correlation of heavy metals is weak for a few heavy metals which mainly come from parent material(such as Cd element in YC core)or which have mutation in deposition process(such as As element in DY core).The accumulation of organic carbon in coastal soils has a strong ability to immobilize exogenous heavy metals.Soil organic carbon in coastal orchards has significant positive correlation with total copper,available copper,organic matter bound and weak acid soluble fractions,which promotes the partitioning of available copper and organic bound copper in soil and increases the potential risk of copper pollution. |
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