The Change Of Land-use Types And Its Influence On Soil Organic Carbon Storage In Nansi Lake

The impact of land use/cover change in wetland ecosystems on the global carbon cycling is an important scientific issue in global climate change.The objectives of this study were to(ⅰ)estimate the change of land use types in Nansi Lakes in the past 40 years,to(ⅱ)investigate the influence of reclamation history and depths on the DOC,LFOC,and HFOC content in paddy-field soil and dry-land soil after their conversion from natural wetlands,and to(ⅲ)calculate the density and storage of organic carbon involving each land use types in Nansi Lake.Based on the remote sensing images of Nansi Lake area in 1978,1987,1998,2008 and 2018,human-machine interactive visual interpretation was conducted using supervised classification to obtain the data of distribution and area of each type of land use in Nansi Lake area for 5 periods.Soil samples were collected in marshes,constructed wetlands,fishponds,paddy fields and dry land.The contents of dissolved organic carbon,light fraction organic carbon,heavy fraction organic carbon and other soil properties(i.e.,moisture content,bulk density,pH,clay,sand,silt,available phosphorous,light fraction nitrogen,and heavy fraction nitrogen)were measured.Moreover,the density and storage of organic carbon of each types of land use were calculated.The main conclusions are as follows:(1)The areas of natural wetlands(lakes,rivers,marshes)in Nansi Lakes area decreased,while the areas of artificial wetlands(fishponds,paddy fields,artificial canals)increased,due to the hydrological changes and interference of human activities during 5 periods.(2)The content of DOC increased 17%in constructed wetland and decreased 39%in fishponds,and the content of HFOC in constructed wetland and fishponds increased 50%and 8%,respectively,compared with that in natural wetlands at 0-20 cm.The HFOC increased in the constructed wetland and fishponds compared with the natural wetland due to the positive effects of carbon input from anthropogenic sources.The constructed wetland had higher DOC content than the natural wetland,while the natural wetland had higher LFOC content than the constructed wetland,indicating that in the constructed wetland microbial,the utilization of DOC was higher and in the natural wetland,plant residue supply was abundant.(3)After the conversion of a wetland,the content of HFOC increased 72%in the paddy fields and decreased 62%in the dry land,while the content of DOC and LFOC decreased in both types.In the paddy fields,LFOC and HFOC content in the surface 20 cm of the soil layer was significantly higher compared to the layer below(from 20 to 60 cm),and there were no significant differences observed in the dry land.The findings suggest that the paddy fields can sequester organic carbon through the accumulation of HFOC.However,the HFOC content decreased 22%after 10 years of cultivation with the decrease of clay content,indicating that paddy fields need to favor clay accumulation for the purpose of enhancing carbon sequestration in the paddy fields.The conversion from wetlands to a paddy fields increases the sequestration of soil organic carbon while conversion from wetlands to dry land decreases the storage of soil organic carbon.(4)The soil organic carbon storage in Nansi Lakes area has undergone significant changes with the change of land use ways.The highest dissolved organic carbon density and light fraction organic carbon density were occurred in the top soils of the marshes,and the highest heavy fraction organic carbon density was occurred in the top soil of paddy fields.The heavy fraction organic carbon storage in the top soil increased by 921.58×103 ton due to the increased area of fishponds and paddy fields,however,the total soil organic carbon storage in the top soil in Nansi Lakes area decreased by 136.34×103 ton because of significant shrinkage of natural wetlands.In order to reduce the impact of natural wetlands on soil carbon storage,it should be advocated to return farmland to wetland.From the perspective of farmland carbon sequestration,paddy fields have a greater potential of mitigating greenhouse gases emission.In summary,the areas of natural wetlands(lakes,rivers,marshes)in Nansi Lakes area decreased,while the areas of artificial wetlands(fishponds,paddy fields,artificial canals)increased from 1978 to 2018 years.The conversion from wetlands to a paddy fields increases the sequestration of soil organic carbon while conversion from wetlands to dry land decreases the storage of soil organic carbon.The total soil organic carbon storage in the top soil in Nansi Lakes area decreased significantly because of significant shrinkage of natural wetlands.The study provides a scientific basis for understanding the change of carbon storage in Nansi Lake wetland sediments and the effects of land use change,and can provide a reference for wetlands management and wetlands carbon sinking.