| Nitrous oxide (N2O) is an important trace gas that causes global warming and stratospheric ozone depletion. Global atmospheric N2O concentration has increased from a pre-industrial value of about 270 ppb to 319 ppb in 2005. More than a third of all N2O emissions are anthropogenic and are primarily due to agriculture. Nitrogen fertilization is considered the primary source of N2O emissions from agricultural soils, and is often increased where available N exceeds plant requirements.Agricultural land in China comprises only 7% of the world’s arable land area but feeds some 22% of the global population. To meet the food demand of growing population, consumption of N-based fertilizers in China has continuously increased since 1970s. China is now the largest consumer of synthetic N in the world, accounting for～30% of the world total use. The use of synthetic N fertilizers will continue to increase for the foreseeable future in an effort to meet food demand, which will inevitably cause additional N2O emissions. Reduction of N2O emissions from synthetic N consumption in China is therefore crucial for mitigating global synthetic N-induced N2O emission.The industrial production of fertilizer also releases carbon dioxide, an important greenhouse gas (GHG), into the atmosphere. It is unquestionable that cutting synthetic N use where available N exceeds plant requirements can reduce not only N2O emission from croplands but CO2 emissions from fertilizer production. However, such a reduction has not been well quantified in China.The objectives of this study are,1) to evaluate the spatial character of N fertilizer use efficiency, and 2) to quantitatively estimate the GHG mitigation potential under various scenarios of N fertilizer use efficiency(NUE) and its regional distribution in mainland China and therefore support policy makers for sensible region-oriented mitigation decisions.This study focused on six cultivated species, including rice, wheat, maize, soybean, cotton and rapeseed. we collected data published in Chinese journals between 1990 and 2000, combined with formal statistical data, including crop yield, planted area, N fertilizer application rate, the precipitation, the location of the measurement, water management, soil physiochemical parameters including soil total N content, soil organic matter,clay content, soil available potassium and soil available phosphorus. Statistical models of percentage of soil fertility contribution(Ps) against soil properties and geographical locations were developed by using stepwise regression. And the models have quite good explanations on the observed datasets.With Ps statistical model and NUE model, Ps, the N fertilizer applied amount and NUE can be evaluated during last 1990s. The major high Ps areas of crops except maize were distributed in the eastern coastal areas, the Huang-Huai basin and Yangtze River Valley, especially in the Central, East China and Sichuan Basin under the suitable conditions of soil and hydrothermal for crop growth. In contrast, the low Ps regions were primarily located in the northern part of North China and the southwest hills under unfavorable environment conditions. The high Ps areas of maize was in northeast China and southwest China. The area-weighted mean rate of synthetic N application were 191,190,189,45,212,172kg ha-1 for rice, wheat, maize, soybean, cotton and rapeseed, respectively. The NUE across mainland China was estimated to be 31% for rice,33% for wheat,32% for maize,39% for soybean,29% for cotton and 31% for rapeseed. The NUE in the eastern coastal areas, North China Plain and Sichuan Basin were about 22%-33%, the lowest in the mainland of each crop.Improving NUE in the areas where the estimated NUE is lower than 30-50% could cut 3.40-7.73×106 t yr-1 of synthetic N use per year, accounting for 18-42% of the total used. The over-applied amout were 3.40,5.74 and 7.73×106 t yr-1 under 30%, 40% and 50% scenarios. Approximately 85% of the total over-applied N came from rice, wheat, and maize cropland. Jiangsu, Henan, Shandong, Hebei, Guangdong, Sichuan, Liaoning, Hunan, Hubei and Anhui Province were the top ten ranked, together accounting for～63% of the national total. On average, every 1% NUE increase could cut synthetic N use by 220×103t yr-1.Based on NUE model, GHG(N2O and CO2) mitigation potential was estimated by using N2O emission model and the emission factor of CO2 in N fertilizer production. The total amount of GHG was estimated to be 187.8 (148.4-220.5) Tg CO2_eq. yr-1 when N2O and CO2 emissions were combined for 6 crops. The N2O mitigation potential ranged from 52.9 Tg CO2_eq yr-1 (30% NUE scenario) to 123.4 Gg N2O yr-(50% NUE scenario) through N fertilizer application, accounting for 16%-38% of the total direct N2O emission. CO2 mitigation potential ranged from 16.4 Tg CO2 yr-(30% NUE scenario) to 37.3 Tg CO2 yr-1 (50% NUE scenario). The direct N2O emission from croplands together with CO2 emission from the industrial production and transportation of synthetic N could be reduced by 17-39%, equivalent to 32.3-74.1 Tg CO2 per year. Improving NUE by 1% can allow the total emission reduced by～2 Tg CO2_eq yr-1. Jiangsu, Anhui, Shandong, Henan, Sichuan and Hubei hold the highest GHG mitigation potential.Uncertainties of this study principally originated from the parameters and models. By validating the results and comparing with other models, the results of this study are comparable and reliable. It should be noticed that the NUE might have been overestimated, since that the organic N supply and N deposition were not taken into account due to insufficient data. Based on available information, the organic N supply and N deposition supply was estimated to be 26±10,29±13,31±14 kg N ha-1and 14±9, 12±7,10±2 kg N ha-1 for rice, wheat, and maize, respectively. The N application rate from both synthetic and organic sources would account to～230 kg ha-1 on the national scale. Accordingly, the NUE would be 25-27%. Thus present mitigation potential might have been underestimated. |
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