Regional Distribution And Variation Of SOC Storage In Agricultural Soils At Different Scales

Carbon(C) storage and sequestration is considered to be an important issue in the study of terrestrial C cycling and global climatic change.Recent studies have shown significant potential for soil C sequestration in croplands.China has a total area of croplands of 130Mhm²,7%to the global total.However,the magnitude and duration of the C sequestration potential of China's cropland soils has not been quantified.The author selected three spatial scales to study the distribution and dynamic of soil organic carbon(SOC) of cropland:Guichi County as the local scale,Anhui province as the provincial scale and China as the country scale.Using the soil series data collected from the 2^nd National Soil Survey estimates the content and density of soil organic carbon in three scales.In this study,we used data of SOC change at monitoring sites from publications available in 1980-2007 to perform a meta-analysis to project the size,duration and potential capacity of soil carbon sequestration in China's croplands.The data set comprises 1099 observations including 416 from rice paddies and 683 from dry croplands across China. The main results were as follows:SOC content and carbon density in paddy soil was higher than that in dry cropland.SOC content and density in paddy soil were 1/3 and 5%higher respectively than that in dry cropland in 1984 while they in paddy soil were 45.18%and 23.73%higher respectively than that in dry cropland in 2005 in Guichi county.According to soil survey data from 2005, statistic analysis showed that total topsoil SOC density was 31.64±16.39tC/hm²,with 36.36±18.75 tC/hm² from the forest soils,17.58±6.07tC/hm² from dry cropland soils and 27.7±6.72 tC/hm² from the plow layer of paddy soil and 14.11±6.44 tC/hm² from plow-pan layer in Anhui province.The content of paddy topsoil organic C was 175%-176%of dry lands in China.The results showed total organic carbon pool of Anhui province was 0.71Pg,and total topsoil SOC pool was 0.28 Pg,with the trends of forest soils＞paddy soils＞dryland soils. According to the geographical division,the size of topsoil carbon pool was in this order: south Anhui area＞Jianghuai hill area＞Huaibei Plain area＞Wanxi Dabie Mountain area＞Along the Yangtze River plain area.Climate and Vegetation dominates the provincial scale distribution of content SOC.There was a significant positive correlation between SOC and precipitation.The difference of SOC in soil subspecies depends on terrain and parent of material.The SOC storage was found closely related to the clay(＜0.002mm) content and N enrichment in soils in plain area.SOC content of 70%samples both paddy soil and dry cropland increased in recent 20 years of China,those samples that changed over 90%ranged from-1%and 4%.Compared 1984 to 2005 in Guichi county,SOC content and density in paddy soil raised 2.01%and 3%, respectively,while dry cropland 1.11%and 1.3%every year.SOC content of cropland was relative to physical geography characteristics,such as physiognomy,vegetation,climate and so on.SOC content ranked as Plain＞Basin＞Mountanious Region＞Hill according to physiognomy type on Guichi county scale.SOC content of cropland in flat physiognomy site was higher than that in hillock and low-lying place,and SOC content in south and east slope were higher than that in north and west slope.SOC density of Anhui provincal region ranked as South Anhui mountanious area＞West Anhui Dabieshan mountainious area＞Changjiang Plain＞Jianghuai hill area＞Huaibei Plain area.SOC distribution was positive relative to precipitation,which was influenced by precipitation and vegetation.SOC distribution was affected by landform and parent materials in soil subspecies.Topsoil SOC content of cropland on national scale was controlled by air temperature and precipitation.There was a close relationship between SOC content and soil property.However,there was difference between dry cropland and paddy soil for the factors impacting SOC content in Guichi County.The positive correlation was shown between SOC and available P, available N and clay content for paddy soil.The results also indicated there was a significant correlation between SOC content of topsoil and total N(R²ï¼ž0.78) on the scale of Anhui province,and clay content affected the SOC sequestration.The estimated effective duration for SOC sequestration is 34-36 year for dry croplands and 27-29 years for rice paddies.Using the average RAIs,a topsoil C stock increase for the whole of China between 1982 and 2006 can be projected between 32.0±65.4 Tg C/yr and 33.6±77.5;Tg C/yr,with a total C stock increase between 0.77±1.57 Pg C and 0.81±1.86 Pg C.There is still a high potential for C sequestration,as the achievable level under current practices is estimated to be at 16.1 g C/kg and at 26.7 g C/kg for dry croplands and rice paddies,respectively.