| Since from the end of the20th century, many scientists more concerned about terrestrial ecosystem carbon cycle and carbon budget by global change. The storage of soil carbon pool was thrice the storage of biological carbon pool in terrestrial ecosystem, and twice the storage of Atmospheric carbon pool. Therefore, soil organic carbon pool was the focus of global terrestrial carbon cycle research of ecological system, and it was one of the core contents of global change research. Existing research results show the global soil carbon storage about was2500Pg, the storage of global soil organic carbon accounted for about60%of the total global carbon pool. The storage of Chain soil organic carbon pool was80-90Pg, it accounting for5%of global organic carbon pool storage. The storage of chain topsoil organic carbon pool was38-40Pg, accounted for42.2%-47.5%of Chain organic carbon pool storage. Existing data indicate that the total soil organic carbon stock of topsoil of20cm depth and of whole soils of100cm depth amounts to0.27Tg and1.00Tg. But the above research results were based on the second national soil survey results, so the estimation results couldn’t reflect the present situation of the soil organic carbon pool. Soil organic carbon pool of farmland was one of the most active and influential carbon pool in terrestrial ecosystems, and its carbon stock accounted for10%of the global terrestrial carbon storage. With estimation, soil organic carbon reserves of farmland accounted for14.4%-16.2%of the total soil organic carbon storage in China. Soil organic carbon fixation of farmland was important carbon sequestration under the pressure of CO2emission reduction. Illy and mountainous regions of Southwest was located on the river source, and its natural environment was complexity and diversity, the geological history was complex and landform. The biodiversity and soil type were special and rich. But the Southwest regions, especially in Chongqing Area, were lack of research on reserves and potential of soil carbon pool of farmland. With the in-depth study on global carbon cycle of farmland ecosystem and in order to further study on dynamics of soil organic carbon change trend, soil organic carbon dynamics model had been applied widely. In the China Northeast Area and China North Area, dynamic models of organic carbon had been verified, for example DNDC model、Roth-C model、CENTUR model. But in the Southwest Hilly Area, there was lack of application of soil organic carbon dynamics model. The oil organic carbon dynamics models which could be applied to purple soil and paddy soil in Chongqing Area were not known. So it was necessary to strengthen the application of organic carbon dynamics model in Chongqing area.In conclusion, Chongqing City was taken as the study area in this paper, and the soil utilization map of Chongqing City、soil type map of Chongqing City、the soil testing data of formula fertilization project from2006-2011、the second soil survey data were collected. According to the soil analysis data under different time points, the method of soil taxonomy was used to estimate soil organic carbon storage and analysis the change trend of soil carbon pool in Chongqing Area’s farmland. Moreover, the soil genuses were taken based as a statistical unit. Form the soil testing data of formula fertilization project, this paper screened out the highest values of topsoil organic carbon content in the same soil genus. This highest values were regarded as the soil carbon sequestration limits. Compared to the Chongqing farmland soil carbon density and storage in2006, the carbon sequestration potential of farmland topsoil in Chongqing City Statistics was calculated. the DNDC model was verified by the monitoring data and research results of the long-term tests. Through the above work, this paper hoped to be able to clarify the soil organic carbon sequestration status and the evolution pattern of Chongqing farmland, and to predict the soil organic carbon sequestration potential in Chongqing City, to provide theoretical reference for research of soil organic carbon pool evolution mechanism. This paper also expected to provide scientific basis on the establishment and implementation of the regional agricultural sustainable development and grain security strategy, as well as international negotiations which related with China global changes.The results are as follows.(1) Nationwide, the SOC density of Chongqing’s farmland soil was medium level in surface SOC density. But the differences of SOC density under different soil types were larger. The SOC density of paddy soil was higher than other soil types in Chongqing Area. The SOC density of purple soil was dignificantly lower than other soil types in Chongqing Area. As the most widely distributed and the most SOC storages soils in Chongqing City, the SOC storages of paddy soil and purple soil were accounted for75.55%in Chongqing city. So paddy soil and purple soil were the most important SOC stock of cropland in Chong City, and these two kinds’soils were the main object for implementation measures to improve SOC content of cropland.SOC density of the cultivated horizon in cropland located in Chongqing City was3.38kg/m2. The order of SOC density of the cultivated horizon in different soil groups were yellow-brown soil (4.25kg/m2)> paddy soil (3.89kg/m2)>calcareous soil (3.88kg/m2)> alluvial soil (3.16kg/m2)> yellow soil (2.68kg/m2)> purple soil (2.44kg/m2). The SOC