Impacts Of Heavy Metal Pollution On The Stability Of Soil Organic Carbon

Soil is the important natural resource for maintaining life because it is the foundation of agricultural development and food security.The soil contamination has become a very serious problem in China,and even contamination of farmland with heavy metals is attracting considerable attention of the world.Androgenic contamination,including irrigation with sewage water,extensive development of mining and metallurgical industry,and the excessive use of chemical fertilizers and pesticides are the main sources of heavy metals in farmland.Soil microbial biomass,which plays an important role in decomposition and mineralization of organic carbon,has been found to be sensitive to increased heavy metal concentration in soil.For the organic carbon of paddy field is the richest in China,its stability has been related to the carbon storage of farmland ecosystem and atmospheric concentration of greenhouse gases.This study took a typical paddy field contaminated with Pb and Cd as the research object.The studied farmland was cultivated from an abandoned metallurgical industry located at Yixing city,Jiangshu province.Here,the effect of heavy metal pollution on the stability of soil organic carbon aggregate was investigated by the means of aggregate grouping,organic carbon analysis and infrared spectroscopy.And the mechanism,including physical protection,chemical stability and the effect of microbial activities,was further studied to supply important reference of realizing the ecological process of soil microorganism or carbon assessment and management which under pollution.The results could be summarized as below:1.The heavy metal pollution can change soil aggregate particles.The results showed that in wheat season the contents of 2-0.2 mm and 0.2-0.02 mm aggregates decreased significantly,whereas the contents of 0.02-0.002 mm and<0.002 mm aggregates increased significantly with the increasing of pollution degree of heavy metals.But it was not happened in the rice growing season because of the different water regime management from that in the wheat growing season.All the data showed that heavy metal pollution decreased the MWD(Mean Weight Diameter)value of the aggregates,which implied that metal pollution could destroy the soil aggregation.But the heavy metal pollution had no significant influences on the content of the organic carbon in bulk soil and aggregates.The study showed that under heavy metal pollution,the composition of soil aggregates in the various growing crop season showed various responses which could be related the various field managements.2.The content of active organic carbon in contaminated soil with heavy metals was significantly higher over that in uncontaminated soil.Microbial carbon content increased with the rising of pollution level first,and then decreased.In other words,sever pollution of heavy metals showed inhibition on microbial activity,whereas mild pollution could promote that Furthermore,the result suggested that the content of organic carbon existing as particle in the soil increased with the increasing of pollution level.3.The heavy metal pollution could change the structure of organic matter in soil.The result indicated that the content of phenol hydroxyl in contaminated soil was significantly higher than that in uncontaminated soil,and the severer the contaminated level the higher the content.As for the content of amino and the polysaccharide,heavy metal contamination showed a contrary effect.In summary,we can find that the heavy metal pollution can change soil aggregate particles and increase the content of the graded aggregate,reduces the large aggregate content,and with the increase of pollution degree,this phenomenon is more obvious;The increasing of the heavy metal pollution in soil can add labile organic carbon content,making it easy to be decomposed,thereby reducing the content of organic carbon,but the treatment changes without significant difference between organic carbon.content.Meanwhile we can also find high levels of pollution reduced microbial carbon content,while moderate and mild pollution can increase the amount of microbial carbon,stimulate the growth of microorganisms.