Quantification And Model Simulation Of Climate Change Mitigation Potential Of China’s Cropland

Anthropogenic greenhouse gas (GHG) emissions have increased rapidly since pre-industrial times, which is a major factor contributing to climate change. Reducing emissions of carbon dioxide (CO2) and other greenhouse gases (GHGs) to earth’s atmosphere and sequestrating soil organic carbon (SOC) in terrestrial ecosystems are identified as two of the most pressing modern-day environmental issues. Agriculture accounted for10-12%of total global anthropogenic emissions of greenhouse gases, and20%of global GHG mitigation potential could be achived from agriculture, which indicates the importance of agriculture in global climate change mitigation. China’s GHG emissions in the agriculture sector in2005were estimated to be820Mt CO2-eq., contributing11%to the nation’s total emissions, and also contributing13%-16%to global GHG emissions in agriculture. Therefore, China’s agriculture plays an important role in global climate change mitigation. Assessment of the SOC dynamics and GHGs emissions accurately in the cropland is important for police makers. The objective of this study was to account and simulate GHG emissions and the mitigation potentials for China’s cropland using various approaches, and then discussed the realization of the GHG mitigation potential. The main results obtained were as follows:(1) Four datasets, included Chinese crop production dataset, national monitoring network dataset and long-term cropland experiments dataset, were built to do the following researches. Chinese crop production dataset includes planting area, crop yield and various agricultural inputs. National monitoring network dataset includes the SOC contents from299sites in the last20years. Long-term cropland experiments dataset were collected from the scientific field studies that reported SOC changes and GHG emissions in Chinese cropping systems under various management practices.(2) Data of topsoil SOC contents from the national monitoring network was used to analyze the SOC dynamics and the sequestration status in China’s croplands. The data set comprises299observations across mainland China. The relative annual SOC changes of80%of monitoring sites were distributed between-1.5%and7.5%. Topsoil SOC of China’s cropland was in a general trend of accumulation with a frequency of79.1%, which were mainly distributed in the North, East and Southwest China’s croplands. The difference of land use effect on soil carbon sequestration was significantly, the SOC content of rice paddy was significantly higher than that of dry croplands, and the frequency of increasing SOC content was also greater than that of dry cropland. The topsoil SOC datasets of China’s cropland were used to calculate the SOC densities in different regions and cropland types. The current average SOC density of China’s cropland was estimated as36.44t ha-1. The greatest SOC density of42.96t ha-1was found in southwest China, and the least was in northwest of China (25.18t ha-1). There was a significant difference in present SOC density between dry cropland and rice paddies, with the latter being greater by almost10t ha-1than the former. The greatest SOC density for dry cropland was in south west (38.45t ha"1) and North-East of China (36.43t ha-1), and for rice paddies in south China (55.97t ha-1).(3) Life cycle analysis-carbon footprint (CF) calculation approach was employed to analyze the changes of CFs in China’s crop production and identify the contributions of various agricultural inputs to total CFs. The mean overall CF of China’s crop production was estimated to be0.78±0.08t CE ha-1yr-1and0.11±0.01t CE t-1yr-1,for land use and bulk production respectively. For the duration the data covered, the carbon intensity under cultivation land use was seen to increase since1993. Among the total, fertilizer induced emissions exerted the largest contribution of-60%. Compared to the UK, the estimated overall CF of China’s crop production was higher in terms of cultivation land use. While there was a significant positive correlation of carbon intensity with total production, carbon efficiency was shown in a decreasing trend during2003-2007. Therefore, low carbon agriculture should be pursued, and the priority should be given to improving fertilizer use efficiency in agriculture of China. Then data of cultivation area, grain yield, application rates of fertilizer, pesticide, diesel, plastic film, irrigated water, etc., for the major grain crops in China were collected from the national statistical archive and CF of direct and indirect carbon emissions associated or caused for these agricultural input was assessed with published emission factors. In general, paddy rice, wheat, corn and soybean in China had the mean CFs with2472,794,781and222kg carbon equivalent (CE) ha-1 in area, and0.37,0.14,0.12and0.1kg CE kg-1yield in grain yield, respectively. For dry crops,78%of the total CFs was contributed by N fertilizer use, with which direct soil N2O emission and indirect emissions from N fertilizer manufacture. For flooded rice paddy, direct CH4emission contributed69%to the total CFs. Moreover, the variations in CFs of dry crops among different provinces could be mostly explained by the difference in N fertilizer application rates while CH4emissions could explain85%of the variation in the CF across provinces for paddy rice. When a reduction in N fertilization by30%is considered, a potential of GHGs reduction of60Mt CO2-eq from production of these crops could be projected. This work highlighted opportunities to gain GHG mitigations in grain crops production associated with good management practices in China.