| Nitrous oxide(N2O),an important atmospheric trace gas,contributes greatly to the global warming and stratospheric ozone depletion.Agricultural ecosystem has been recognized as one of the major sources of atmospheric N2O.China’s crop production,characterized by vast cultivation area and high fertilizer N input,has become a hotspot for terrestrial ecosystem N2O emission in global greenhouse gas research.The Intergovernmental Panel on Climate Change Guidelines for National Greenhouse Gas Inventories[IPCC(1997,2000,2006)],devide the N2O emission from cropland into two parts including fertilizer induced N2O emission and background N2O emission from soil nitrogen transformation.The Emission Factor(EF)has been introduced by IPCC to quantify the fertilizer induced N2O emission,indicating that the linear regression can be employed to simulate the direct N2O emission from China’s cropland.Rice and vegetable are China’s two major crops representing paddy and upland productions.According to China’s statistic yearbook,the 30.06 million hectare of rice and 19.64 million hectare of vegetable planted in 2010 respectively comprise 18.5%and 12.1%of China’s total crop cultivation area(162.28 million hectare).The International Fertilizer Industry Association(IFA)estimates that 5.02 Tg and 6.55 Tg of fertilizer N were applied to China’s rice and vegetable production in 2010-11,representing 15.4%and 20.1%of China’s chemical fertilizer N consumption.Therefore,the N2O emissions from China’s vegetable and rice growing fields have been an important part of national greenhouse gas inventories,which need a good estimate as accurate as possible.Based on the IPCC emission factor methodology,this thesis summarized and analyzed field measurements of N20 emission from upland vegetable and rice growing paddy fields that were published in peer-reviewed English and Chinese journals,therefore determined the N2O emission factors and background N2O emissions by Ordinary Least Squares(OLS)regression analysis.Statistics about China’s rice and vegetable production were then collected and input into the direct N2O emission models to estimate the total amount of N2O direct emissions from China’s rice and vegetable production.Various water management regimes,including continuous flooding(F),flooding-midseason drainage-reflooding(F-D-F),and flooding-midseason drainage-moist intermittent irrigation but without water logging(F-D-M),have been practiced in China’s paddy rice production,which often pose impact on the direct emission of greenhouse gas N2O.A total of 142 measurements from 34 paddy rice field studies,were summarized and analysed using OLS linear regression method,which showed that seasonal total N2O was roughly 0.05%of the nitrogen applied in the continuous flooding rice paddies,without obvious linear relationship;while under the water regime of F-D-F or F-D-M,seasonal N2O emissions increased linearly with N fertilizer applied in rice paddies,with N2O emission factor averaged 0.25%for F-D-F and 0.76%for F-D-M water regime.During the paddy rice growing season,the background N2O emissions were 0.37 kgN2O-N-ha-1 and 0.43 kgN2O-N·ha-1,for F-D-F and F-D-M water regime,respectively.These statistic models were then adopted to estimate changes in direct N2O emissions from paddy fields during rice growing season in mainland China.According to statistics of paddy rice cultivation area,chemical fertilizer nitrogen input and water regimes in China’s mainland,the models estimated that seasonal N2O emissions have increased from 10.06 GgN2O-N each year in the 1950s to 28.19 GgN2O-N each year in the 1990s,then decreased to 24.92 GgN2O-N each year for period 2005-2014.The uncertainties in N2O estimate in this study were estimated to be 59.8%in the 1950s and 37.5%during 2005-2014.The averaged N2O emission rate is highest in single-rice field of North China Plain and central&western areas,due to its high fertilizer nitrogen application rate.While the Middle and Lower Reaches of Yangtze River contributes the greatest share(51%-54%)to national paddy rice N2O emissions,mainly due to its large rice planting area,high fertilizer N application amount,and high ratio of F-D-M water regime.The relative low ratio of N2O emission from paddy rice fields to annual total emission from croplands,ranging 7%-10%in the 1990s and 3%-8%in the 2000s,indicates that paddy rice development in the past decades could have greatly contributed to mitigating agricultural N2O emissions.The N2O emission from vegetable fields contributes to national greenhouse gases budget.However,characteristics of high N application inputs,frequent irrigation,intensive production and multiple planting-harvest cycles during the vegetable growing year,make it quite different for N2O emission pattern and quantity compared to paddy rice or other upland crop productions.Furthermore,China’s vegetable cropping systems increasingly relies on facilities,such as plastic film greenhouses or glasshouses.Yet reliable estimation of N2O emissions from vegetable production in the world has been lack.The 78 N2O measurements from 26 field studies of vegetable production compiled from previously published English and Chinese journals showed that annual N2O emissions increased with N fertilizer applied in vegetable fields.The OLS linear regression model produced that the N2O emission factor averaged 0.51%and 1.79%,with background N2O emission of 2.20 kgN2O-N-ha-1 and 1.97 kgN2O-N-ha-1,for greenhouse and open-air vegetable cropping system,respectively.Integrated with the input data on vegetable cultivation area,chemical fertilizer N application rate,multiple cropping index,the models estimated that the vegetable production emitted 115.70 GgN2O-N in 2011,including 97.65 GgN2O-N induced by chemical fertilizer N input and 18.05 GgN2O-N of background emission,with the uncertainty estimated to be 32.8%.The areas of Northern and Central China contribute the greatest share to national N2O emission from vegetable cultivation.The uncertainties in N2O estimate of this study were estimated to be as high as 95.1%,implying the pressing need for underlying mechanism study of N2O emission from vegetable fields.Vegetable field contributes 7%to China’s total cropland area,yet 13.2%-42.1%of China’s annual agricultural N2O emission in 2011,which indicates that vegetable development in the 21st century could have greatly contributed to the increase of agricultural N2O emissions.Although vegetable cultivation area(19.64 million hectare)is only roughly 2/3 of that of paddy rice(30.06 million hectare),the amount of chemical fertilizer N applied in vegetable production(6.50 TgN)has already exceeded that of paddy rice development(5.42 TgN)in 2011.In addition,the practice of flooding is still mainstream in China’s rice production,though it is more and more interrupted by short term drainage or moist irrigation,which also gives reason for the low N2O emission from rice production(24.92 GgN2O-N in 2011)compared with that from the upland vegetable development(115.70 GgN2O-N in 2011).Relative to paddy rice production,the vegetable development have greatly contributed to China’s agricultural N2O emissions,which deserves more attention from climate change researches. |
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