Estimating the volatilization of ammonia from synthetic nitrogenous fertilizers used in China
Introduction
Over the past 25 years a growing body of scientific evidence indicates that ammonia (NH3) released from agricultural production processes may have significant environmental impacts, including affects on the local, regional and global nitrogen cycles (Matthews, 1994, Vitousek et al., 1997, Zheng et al., 2002, Galloway et al., 2008, Gruber and Galloway, 2008). Ammonia is a major alkaline constituent in the atmospheric boundary layer, and plays a certain role in local atmospheric conditions, particularly in alleviating the impacts of acidic emissions to the regional environment and agro-ecosystem (Li and Sun, 1996; Su et al., 2003, He et al., 2007, Zhang et al., 2008). In addition to nutrient enrichment (eutrophication) of terrestrial and aquatic systems and global acidification of precipitation (through oxidation of ammonium sulfate to nitric and sulfuric acid), NH3 has also been shown to be a major factor in the formation of atmospheric particulate matter and secondary aerosols (Anderson et al., 2003, Kim et al., 2006, Pinder et al., 2007, Sharma et al., 2007), leading to potential adverse effects on human and ecosystem health (Powlson et al., 2008).
Ammonia volatilization after fertilizer application is an important pathway of nitrogen (N) movement from the soil, resulting in large losses of soil-plant N (Harrison and Webb, 2001, Pacholski et al., 2008). The volatilized NH3 represents a significant fraction of current emission inventories and is typically considered the second most important source of NH3 in many countries. Anderson et al. (2003) reported that, in developed countries, 10–20% of the emissions may originate from applied fertilizer. In developing countries, NH3 volatilization from nitrogen fertilizer ranges from 10% to 50% (Streets et al., 2003).
Previous research has shown that large quantities of atmospheric NH3 in China originate from agricultural fertilizers. Zhao and Wang (1994) estimated that NH3 derived from fertilizer accounted for 46.1% of the total emissions in China in 1990, and represented up to 25.5% of all anthropogenic sources in Asia. Streets et al. (2003) reported similar estimates, with fertilizer-derived NH3 accounting for 50% of the anthropogenic emissions in China and about 24.5% of the total emissions in Asia in 2000. The growing volumes of fertilizers that are used in agriculture are the primary driver behind the current situation (Bouwman et al., 2002a, Zheng et al., 2002, Ju et al., 2004). The total amount of N nutrient in synthetic fertilizers used in China increased from 9.43 Tg (Tg) in 1980 to 26.86 Tg in 2005, with an annual growth rate of 7.9% (Editorial Board of China Agriculture Yearbook, 2006). In 2005, 26.86 Tg of N nutrients in synthetic fertilizers were consumed in China, accounting for 56.4% of national total nutrient consumption and 34.7% of the total amount of the world N consumption (Editorial Board of China Agriculture Yearbook, 2006). The annual amount of N applied to all of the arable land in China reached 232 kg ha−1 in 2005, which was more than twice the world average (NBSC, 2007). The dominant kind of synthetic nitrogen fertilizer that is being applied in China has gradually shifted over 30 years (Fig. 1). While ammonium bicarbonate (ABC, 17% N) was the dominant form in the mid-1970s, urea (46% N) became the more commonly used form after 1997 (Sun and Wang, 1997). By 2005, urea and ABC accounted for about 50 and 40%, respectively, of the fertilizer N applied in China.
Several factors have contributed to the dramatic rise in the use of nitrogenous fertilizers in China. Use of synthetic fertilizers in many countries grew substantially in the latter half of the 20th century as farmers forced higher crop production out of the same, or fewer, hectares of farmland. China has been particularly successful in increasing agricultural production, generating enough grain for 21% of the global population with only 9% of the world’s arable land. Unfortunately, as a consequence of this increasing demand for food, Chinese farmers tend to apply N fertilizers at excessively high rates (Zhu and Chen, 2002, Xiong et al., 2008). This is especially true in the eastern regions (Richter and Roelcke, 2000, Ju et al., 2004, Ju et al., 2009, Yan, 2008). The over-reliance on N is not surprising considering that N fertilizers are more effective than P and K fertilizers in increasing crop yields, have no obvious adverse effect on soil and are recommended by the government (Zhang et al., 2006). Finally, as in many countries, application of organic manures (e.g., waterlogged compost) has been gradually abandoned in most parts of China, mainly due to high labor costs and rising incomes from off-farm activities (Ma et al., 2009, Ju et al., 2009).