(4) Carbon trading has been developed rapidly in recent years under the context of climate change mitigation, and quantifying carbon sequestration and GHGs emission reduction in the projects is the basis for carbon trading. Therefore, it is vital to develop the methodologies for quantifying the projects. Recommended fertilization use fertilizers rationally to increase production, and also reduce greenhouse gas emissions and improve soil carbon storage at the same time. The methodology on quantifying carbon sequestration and GHGs emission reduction was discussed from the aspects of the setting of the boundary and baseline, the selections of carbon pool and key GHGs emission sources and measurement methods, with the purpose of preparing for the recommended fertilization methodology in future. A methodology framework was identified to quantify the GHG emission reduction in recommended fertilization. The GHG emissions in conventional fertilization was set as a baseline and the boundary was scaled in field plot. In this framework, the key GHG sources include N fertilizer induced N2O emissions, CH4emission from rice paddy and the GHG emissions involved in fertilization equipment, and the soil carbon pool was selected to be quantified. GHG emissions during the process of fertilizer formula determination was regared as a leakage.(5) To assess the topsoil carbon sequestration potential (CSP) of China’s cropland, two different estimates were made:a) a biophysical potential (BP) using a saturation limit approach based on SOC accumulation dynamics, and a storage restoration approach from the cultivation-induced SOC loss, and b) a technically attainable potential (TAP) with a scenario estimation approach using SOC increases under best management practices (BMPs) in agriculture. Thus, the BP is projected to be the gap in recent SOC storage to either the saturation capacity or to the SOC storage of uncultivated soil, while the TAP is the overall increase over the current SOC storage that could be achieved with the extension of BMPs. The recent mean SOC density of China’s cropland was estimated to be36.44t ha-1, with a BP estimate of2.21Pg C by a saturation approach, and2.95Pg C by the storage restoration method. An overall TAP of0.62Pg C and0.98Pg C was predicted for conservation tillage plus straw return, and recommended fertilizer applications, respectively. This TAP is comparable to40%-60%of total CO2emissions from Chinese energy production in2007. Therefore, carbon sequestration in China’s cropland is recommended for enhancing China’s mitigation capacity for climate change. However, priority should be given to the vast dry cropland areas of China, as the CSP of China is based predominantly on the dry cropland.(6) The prediction of CH4emissions from rice paddies could play a key role in GHG mitigation efforts associated with agriculture. We describe a methanogenesis sub-model that has been developed in the DAYCENT ecosystem model for estimating CH4emissions and assessing mitigation potentials for rice paddies. Methanogenesis is modeled based on the simulation of soil hydrology and thermal regimes, rice plant growth, SOM decomposition, and CH4transport from the soil to atmosphere. A total of97sites from China’s rice paddies were used to develop and evaluate the model, in which25sites (91observations) were used for parameterization and72sites (204observations) were used for model evaluation. Comparison of modeled results with measurements demonstrated that CH4emissions in rice paddies of China can be successfully simulated by the model with an overall R2of0.83, and included an evaluation of CH4emissions for a range of climates and agricultural management practices. The model was most sensitive to parameters influencing the amount of labile C available for methanogenesis(7) We uses the DAYCENT ecosystem model to predict GHG mitigation potentials associated with soil management in Chinese cropland systems. DAYCENT was evaluated with data from350experiments in China’s cropland, including measurements of N2O, CH4emissions and SOC stock changes. In general, the model was reasonably accurate with R2values for model predictions versus observations ranging from0.71to0.85. Modeling efficiency varied from0.65for SOC stock changes to0.83for crop yields. Mitigation potentials were estimated on a yield basis (Mg CO2-eq. Mg-1Yield). The results demonstrate that the largest decrease in GHG emissions in rainfed systems are associated with combined effect of reducing mineral N fertilization, organic matter amendments and reduced-till coupled with straw return, estimated at0.31to0.83Mg CO2-eq. Mg-1yield. A mitigation potential of0.08to0.36Mg CO1-eq. Mg-1yield is possible by reducing N chemical fertilizer rates, along with intermittent flooding in paddy rice cropping systems.In this thesis, a database was built to develop and apply the accounting and simulating methods, and both of soil carbon sequestration and greenhouse gas emissions were considered. This thesis improved the approach for evaluating and predicting of GHG emissions reduction potential in the cropland, developed an agricultural carbon accounting methodology, and constructed an analysis framework for assessing carbon footprint of crop production. The characteristic of high carbon input and low carbon efficiency in Chinese agricultural production was proposed, and the GHG emission reduction potential and its technical approach were revealed. This thesis provide a solid scientific basis and technical support for carbon sequestration and GHG mitigation in Chinese agricultural production.