| Agriculture has a very high share in the anthropogenic emissions of greenhouse gases, especially N2O. In 2005, the direct N2O emission from agricultural soils accounted for 37% of the total N2O emission in China. There are significant opportunities for greenhouse gases mitigation in agriculture. Nitrogen (N) fertilization is the primary source of N2O emission from agricultural soils. And greenhouse gases emission also come from manufacturing and transport of synthetic N fertilizer. In this research, the over-applied amount of synthetic N fertilizer and mitigation potential (MP) of greenhouse gases under different scenarios of recommended synthetic N application rate for main grain crops in China were estimated based on the analysis of national statistical data, and the effects of substituting synthetic fertilizers with composted residues from rice straw-pig manure composting in greenhouse for CO2 fertilization on greenhouse gases emission reduction were also evaluated using data mining method. The main research results are listed as follows:1. The over-applied amount of synthetic N fertilizer under different scenarios of recommended synthetic N application rate for main grain crops in ChinaRelated research suggests that recommended synthetic N application rates for main grain crops in China should be in the range of 130-150 kg ha-1. This study aimed at estimating the amounts of over-applied synthetic N under different scenarios of recommended synthetic N application rate (150,140 and 130 kg ha-1) for main grain crops in China based on the provincial level statistical data. Our research indicates that the total consumption of synthetic N on main grain crops in China can be reduced by 6.11-7.54 Tg yr-1 (36-45% of the total consumption) under the recommended synthetic N application rate scenarios of 130-150 kg ha-1. On average, lowering synthetic N application rate by 10 kg ha-1 can save synthetic N by 0.71 Tg yr-1.In terms of the total amounts of over-applied synthetic N for main grain crops at the provincial level, Shandong, Henan, Jiangsu, Hebei, Guangdong, Anhui, Shannxi, Sichuan, Hubei and Hunan provinces were ranked as the top ten. And the total estimated amounts of over-applied synthetic N for these provinces under synthetic N application rate scenarios of 150,140 and 130 kg ha-1 were 4.63,5.04 and 5.46 Tg yr-1, respectively, together accounting for 76%,74% and 72% of the national total.2. Mitigation potential of greenhouse gases under different scenarios of recommended synthetic N application rate for main grain crops in ChinaSynthetic N fertilization is recognized as the most significant factor contributing to the N2O emission from soil. Moreover, there are hidden carbon "costs" in the process of synthetic N production, transportation and application. If the synthetic N application rates for main grain crops in China could be controlled at 130-150 kg ha-1, the estimated total MP of greenhouse gases, including direct N2O emission (79.58-97.02 Gg N2O-eq yr-1) from croplands and indirect greenhouse gases emission (51.53-62.60 Tg CO2-eq yr-1) from production and transportation of synthetic N, ranges from 75.24 to 91.51 Tg CO2-eq yr-1. Reducing synthetic N application rate by 10 kg ha-1 can mitigate the total emission of greenhouse gases by 8.1 Tg CO2-eq yr-1.It was concluded that reducing synthetic N application rate for main grain crops in China to a reasonable level of 130-150 kg ha-1 can greatly reduce the emission of greenhouse gases, especially in the major grain-crop production provinces such as Shandong, Henan, Jiangsu, Hebei, Anhui, Shannxi, Guangdong, Sichuan, Hubei and Liaoning. And the estimated MP of greenhouse gases for these provinces under synthetic N application rate scenarios of 150,140 and 130 kg ha-1 were 56.95,61.94 and 66.93 Tg CO2-eq yr-1, respectively, together accounting for 76%,74% and 73% of the national total.3. Analysis of greenhouse gases MP for substituting synthetic fertilizers with composted residues from rice straw-pig manure composting in greenhouse for CO2 fertilizationIn recent years, the total output of crop straw in China was estimated to be 0.8 billion t, however, approximately 20% of which was burnt openly in the field after harvesting. At the same time, the annual output of animal manure generated from Chinese livestock and poultry breeding exceeds 2.0 billion t. Open burning of crop straw is an important source of air pollutants. And the direct discharge of animal manure into waterbodies has become a major cause of water pollution. In order to create a more holistic approach for utilizing agricultural organic wastes and nutrients resources, a new strategy for CO2 fertilization was developed by composting rice straw and pig manure in greenhouse.The annual processing rate for rice straw-pig manure composting was approximately 30 t ha-1 of rice straw and 10t ha-1 of pig manure. Taking composting rice straw and pig manure in greenhouses for CO2 fertilization in Zhejiang province as a case study, under an assumed extension area of 4000-20000 ha, the composted residues could provide 793-3963 t N yr-1,476-2382 t P2O5 yr-1 and 251-1254 t K2O yr-1. Partial replacement of synthetic fertilizers by composted residues could reduce 7.89-39.47 Gg CO2-eq yr-1 emission from fertilizer industry. Along with the avoided 43.13-215.66 Gg CO2-eq yr-1 emission from the soils caused by open burning of rice straw, the greenhouse gases MP from rice straw-pig manure composting would range from 54.90 to 274.50 Gg CO2-eq yr-1.In the case of additional extension area of 1000 ha, replacement of synthetic fertilizers by composted residues could reduce 1.97 Gg CO2-eq yr-1 emission. And the contribution of N, P2O5 and K2O would be 1.64,0.28 and 0.04 CO2-eq yr-1, respectively. In addition, this strategy has characteristics of low cost, easy operation and remarkable economic benefits, which would encourage farmers to adopt this technique. Therefore, this strategy could be an environment-friendly practice for promoting optimization of N management and reduction of greenhouse gases emission from agriculture.It was concluded that reducing synthetic N application rates for main grain crops in China has a great potential for mitigating the emission of greenhouse gases. And the utilization of composted residues from composting rice straw and pig manure for CO2 fertilization could help to reduce the consumption of synthetic N fertilizer. This study could provide scientific basis for Chinese central and regional governments to make reasonable policy decisions for mitigation of greenhouse gases at the national or provincial level. Uncertainties of this study principally originated from uncertainties in relevant parameters adopted. By comparing our results with results of other research, the results of this study are reliable. |
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