Reactive nitrogen releases and greenhouse gas emissions during the staple food production in China and their mitigation potential

Reactive N (Nr) releases are closely linked with greenhouse gas (GHG) emissions, and the simultaneous evaluation of them can help to develop overall effective mitigation options. In this study, we evaluated the characteristics of the Nr and GHG releases from staple food (rice, flour and corn-based fodder) production in China (2001-2010) and explored their mitigation potential. Results showed that there was a high spatial variation in the Nr and carbon footprints. Provincial Nr footprints had a significant linear relationship with carbon footprints, attributed to large contribution of N fertilizer use to both GHG and Nr releases. NH3 volatilization and N leaching were the main contributors to the Nr footprints, while synthetic N fertilizer applications and CH4 emissions dominated the GHG (carbon) footprints. About 10 (95% uncertainty range: 7.4–12.4) Tg Nr-N and 564 (404–701) Tg CO2 eq GHG were released every year during 2001–2010 from staple food production in China. This caused the total damage costs of 325 (70–555) billion ¥, equivalent to nearly 1.44% of the Gross Domestic Product of China. A reduction of 92.7 Tg CO2 eq yr and 2.2 Tg Nr-N yr could be achieved by reducing synthetic N inputs by 20%, increasing grain yields by 5% and implementing off-season application of straw and mid-season drainage practices for rice cultivation. Key Words Reactive nitrogen, greenhouse gas, food production, damage costs Introduction Staple food (rice, flour and corn-based fodder) production in China are projected to release substantial reactive N (Nr) and greenhouse gas (GHG), due to the excessive use of N fertilizer and other agricultural material (e.g., manure), and the lower current N use efficiency (Yan et al., 2005; Chen et al., 2014). Increases in Nr and GHG emission will cause a cascade of environmental problems, such as air pollution, stratospheric ozone depletion and eutrophication (Galloway et al., 2008). There is increasing evidence that Nr release (Nr footprint) is closely interlinked with GHG emission (carbon footprint) (Cui et al., 2013). For example, NH3 volatilization, N2O emission and N leaching, could be promoted linearly or exponentially, through N fertilization use, where N fertilizer production is an important contributor to carbon footprint (Zhang et al., 2012). But few studies have evaluated both the Nr releases and GHG emissions through the perspective of life-cycle analysis in regional scales. This impairs the development of mitigation options for the simultaneous reduction of both Nr and GHG emissions. In this study, we did a preliminary evaluation of the Nr and carbon footprints of staple food production in China. We then assessed the total Nr and GHG releases, and their associated damage costs to environment and humans, as well as the mitigation potentials. Methods System boundaries The system boundaries of the Nr and carbon footprints in our study were set as the period from the production of agricultural inputs to the distribution of the food to market. The food production and agricultural inputs data (between 2001 and 2010) come from the website of the National Bureau of Statistic of the People's Republic of China (http://www.stats.gov.cn/). Nr and carbon footprint The Nr footprint (g N kg food) was calculated using the following equation: Nr footprint = ( AI#$% m i=1 + FC($%+ n j=1 FP* $%) / food yield g=1 , (1) where AI#$%denotes the Nr that is lost during the production and transportation of agricultural inputs (AI); FC($%represents the Nr lost during farm cultivation(FC); and FP* $% denotes the Nr discharged food processing (FP) and transportation. The GHG footprint (g N kg food) was calculated using the following equation: