Several reviews have discussed N2O emission mitigation options from grazed agriculture (e.g.
Oenema et al. 1997, Dalal et al. 2003, de Klein and Ledgard 2005, de Klein and Eckard 2008, Eckard et al. 2010, Luo et al. 2010). Most reviews suggested increasing the N efficiency of the animal so less N was excreted. Opportunities to increase N efficiency relied on increasing animal production for a given amount of feed, or reducing the amount of N ingested by the animals (e.g. Luo et al. 2008a). Other options included increasing the proportion of N excreted in faeces rather than urine, or diluting the animal’s urine by providing a diuretic such as salt (NaCl) (de Klein and Eckard 2008, Spek et al. 2012). Furthermore, the dry matter content of animal feed can influence the amount of water intake, and subsequently affect the amount and N concentration of urine voided by dairy cows (Khelil-Arfa et al.
2012).
On-farm management methods identified to mitigate N2O emissions generally followed
industry best-practice guidelines, such as avoiding wet season grazing, irrigating efficiently, and managing soil water content through surface or subsurface drainage (Oenema et al. 1997, de Klein and Ledgard 2005, Monteny et al. 2006, de Klein and Eckard 2008, Eckard et al. 2010).
Adjusting grazing management by utilising a stand-off pad may also be useful for reducing N2O
emissions from winter grazed systems. Not only is the potential for trampling-induced denitrification reduced when using a stand-off pad, but less N is excreted onto the soil as well (Luo et al. 2008b). Considerably less N2O was emitted from a stand-off pad than from pasture for the same given urine-N
input (Luo and Saggar 2008).
Nitrification inhibitors have also been recommended as a cost-effective means to reduce the amount of N2O emitted from grazed soil (e.g. Monteny et al. 2006, de Klein and Eckard 2008, Doole
Chapter Two 2014). Dicyandiamide (DCD) is a nitrification inhibitor that has received considerable research interest as it not only can reduce N2O emissions, but also reduce NO3- leaching (Di and Cameron 2002,
Monaghan et al. 2013, Cameron et al. 2014).
2.15.1 DCD
Dicyandiamide (DCD, C2H4N4) inhibits nitrification by binding to the receptor site for the
ammonia monooxygenase enzyme, preventing microorganisms from carrying out nitrification. By slowing the process of nitrification, DCD reduces soil NO3- content, reducing the substrate available for
denitrification, and therefore reducing the amount of N2O emitted via denitrification (de Klein and van
Logtestijn 1994).
Weiske et al. (2001), Smith et al. (2008a), and Singh et al. (2009) have shown DCD to be a powerful tool for reducing N2O emissions from grazed agricultural systems. When reviewing published
research, Clough et al. (2007) found that, on average, DCD reduced net N2O emissions by 72%. Luo et
al. (2013b) also reported that using DCD together with a winter stand-off pad was more effective at reducing N2O emissions than using either option alone.
Evidence suggests DCD would be an ideal mitigation strategy to use in a winter forage grazing system. Recently, the N2O emission mitigation efficiency of DCD was found to increase with increasing
urine-N load (Selbie et al. 2014), and Ball et al. (2012) found the effectiveness of DCD was not hampered by trampling. In addition, the rate of degradation of DCD is temperature dependent, and therefore DCD is most effective during the cooler winter months when soil temperatures are below 10 °C (Di and Cameron 2004a, Kelliher et al. 2008). The high urine-N deposition rate, together with the cool temperatures experienced during winter, would suggest that DCD would be a suitable option for mitigating N2O emissions from winter forage grazing. In contrast to the large number of studies
where DCD has been applied to grazed pasture, there is limited research on the effect of DCD to reduce N2O emissions from grazed winter forage systems.
