Nitrification inhibitors (NI) such as dicyandiamide (DCD), nitropyrin, and 3,4 dimethyl pyrazole phosphate (DMPP) slow the activity of Nitrosomonas, the genus of nitrifying bacteria responsible for the oxidation of NH4+ to NO2− and this helps to retains N in the NH4+ form longer in soil providing more chance plant to uptake NH4+ (e.g., Abbasi and Adams, 2000; Di et al., 2007). Thereby NI can inhibit nitrification and further denitirification producing N2O and it was found that NI reduce N2O emissions 30 to 80% (e.g., Abbasi and Adams, 2000; Di et al., 2007; Zaman et al., 2009; Saggar et al., 2009; Akiyama et al., 2010). Nitrous oxide contributes to greenhouse effect (Wang et al., 1976) and the global warming potential of N2O is 298 times that of carbon dioxide (CO2) and 25 times that of methane (CH4) in a 100-year time horizon (Forster et al., 2007). Of the entire anthropogenic N2O emission (5.7 Tg N2O–N yr−1), agricultural soils provide 3.5 Tg N2O–N yr−1 (IPCC, 2006). Therefore, it has been suggested that NI use can be potent mitigation option for GHG emissions in agricultural lands (e.g., Bolan et al., 2004; Klein and Ledgard, 2005; Akiyama et al., 2010).
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Soil ammonia emissions
Soil NH3 emission is primarily produced when NH4+ dissociates into gaseous NH3 under alkaline conditions, and soil pH and CEC are important soil factors in determining the magnitude of NH3 emissions from N fertilizer applied to agricultural soils (e.g., Nelson 1982; Francis et al. 2008). Soil NH3 emissions are a concern since they constitute a significant loss of N in agricultural soils (e.g., Nelson 1982; Francis et al. 2008) and cause soil acidification (e.g., Van der Eerden et al. 1998; Rennenberg and Gessler 1999) and eutrophication (e.g., Bobbink et al. 1992) through atmospheric deposition. Emitted NH3 is also an indirect source of N2O (e.g., Martikainen 1985).
The effect of nitrification inhibitors on soil ammonia emissions
It has been reported that NI increased NH3 emission in field experiments (e.g., Cornforth and Chesney, 1971; Prakasa Rao and Puttanna, 1987; Zaman et al., 2009) and lab incubation experiments (e.g., Bundy and Bremner, 1974; Rodgers, 1983; Asing et al., 2008).
The mechanisms of increase in NH3 emission with NI have been explained as follows:
First, NH4+ retains longer in soil owing to NI and may increase NH3 emission (e.g., Asing et al., 2008; Zaman et al., 2009).
Second, the DCD-induced reduction in nitrification would reduce soil acidification resulting in a prolongation of the elevated pH and a consequent increase in NH3 emission losses (Fox and Bandel, 1989).
Third, by maintaining the NH4+ in the soils, it caused a priming effect with a subsequent increase in the rate of soil organic matter mineralization and an extra release of soil organic N (Gioacchini et al., 2002).
The mechanisms of increase in NH3 emission with NI have been explained as follows:
First, NH4+ retains longer in soil owing to NI and may increase NH3 emission (e.g., Asing et al., 2008; Zaman et al., 2009).
Second, the DCD-induced reduction in nitrification would reduce soil acidification resulting in a prolongation of the elevated pH and a consequent increase in NH3 emission losses (Fox and Bandel, 1989).
Third, by maintaining the NH4+ in the soils, it caused a priming effect with a subsequent increase in the rate of soil organic matter mineralization and an extra release of soil organic N (Gioacchini et al., 2002).
The concern of increased NH3 emission by NI treatments appeared in literature from late 1960 (Hauck and Bremner, 1969) and it was argued that NI use may trade-off for mitigating N2O to increasing NH3 emissions (Mkhabela et al., 2006) that are indirect source of N2O (e.g., Martikainen, 1985) and cause soil acidification (e.g., Van der Eerden et al., 1998; Rennenberg and Gessler, 1999) and eutrophication (e.g., Bobbink et al., 1992).
In contrast, a few studies showed no significant increase in NH3 emission after NI treatments in field experiments (Clay et al., 1990) and lab incubation experiments (e.g., Clay et al., 1990; Dendooven et al., 1998; Mkhabela et al., 2006).
In contrast, a few studies showed no significant increase in NH3 emission after NI treatments in field experiments (Clay et al., 1990) and lab incubation experiments (e.g., Clay et al., 1990; Dendooven et al., 1998; Mkhabela et al., 2006).
Soil properties and ammonia emissions in nitrification inhibitors treatment
It can be preliminary summarized that NI treatment increased NH3 emission in the soil which has relatively higher range of pH (5.4 to 7.9) and lower range of CEC (5.7 to 16.8 meq 100 g–1 soil) compared to the soil of studies reporting no increase in NH3 emission (pH 4.7 to 6.2 and CEC 10.0 to 24.0 meq 100 g–1 soil).