The high amounts of fertilizer used in China contribute to significant volatilization of NH3. Unfortunately, there are few studies from China that provide substantial quantitative estimates of the amount of NH3 lost to the atmosphere. While global or regional assessments of NH3 volatilization from fields in China or East Asia have been completed using simplified emission factors, those estimates are very approximate and associated with a high degree of uncertainty (Matthews, 1994, Zhao and Wang, 1994, Bouwman and Van Der Hoek, 1997, Bouwman et al., 1997, Bouwman et al., 2002a, Bouwman et al., 2002b, Bouwman et al., 2005a, Bouwman et al., 2005b, Van Aardenne et al., 2001, Streets et al., 2003). Most of these estimated inventories were compiled simply by multiplying defined emission factors (e.g., emissions per unit of urea applied) obtained from developed countries by the corresponding agricultural activity data. These methods for estimating NH3 emissions are associated with considerable uncertainty since emissions are site- and season-specific; it is not surprising that they have drawn considerable criticism (Battye et al., 2003, Goebes et al., 2003, Streets et al., 2003, Kim et al., 2006). Webb et al. (2009) have proposed that, where NH3 emissions are estimated using factors measured elsewhere, the degree of error may be as high as 100%. Such a large amount of uncertainty and potential error can lead to highly inaccurate mapping of emissions which can, in turn, result in significant errors in the assessment of potential environmental impacts. Kim et al. (2006) found that calculated NH3 emissions in East Asia may have been overestimated by a factor of 1.1–3.8.
There is an ongoing need for more realistic emission inventories based on accurate factors (Hutchings et al., 2001, Fischer et al., 2010). Given this critical demand, there is a strong need for high resolution estimates of NH3 volatilization based on the distribution of all types of N fertilizers and their emission factors from different crops and during different seasons. The purpose of the research described in this paper was to quantify the regional distribution and seasonal variations, as well as the overall magnitude of NH3 emissions, in China. The base year for our analysis was 2005.
Section snippets
Agricultural activity data
Agricultural activity level was based on statistical data and farmers’ traditional agronomic practices. Total nitrogen fertilizer consumption was determined using Chinese county census data for 2005. The total quantity of N-containing fertilizers applied and the sown area for 2448 counties in China were collected from available county- and provincial-level yearbooks1
Evaluation of the NARSES model
Emission factors obtained from the NARSES model were needed for the construction of an accurate inventory of NH3 losses and, subsequently, to estimate environmental impacts. The more robust the modeled emission factors, the more confidence can be placed on the resulting impact assessments.
Data on volatile NH3 losses following traditional crop management and nitrogen fertilizer application methods under field conditions were selected. These experiments simulate the traditional application
Conclusions
The anthropogenic emissions of NH3 from synthetic nitrogenous fertilizers used in China were calculated for 2005. The total NH3 emission for China was estimated to be 3.55 Tg N, with an estimated 13.23% of nitrogen in fertilizer volatilizing as NH3, with an uncertainty of ±50%. It was estimated that 35.5 kg (NH3–N) ha−1 arable land were emitted in 2005. Ammonia emissions from fertilizers vary seasonally; the inventory accounts for these fluctuations by presenting emission estimates on a monthly
Acknowledgements
This work was financially supported by the Chinese National 863 Key High-Tech Scientific Programs via grants 2006AA06A309 and the National Key Technology R&D program in the 11th Five-Year Plan (Project number: 2007BAC16B04).
The authors appreciate Dr. Eugene Y. Leong and Dr. David Pillard very much for their kind help in providing valuable comments and editing the manuscript that improved the manuscript greatly.
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