Temporal And Spatial Simulation Of Paddy Soil Carbon Pool And The Carbon Footprint Of Rice Production In Western Jilin

Soil is the most active carbon pool in the Earth's surface, and its variation diretctly impact on the concentrations of greenhouse gases in atmosphere. Since the industrial revolution, because of the human beings'excessive request for the natural resources such as fossil fuels, land resources and woods, the carbon balance between atmosphere and soil was broken. And the unbalanced statu directly caused CO₂ concentration in atmosphere increasing which directly led to the global warming and other interrelated issues. Based on concerns of the greenhouse effect, countries all around the world are seeking for the way to reduce CO₂ emissions and increase CO₂ sequestration. But it is difficult to find effective ways to reduce CO₂ emissions in industry, camparing with the relatively simple ways in agricultural carbon sequestration. Under this background, the farmland ecological system, especially paddy fields, because of its wide distribution, huge area and strong carbon sink capacity, became one of the hottest topics in global change researches. Western Jilin Province, as a national important grain production base, because of its special environment and the typical developing process of paddy fields was selected as the study area. Moreover, since the implementation of local land development projects in 2007, paddy fields area had been increasing rapidly. In this condition, reseach on the carbon dynamics in paddy soils and the carbon footprint of rice production has important meaning.Research on carbon dynamics of croplands is prevalent in both domestic and international studies. A series of achievements had been acchived and a number of carbon dynamics simulation models had been established. However, in previous researches, paddy soil organic carbon (SOC) was studied in one certain aspect, such as the spatial divisions, vertical distributions and temporal variations. But in entirety, one aspect of regional paddy fields carbon dynamics was inadequate. And the achievement of rice carbon footprint was still rarely.The study was supported by the National Natural Science Foundation named "Effect of LUCC on soil organic carbon and the driving mechanism during the past 50 years in Western Jilin Province". By interpreting the remote sensing (RS) images, land use / cover change (LUCC) situation in western Jilin Province between 1989 and 2008 was achieved. Through the paddy soils sampling and analysis, SOC accumulation rule and vertical distribution were obtained. And by simulation of a biogeochemical model named Denitrification-Decomposition model (DNDC model), the SOC pool and temporal and spatial distribution were achieved, as well as the amount of greenhouse gases (GHGs). And what's more, the total global warming potentials (GWPs) of the 5 years were calculated. Based on all of the results above, the carbon footprint of rice production was assessed.Firstly, TM images of the study area in 1989,1996,2000,2004 and 2008 was purchased, after the steps of atmospheric modification, geometrical correction, information enhanced, images mosaic, unsupervised classification, the LUCC status was acchieved by a manual visual interpretation according to a certain land use classification system. The results were highly accurate according to the accuracy testing. And the area variations, spatial distributions and conversion area between different land use types were gained. Results showed that: the area of dryland, grassland and saline-alkaline land were the main land use types of the study area. Farmland, forest land, urban land, saline-alkaline land kept increasing in the 19 years, while grassland, wetland, water area, beach land and sandy land were decreasing at the same period. Since 2000, particularly since 2004, the regional ecological environment has been recovered to a certain extent. But the whole trend was still getting worse. The distribution of land use showed a typical characteristic of ecotone between agriculture and animal husbandry. The major conversions between land use types were: grasslands to farmlands, high cover ratio grasslands and wetlands to medium cover ratio grasslands and low cover ratio grasslands, co-transformation between s saline-alkaline lands (sandy lands) and the low cover ratio grasslands, water area to grasslands or paddy fields, etc.Secondly, adopting the concept of space as a substitute for time, paddy soil samples covered different developmental