density of alluvial soil、yellow soil and purple soil were lower than the average lever of Chongqing City. The SOC storage in0-20cm of cropland located in Chongqing City was70.442Tg. The SOC storage in paddy field was37.773Tg, and the SOC storage in dry land was32.669Tg. The SOC storages of paddy field and dry land accounted for53.62%and46.38%respectively. The order of SOC storage of the cultivated horizon in different soil groups are paddy soil (37.773Tg)> purple soil (15.448Tg)> yellow soil (9.2030Tg)>calcareous soil(6.9400Tg)>yellow-brown soil(0.71140Tg)> alluvial soil (0.36660Tg). In the cultivated horizon of Chongqing city, the order of the indexes of SOC abundance in different soil groups are yellow-brown soil(AI0-20=205)> calcareous soil(AI0-20=199)> paddy soil (AI0-20=185)> yellow soil (AI0-20=137)> alluvial soil (AI0-20=107)> purple soil (AI0-20=103). The SOC storage capability of yellow-brown soil was the strongest in0-20layer of soil, secondly for calcareous soil, and purple soil was the weakest. In different soil types, the SOC storage capability was different. As the second laggest distributed soil in Chongqing City, purple soil was the main object for implementation measures to improve SOC content of0-20cm cropland.(2) The SOC density and SOC storage in different county changed relatively greatly. In different counties, the SOC dendity of same soil type had siginificant difference, and the percentage of SOC storage which was same soil type also was different. Form the geographical perspective, the SOC reserve of Yuxi Region was much larger than the SOC reserve of Yudongbei Region. It showed that the SOC distribution was uneven in Chongqing Area. The importance of soil types in different geographical region’s farmland carbom pool was also different.In different counties of Chongqing city, the maximal value of paddy soil’s SOC density of the cultivated horizon in cropland appeared in Chengkou County, the maximal value of purple soil’s SOC density appeared in Youyang County, the maximal value of yellow soil’s SOC density appeared in Rongchang County, and the maximal values of calcareous soil’s and alluvial soil’s SOC density respectively appeared in Qijiang County and Xiushan County. The minimum values of different soil groups(paddy soil、purple soil、yellow soil、calcareous soil、alluvial soil)respectively appeared in Yubei District、Yunyang County、Kaixian County、Liangping County、Fuling District. The SOC storage in different county changed relatively greatly, which had the amplitude of4.0810Tg. The SOC storage in0-20cm of cropland located in Youyang County was4.5250Tg, and the SOC storage in Wangsheng District was0.44320Tg. These SOC storages respectively were the maximum and minimum values. The SOC storage of most counties were located in the1-2Tg range, and paddy soil’s and purple soil’s SOC storage had great proportion in total SOC storage. But In a few counties, yellow soil’s and calcareous soil’s SOC storage also had great proportion in total SOC storage. The thirty-three counties of Chongqing were divided into five geographical regions from the perspective of geographical area. The order of SOC storage in different geographical area were Yuxi Area> Yudongnan Area> Yudong Area> Yudongbei Area> Yunan Area. The SOC storages in different geographical area respectively accounted for28.58%、21.04%、18.80%、16.35%、15.22%in Chongqing city’s total SOC storage. In Yudong Area、Yunan Area and Yuxi Area, the order of SOC storage of the cultivated horizon in different soil groups were similar, which were paddy soil> purple soil>yellow soil>calcareous soil>alluvial soil. But in Yudongbei Area and Yudongnan Area, the order respectively were paddy soil>yellow soil>calcareous soil>purple soil>yellow-brown soil>alluvial soil and yellow soil> paddy soil> calcareous soil> purple soil> alluvial soil>red soil.(3) in parallel ridge-and-valley area, the basin edge hill area and Wuling mountain area of Chongqing City, the farmland SOC densities of three typical counties showed a rising trend. But the change trends of different soil types were different. The SOC densities of paddy soil and yellow soil had overall upward trends in Chongqing Area. But the SOC density of purple soil had a trend of decline. The change trend of SOC stroage was overall upward in Chongqing City. So in the past thirty years, the farmland topsoil showed a effect of sink enchancement in the carbon cycle in Chongqing CitySince1980, artificial disturbance had complex effect to SOC accumulation in cropland. In three typical counties, the SOC storages of Dianjiang County and Youyang county respectively increased of0.41456Tg and0.67140Tg, the growth rate was26.1%and17.4%respectively. But the SOC storages of Qijiang County rasied0.23149Tg, the rate was9.60%. In paralleled ridge-valley area and basin margin-hilly lands area, the value added of SOC storage in paddy filed had biggest effect on the growth of the county’s SOC storage, and SOCR0-20volume growth of paddy soil accounted for59.0%and75.8%in regional total SOCR0-20volume growths. In Wuling Mountainous Area, the soil group was yellow soil. But the value decreased of SOC storage in purple soil had biggest effect on the decrease of basin margin-hilly lands area’s SOC storage.