It has been well known that soil pH and CEC are important soil factors determining the magnitude of NH3 emissions from N fertilizer applied to agricultural soils (e.g., Nelson, 1982; Francis et al., 2008). Ammonia losses increase with higher soil pH because high H+ concentration in soil increases dissociation of NH4 to NH3, thus increasing the potential for volatilization (Sharpe and Harper, 1995; Francis et al., 2008).
However, significant amount of NH3 can be lost at soil pH values as low as 5.5 if large amount of urea or NH4+ are applied in soil surface (Nelson 1982). This can explain the observed significant increase in NH3 emission in NI treatment with low pH soils. High soil CEC reduce NH3 emission because CEC provides a mechanism by which NH4+ are removed from soil solution, thereby reducing NH4+ in the soil solution that is subject to volatilization (e.g., Hargrove, 1988; Al-Kanani et al., 1991; Whitehead and Raistrick, 1993; Sommer et al., 2003). These results indicate that soil which has properties for high potential of NH3 emission from N fertilizer also has high potential of increase in NH3 emission in NI treatments.
It has been known that high organic matter (OM) and clay content and fine-textured soils reduce NH3 emission (Nelson, 1982; Al-Kanani et al., 1991; Francis et al., 2008) because of the relative contribution of the OM and clay components to the CEC of the soil (Al-Kanani et al., 1991). Therefore, it is suggested that NI treatment in low soil pH and high CEC, OM and clay content and fine-textured soils may decrease the potential of increase in NH3 emission.
Application method and ammonia emissions in nitrification inhibitors treatments
The use of NI with liquid fertiliser or animal urine or their fertiliser placement in subsurface soil (i.e., 2 cm and 5 cm depth) either reduced or had little effect on NH3 emission (e.g., Rodgers 1983; Prakasa Rao and Puttanna 1987). As the liquid N input and NI moves quickly into the subsurface soil, NH4+ is sorbed onto the exchange sites and has less chance for NH3 emission (Di and Cameron 2004). These results suggest that increase in NH3 emission by NI can be mitigated by managing the application method, including liquid type or sub-surface placement.
Ammonia emissions in combined use of nitrification and urease inhibitors
Combined use of NI and UI [e.g., N-(nbutyl) thiophosphoric triamide (nBPT)] reduced NH3 emission compared with NI treatment only (e.g., Clay et al., 1990; Zaman et al., 2009; Zaman and Blennerhassett, 2010).
Clay et al. (1990) reported that NH3 emission in treating urea with nBPT (8 kg ha−1) and DCD (20 kg ha−1) was around 100 times less than in treating urea with only DCD for bare and residue-covered soil. Zaman et al. (2009) found combined application of nBPT (3 L ha−1) and DCD (7 kg ha−1) reduced NH3 emission by 9 to 78% over urine alone while application of DCD (7 kg ha−1) increased NH3 emission by 9 to 56% over urine alone. Similarity, Zaman and Blennerhassett (2010) found combined application of nBPT (1 L ha−1) and DCD (7 kg ha−1) reduced NH3 emission by 48 to 51% compared with urine alone treatment while only DCD (7 kg ha−1) application increased NH3 emission by 18 to 41% over urine alone.
The reduced NH3 emission by combined application of UI and NI compared with NI application alone can be explained by decreased urea hydrolysis by UI (Singh et al., 2008; Zaman et al., 2009; Zaman and Blennerhassett, 2010). Urease inhibitors decreasing urea hydrolysis retard release of NH4+ from urine and increase in soil pH (e.g., McCarty et al., 1987; Watson et al., 2008; Zaman and Blennerhassett, 2010), both of which are known to accelerate NH3 emission rate (e.g., Nelson, 1982; Francis et al., 2008). These results suggest that combined application of UI and NI can be a solution to prevent increase in NH3 emission by NI treatment and more studies are needed in various climate conditions and soil environments to assess the efficacy.
Clay et al. (1990) reported that NH3 emission in treating urea with nBPT (8 kg ha−1) and DCD (20 kg ha−1) was around 100 times less than in treating urea with only DCD for bare and residue-covered soil. Zaman et al. (2009) found combined application of nBPT (3 L ha−1) and DCD (7 kg ha−1) reduced NH3 emission by 9 to 78% over urine alone while application of DCD (7 kg ha−1) increased NH3 emission by 9 to 56% over urine alone. Similarity, Zaman and Blennerhassett (2010) found combined application of nBPT (1 L ha−1) and DCD (7 kg ha−1) reduced NH3 emission by 48 to 51% compared with urine alone treatment while only DCD (7 kg ha−1) application increased NH3 emission by 18 to 41% over urine alone.
The reduced NH3 emission by combined application of UI and NI compared with NI application alone can be explained by decreased urea hydrolysis by UI (Singh et al., 2008; Zaman et al., 2009; Zaman and Blennerhassett, 2010). Urease inhibitors decreasing urea hydrolysis retard release of NH4+ from urine and increase in soil pH (e.g., McCarty et al., 1987; Watson et al., 2008; Zaman and Blennerhassett, 2010), both of which are known to accelerate NH3 emission rate (e.g., Nelson, 1982; Francis et al., 2008). These results suggest that combined application of UI and NI can be a solution to prevent increase in NH3 emission by NI treatment and more studies are needed in various climate conditions and soil environments to assess the efficacy.
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