years were collected. And the SOC content was tested by potassium dichromate - sulfuric acid method. Then the temporal and vertical varations of SOC had been analyzed. The results were compared with other land use types. The results indicated that: both SOC content and soil organic carbon density (SOCD) of paddy field increased with the increasing of the developmental years, the accruable laws with years showed an obvious exponential curve; the vertical distribution of paddy profiles showed a significant exponential curve too; SOCD range in a diminishing sequence in different soil layers was surface soils, subsoils and bottom soils. Compared with other land use types, paddy soils had a higher SOC content averagely.Thirdly, the LUCC data and soil data were collected to construct a coupled database. The database, combined with farm management, weather and other data, were inputted together into the DNDC model to estimate the SOC pool of paddy soils in 1989,1996,2000,2004 and 2008. The temporal and spatial variations were analyzed too. And based on simulative results, the carbon sequestration of paddy soil during the 19 years was educed. The results showed that: soil organic carbon storage in the years of 1989,1996,2000,2004 and 2008 were 1,060.67×10⁴t, 1,883.00×10⁴t, 2,217.39×10⁴t, 2,839.10×10⁴t and 3,403.20×10⁴t, respectively, and the average SOCD of the five years were 93.97t/hm², 111.59t/hm², 119.69t/hm², 125.21t/hm² and 129.46t/hm². The region with highest SOCD located at joint of Qianguo County and Songyuan City. SOCD of Taoer river irrigation district was 110-160t/hm² in the year of 2008. Zhenlai irrigation district had a shorter developmental histroy and its average SOCD was lower. Total carbon sequestration by paddy soils during 1989-2008 was 9.95×10⁶t, and the carbon sequestration rate was 1.99t/hm·yr. Overall, the development of paddy fields was a process of carbon sequestration in the aspect of soils, in which a longer period of paddy field had a smaller carbon sinks amount, while the shorter developmental period paddy had a larger amount of paddy carbon sinks.Fourthly, synthetically considering the emissions of GHGs and carbon sequestration in paddy soils, 100-year GWPs was calculated according to the IPCC method. The statistical relation between average SOCDs and GWPs was analyzed, too. The results showed that: the average emission rate of CH₄ wre 0.33t/hm²Â·a, 0.45 t/hm²Â·a, 0.44t/hm²Â·a, 0.43t/hm²Â·a and 0.46 t/hm²Â·a in the years of 1989, 1996, 2000, 2004 and 2008 as well as the emission rates of N₂O were 3.64 kg/hm²Â·a, 2.82 kg/hm²Â·a, 2.58 kg/hm²Â·a, 2.20 kg/hm²Â·a and 2.04 kg/hm²Â·a, respectively. The total amount of paddy soil GWPs in the five years in were 606,512 tCO₂-eq., 1,393,681 tCO₂-eq., 1,553,270 tCO₂-eq., 1,744,756 tCO₂-eq. and 2,448,500 tCO₂-eq., showed an increasing trend. And the average production rate showed a continuing upward trend too. The highest GWP production rate region located at the northeast of Qianguo County. Average SOCDs and GWPs of paddy fields in the five years showed a significant linear relationship.Finally, the carbon footprint of rice production was assessed by using the method of life cycle assessment. The standard steps of life cycle assessment (LCA) set by International Organization for Standardization (ISO) were: goal scope and definition, inventory analysis, impact assessment and interpretation. The results indicated that: production of 1000 kg rice in West Jilin Province consumed a total energy of 2,926.89MJ, it was higher in the whole country and the main difference was the growing phase. Carbon footprint of 1,000 kg rice yields was 1,330.26 kg CO₂-eq. and the carbon footprint of rice planted in saline- alkali paddy was 1,611.13 kg CO₂-eq., and it was a lower level because of the less emission of the GHGs. The production and application of urea was the key element in both of energy consumpion and carbon footprint.In this study, carbon effect of paddy soil and rice production in western Jilin was set as the research goal. The research method included RS image interpretation, laboratory testing, model simulation, and it was a miscellaneous investigation. In the apects of multi-angle study of paddy soil carbon and the carbon footprint of rice production, the paper had a certain progress of creation. It had great significance in the aspects of enriching the technical approaches of paddy SOC dynamics, enhancing the knowledges of carbon cycle in regional scale, and guiding the development of low-carbon agriculture.