(4)SOC content of cropland was relative to physical geography characteristic, for example physiognomy, climate, soil physicochemical property and so on. The SOC content of0-20cm soil layer rank as Pingba Gentle Hill> The Lower Hill> The Foothills and Slope Waist Gently> The Top Hill. The order of SOC content of the cultivated horizon in different altitude were600-700m>500-600m>300-400m>400-500m. In a word, environmental factors had a minor effect on SOC contents. The correlationof SOC content of topsoil with soil total nitrogen content was significantly, but it hadn’t correlation with other soil chemical properties. Soil physical properties greatly effects on SOC content. The SOC content in different soil texture rank as, clay soil textures> loam> sandy loam> light loam. The order of SOC content in different soil structure were microaggregates> aggregates> crumb> granular> block. Cropland management, such as farming system, could significantly affect SOC of the cultivated horizon. In different farming system, the order of SOC content were One-crop> Double-crop> Perennial-crop> Three-crop. The result of T test in four kinds of farming systems showed that farm system of one-crop had the extremely significant difference with other farming systems.(5) The soil carbon sequestration potential of farmland topsoil was great in Chongqing Area. But the regional differences and the differences of soil types of carbon sequestration potential were obvious. The Yuxi region was the largest geographical region in carbon sequestration potential. The carbon sequestration potential of paddy soil and purple soil were much higher than other soil types. DNDC model could provide an execellent simulation of SOC dynamic change in Chongqing City. So a new method of study on carbon sequestration potential and the SOC evolution trend was offered in Chongqing City.The SOC density and storage in2006were used for basic point and the present area of arable area was maintained, SOC density potential and SOC storage potential in0-20cm soil layer were6.69kg/m2and227.23Tg, increased by198%and322%. It showed that there was huge potential of SOC sequestration in0-20cm cropland located in Chongqing City. The SOC storage potential of paddy soil and purple soil respectively were68.714Tg and137.09Tg, which accounted for60.3%and30.2%. It showed that the SOC storage potential of cultivated horizon mainly existed in purple soil and paddy soil. In different soil types, the soil genus of gray-grown-purple soil and purple rice soil had the highest potential of SOC sequestration in0-20cm soil layer of Chongqing City. A long-term experiment and DNDC model were conducted to study the effects of different tillage systems on accumulation and mineralization of organic carbon in purple paddy soil. The analog value of DNDC model was same as measured values of different treatments. The RMSE (root mean square error) values between the analog values and measured values were smaller than10%, and the correlativity of all treatments were extremely correlativity in0.01notable level. It indicated that the simulates results of four kinds cultivation system by DNDC model were extremely well. As displayed by the long time stimulant results:if keeping present cultivation system and cultivate pattern, soil organic carbon contents of Beibei District would stably increase during100years. The SOC storage potential of Beibei District was calculated by DNDC model. The analog value was lower than the present SOC sequestration potential. It may be caused by the DNDC model doesn’t consider equilibrium time of SOC increased and the limit value of SOC sequestration potential.In a word, the recent soil survey data and analysis results of Chongqing region were used in this paper, soil organic carbon storage status and space-time distribution of Chongqing City were estimated. The result showed that topsoil organic carbon pool of farmland occupied an important position in the soil organic carbon pool in Chongqing Area.Compared with the previous estimated results which used the data of the second soil survey, this paper could better reflect the status of organic carbon pool in Chongqing Area, and provide data support for correctly evaluate the position of Chongqing City’s farmland carbon pool in national carbon pool. At the same time, the paper proved the important status of different soil types on farmland soil organic carbon pool in Chongqing Area. It provided reference basis for the regional farmland management measures and soil carbon regulation policy; the results of this study found that farmland topsoil had a great carbon sequestration potential in Chongqing region. The different soil types of farmland had differences on carbon sequestration potential. This paper also clearly estimated spatial distribution of farmland soil carbon sequestration potential in Chongqing Area. The above results provides theoretical reference for the strategy formulation and technology selection of agriculture carbon sequestration in Chongqing region, and offered scientific basis on the international negotiations which related with global changes; through the combination of long-term located experiment with soil organic carbon dynamics model, this paper tested and verified the dynamic model of soil organic carbon. Through this works, the soil organic carbon dynamic model which suitable for Chongqing Area was found. It supplied a new research idea and method for research changes trend of soil organic carbon and prediction on soil carbon sequestration potential in Chongqing City.In the future works, the following research could be strengthen:... |
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