Dynamic Characteristics Of Soil Organic Carbon And Carbon Sequestration Effect Of Conservational Farming Systems In Paddy Soil Of Southchina

As the global environmental change is going on,especially global warming,great emphasis is given on how to slow down the trend by the scientists and politicians all over the world.The storage of soil organic carbon is about 2 to 2.5 times of that in the atmosphere and the above of the terrestrial ecosystem and its dynamic changes are related to the concentration of CO₂ and then the global environmental change.Farming system which includes both the cropping system and soil management plays an important role in the soil organic carbon stability and accumulation.Researches showed that the farmland soil had been carbon sequestration storage under the farming system evolution about recent 20 years,especially in the paddy soil of South-China whereas little research had been done. Under the condition of knowing the characteristics of southem region adequately,and also based on the historic data,published literature,field experiments and household survey data, the characteristics of soil organic carbon and nitrogen in the paddy soil of south-China,the effect of anthropogenic disturbances on the paddy soil carbon and the relationship between farmer behavior and soil organic carbon and nitrogen was studied through the integration of qualitative analysis and quantitative analysis.The main results were as follows:The results suggested that anthropogenic disturbances had great impacts on soil organic carbon density in paddy soils,indicating a high control potential of soil organic carbon density.Results showed that soil organic carbon density in the plow layer was about 3.15 kg/m² in average which ranged from 0.81 kg/m² to 12.68 kg/m².The variation coefficient of soil organic carbon density in the plow layer of farmland was 57%,which was 35 percentages lower than that of non-farmland soils.Compared to soil organic carbon density in the dry land,SOC density in paddy soils was 13 percentages higher with a lower variation coefficient between different regions.In addition,the relationships between the climatic factors(annual average temperature and annual precipitation) and soil organic carbon density were lower in farmland than those in non-farmland soils,as well as in paddy soils than that in dry land of farmland.There existed close coupled relationship between soil organic carbon and total nitrogen in the plow layer of farmland,and maybe more carbon could be sequestrated in the paddy soil than that of in the upland soil under the same nitrogen level.Results showed that soil organic carbon and total nitrogen contents were higher in paddy fields than in upland fields by 47.8%and 45.5%,respectively,but spatial variation of soil organic carbon and total nitrogen were found higher in upland than in paddy fields.Soil organic carbon and total nitrogen ratio(C/N) was about 10.8 in paddy fields,higher than 9.9 in upland fields.The highest C/N ratio value was found in the paddy fields in Northeast China while the lowest in the up land fields in Northwest China and East China.Meanwhile,results also show significant regional variation of C/N ratio in up land fields,but little significant variation in paddy fields.The soil organic carbon and total nitrogen characteristics and its key effect factors were differ between different paddy soil types,so the different technologies must be selected according to the variation of regional soil types.Results showed that the highest soil organic carbon density was found in the gleyed paddy soil,which was about 75.97%higher than the lowest(submergenic paddy soil).The gap of the total nitrogen between the above two types of soil was 1.31,and showed significant difference between them.Correlation analysis indicated that the soil organic carbon density was significantly and positively correlated with the plow layer depth and total nitrogen content.But to the carbon and nitrogen ratio,the key factor were much more complicated and differed for each other.Integrated fertilization with organic mature and chemical fertilizers was the key technology to the carbon sequestration and nitrogen utilization.Straw application could increase the soil organic carbon density but with limited extent.Long application of integrated fertilization with organic mature and chemical fertilizers could not only enhance the soil organic carbon density but the carbon and nitrogen ratio whether in double cropping rice region or single cropping rice region.Compared to the region where only the chemical fertilizers was used,the soil organic carbon density and C/N ratio in the integrated fertilization region was about 20.46%and 6.43%higher,respectively.But it was only 3.37%and 4.15%respectively in the single cropping rice region where the soil was much fertilizer.In addition,the carbon sequestration effect was not so significant through the straw applications when the quantity of the residues reached to some level.Conservational farming system technology was beneficial to the increasing of carbon and nitrogen in the soil,but the effect of each technology differed from each other.Taken the Yangtze delta plain for example,results showed that the increases in sown oilseed rape area,reduced-tillage wheat area and straw application area had increased organic carbon about 0.94 Tg,2.76 Tg and 3.95 Tg in surface soils(15 cm),respectively.The highest effect of carbon sequestration occurred under the transformation of Wheat-rice system to Oilseed rape-rice system.Based on the econometric model,results showed that the farming production behaviors and the land policy had great effect on soil organic carbon and soil nitrogen.Farmer behavior was closely correlation with the farmer's age,their educational level and land policy.Besides those,different individual opted dissimilar land management especially in the application of organic manure which was greatly related to the soil organic carbon and nitrogen content.Consequently,Perfecting the land use and enlarging the land scale may promote the farmer to increase the investment on the soil and select appropriate technology which will contribute to the enhancement of soil quality.