In grazed winter forage systems, Smith et al. (2008b), van der Weerden et al. (2012b), and Monaghan et al. (2013) have shown DCD to be effective in reducing N2O emissions from fine-textured
Pallic soils, but similar research on free draining stony soil in Canterbury has not yet been reported. As well as the 25% reduction in N2O emissions from the forage crop, Monaghan et al. (2013) noted that
the application of DCD conserved soil-N during winter, which resulted in increased yields from the pasture planted the following season.
In addition to DCD, several other nitrification inhibiting substances exist, such as karanjin, nitrapyin, and 3,4-dimethylpyrazole phosphate (DMPP) (Bedard and Knowles 1989, Majumdar 2002,
Chapter Two
32
Cameron et al. 2013). Besides nitrification inhibitors, a large body of evidence is growing that advocates the use of charcoal to supress N2O emissions from agricultural soil.
2.15.2 Biochar
The Earth’s soil contains more than three times as much carbon as does the atmosphere and all plants combined (Schmidt et al. 2011). Consequently, many authors have proposed humans manipulate the carbon cycle and use soil as a sink for atmospheric CO2, and in doing so, reduce the
threat of global climate change (Lehmann 2007b, a, Steinbeiss et al. 2009, Clough and Condron 2010, Sohi et al. 2010, Woolf et al. 2010). When organic matter is heated in the absence of oxygen, charcoal is formed. The term biochar has been associated with charcoal used as a soil amendment for agronomical benefit (Lehmann and Joseph 2009). In addition to carbon sequestration, other benefits have been noted when biochar is worked into soil, including improved soil nutrient retention through increased cation exchange capacity (CEC) (Lee et al. 2010, Van Zwieten et al. 2010a, Peng et al. 2011); increased water holding capacity (Jeffery et al. 2011, Case et al. 2012, Karer et al. 2013); the creation of a potential refuge for soil biota (Steinbeiss et al. 2009, Lehmann et al. 2011); and a decrease in soil bulk density/increase in porosity (Chan et al. 2007, Atkinson et al. 2010, Case et al. 2012). Moreover, N adsorbed by the addition of biochar to soil has been shown to be plant available, suggesting biochar has the potential to increase plant growth (Taghizadeh-Toosi et al. 2012, Zheng et al. 2013).
While several studies have had success using biochar as a soil amendment to suppress N2O
emissions (e.g. Atkinson et al. 2010, Singh et al. 2010, Van Zwieten et al. 2010b, Taghizadeh-Toosi et al. 2011), other studies question the effectiveness of biochar and its ability to reduce N2O emissions
(e.g. Clough et al. 2010, Scheer et al. 2011, Ameloot et al. 2013, Angst et al. 2014). The mechanisms by which biochar suppresses N2O emissions is the subject of considerable ongoing research. Biochar
is thought to produce physical, chemical, and biological changes within soil, leading to various alterations of the N cycle (Lehmann and Joseph 2009). However, in recent reviews, both Clough et al.
(2013a), and Cayuela et al. (2014) described the controversy and lack of understanding surrounding biochar’s ability to influence N2O emissions. In light of such conclusions, the effect of biochar on
denitrification is not well known. Improved soil aeration may reduce the incidence of denitrification (Case et al. 2012), but conversely, Cayuela et al. (2013) suggested the mild liming effect when biochar was added to soil may combine with an "electron shuttle" effect that assists in transferring electrons to denitrifying microorganisms, which may enhance the reduction of N2O to N2. On the contrary, the
addition of C may facilitate denitrification in soil with a low C content and may promote N2O
Chapter Two Biochar has been shown to remove NH4+ from solution and dairy effluent (Ding et al. 2010,
Hale et al. 2013, Hollister et al. 2013, Sarkhot et al. 2013). The application of DCD to urine affected soil increases the amount of time that the urine remains in the NH4+ form. When applied together,
DCD and biochar may have a complementary effect, retaining urine-N in the soil and reducing the potential for N2O emission. However, the effectiveness of using a combination of biochar and DCD as
a method of reducing N2O emissions has not been tested.