Ammonia volatilization from synthetic fertilizers and its mitigation strategies: A global synthesis
Introduction
The excessive release of reactive nitrogen (Nr) poses adverse impacts on the natural biogeochemical cycle of N, causing negative consequences on water, air and land (Fowler et al., 2013, Galloway et al., 2008). As a species of Nr, ammonia (NH3) is lost via volatilization as one of the main pathways of N loss in agricultural systems. While >40% of the applied N is reported to be lost as NH3 under certain environmental and edaphic conditions (Singh et al., 2013), an average of 1014% of N is lost via volatilization from synthetic fertilizers (Bouwman et al., 2002, De Klein et al., 2006). Globally, the approximated demand for N fertilizers was 112 million tons of N in 2014 (FAO, 2015). Using the IPCC default value (10%, De Klein et al., 2006) and Bouwman et al. (2002)s average value (14%) for volatilization from applied N, 11.215.7 million tons of fertilizer-N are lost as NH3-N globally. This N loss as NH3 represents a substantial financial cost to farmers. Furthermore, based on the IPCCs default emission factor for indirect N2O emission from N volatilization and deposition (EF4) of 1% (De Klein et al., 2006), this loss equates 0.10.16 million tons of indirect N2O-N emission, or 52.473.5 million tons of carbon dioxide equivalent (CO2-e). However, this indirect connection between NH3 and N2O emissions is often neglected, and in most countries, there are no regulations or incentive programs to tackle the challenge of NH3 volatilization (Behera et al., 2013).
Mitigation strategies for NH3 volatilization from N applied in agricultural systems are widely studied. The 4R nutrient stewardship concept (right fertilizer source, rate, place and time) was introduced by Bruulsema et al. (2009) to achieve cropping system goals with economic, social and environmental benefits. For example, when compared to urea (the most commonly used N fertilizer), alternative N source such as ammonium sulphate, diammonium phosphate and calcium ammonium nitrate decreased NH3 volatilization by 2255% (Bayrakli, 1990). It is widely reported that NH3 volatilization increased with N application rate (Black et al., 1985, Bosch-Serra et al., 2014, Turner et al., 2012). Sub-surface banding or deep placement of urea reduced NH3 volatilization when compared to surface broadcast of urea on calcareous or well-buffered soils (Cai et al., 2002, Rao and Batra, 1983). Rochette et al. (2013) found that urea applied at depths >7.5 cm resulted in negligible NH3 volatilization. Split, band application decreased NH3 volatilization when compared to a single, surface application (Junejo et al., 2013). However, in Rodgers et al. (1984)s study, NH3 volatilization tended to increase under split application in summer.
Farming practices such as adjusting irrigation amount can mitigate NH3 volatilization by 4790% (He et al., 2014, Holcomb et al., 2011, Zaman et al., 2013). In contrast, compared to zero water application, NH3 volatilization was increased by 9% when a 3 mm water was added to the soil immediately after urea application (Sanz-Cobena et al., 2011). The retention of crop residues on the soil surface is a common feature in conservation farming. Nevertheless, this may form a barrier which prevents urea from reaching the mineral soil, and may increase NH3 volatilization (Su et al., 2014). Recent studies focused on mitigating NH3 volatilization using inexpensive amendments such as natural mineral or industrial by-products or chemicals that have high ammonium binding capacity e.g. zeolite (Ahmed et al., 2006b, Bundan et al., 2011) or acidifying effects e.g. humic acid or fulvic acid (Ahmed et al., 2006a, Reeza et al., 2009).
In addition to farm management practices, there have been numerous studies on enhanced efficiency fertilizers in mitigating NH3 volatilization from agroecosystems. For example, urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) was found to be more effective than phenyl phosphorodiamidate in retarding urea hydrolysis and more widely used because NBPT works at a low concentration and is easy to store (Chien et al., 2009, Saggar et al., 2013). Nonetheless, the effects of urease inhibitors varied with edaphic and environmental conditions (Suter et al., 2013). Controlled-release fertilizers such as polymer sulphur-coated urea and polyolefin-coated urea can improve N use efficiency, grain yield and pasture quality (Chen et al., 2008). However, Hawke and Baldock (2010) found that sulphur coated urea showed higher NH3 volatilization (10%) compared to uncoated urea. Nitrification inhibitors prevent or slow the microbial conversion of ammonium (NH4+) to nitrate (NO3) (Lee et al., 1999). Although nitrification inhibitors are designed to target N2O emissions, the use of these inhibitors may prolong the retention of NH4+ in the soil resulting in NH3 volatilization (Kim et al., 2012, Lam et al., 2016, Ni et al., 2014).
While studies on the mitigation strategies for NH3 volatilization are sometimes inconclusive, a systematic synthesis of these studies is lacking. To fill this knowledge gap we report the results of a quantitative synthesis of the current literature on the mitigation strategies of NH3 volatilization to provide critical information on how to minimize N loss as NH3 in agricultural systems. The information is crucial for improving global fertilizer N use efficiency, environmental quality, and climate change mitigation.
Section snippets
Database compilation
This meta-analysis was conducted based on studies of the effects of mitigation strategies (the 4R nutrient stewardship, farming practices and enhanced efficiency fertilizers) on NH3 volatilization in cropping and pasture systems published from 1971 to February 2016. We performed extensive keyword searches of several databases (Web of Science (ISI), SCOPUS, CAB Abstracts (ISI), Academic Search complete (EBSCO) and Google Scholar), and the reference list of cited references. The keywords used in
Percentage of N loss as NH3
Globally, the average NH3 volatilization losses ranged from 0.9 to 64% (a mean of 17.6%) of the applied N. The percentage of N loss as NH3 was the highest in South Asia (30.7%), followed by North America (17.5%) and Southeast Asia (16.1%). The regions in Europe represent the lowest percentage of N loss as NH3 (13.0%) (Table 1). In terms of amounts, NH3 volatilization accounted for 0.696 (a mean of 19.1) kg N ha1 per cropping season. The amount of NH3-N volatilized per cropping season was the
Ammonia as a major N loss pathway
Ammonia volatilization from N applied in agricultural systems is a global issue that needs to be resolved. We found that the global average percentage of N loss as NH3 was 17.6%, which is comparable to the range of 1014% reported by the IPCC (De Klein et al., 2006) and Bouwman et al. (2002). The values for specific continents are similar to those reported by Sutton et al. (1995), Bouwman et al. (2002) and Yan et al. (2003). It is worth noting that up to 64% of N applied could be lost as NH3 (
Conclusion
This study highlights the significance of mitigating NH3 volatilization from agriculture and evaluates the potential of various strategies for reducing the loss. The use of non-urea based fertilizers, reduced fertilizer application rate, deep placement of fertilizers, irrigation, urease inhibitors and controlled release fertilizers are effective in reducing NH3 volatilization. Adopting these mitigation strategies can also reduce indirect greenhouse gas emission. These strategies should be
Acknowledgements
This work was supported by the BIP reinvestment funds of the Faculty of Veterinary and Agricultural Sciences of the University of Melbourne, and the Australia-China Joint Research Centre jointly funded by Australian Government Department of Industry and Science, and the Chinese Ministry of Science and Technology.
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