Many research trials have confirmed inhibitors are effective in delaying the conversion of either urea to NH/ (Uls) or NH/ to N03- (Nls). The majority of research indicates the application of Uls to soils with fertiliser urea or urine reduces NH3 volatilisation while the application o f Nls reduces N03- leaching and N20 emissions. Some studies also show Nls increase NH3 volatilisation (Davies & Williams
1 995; Nastri et al. 2000).
Treating urea with UI (NBPT) reduces NH3 loss from surface applications (Clay
et al. 1 990; Brernner et al. 1 99 1 ). Laboratory (Carmona et al. 1 990; Vittori-Antisari et al. 1 996) and field studies (Watson et al. 1 994a; Rawluk et al. 2001 ) have shown increased inhibition of urease activity with an increasing rate of UI that followed the law of diminishing returns (Watson et al. 1 994b). NBPT can reduce NH3 vo1atilisation significantly in urea, with concentrations as low as 0.005 % (w/w) (Cannona et al.
1 990). Christianson et al. ( 1 990) observed 68% inhibition of urea hydrolysis at 0.0 1 % N B PT (w/w) and NH3 losses l .5 to 3 times lower when the rate was increased t o 0. 1 %. The optimum concentration of NB PT for temperate grassland soils to inhibit urea hydrolysis is 0. 1 % of urea (w/w) (Watson et al. 1 994b). However, it has been observed that NBPT is less effective at higher temperatures (Bremner et al. 1 99 1 ) and in soils with high levels of organic carbon (Carmona et al. 1 990; Wang et al. 1 99 1 ). A soil incubation study, using a wide range of soil types, indicated the effectiveness of NB PT
in lowering NH3 volatilisation was the greatest in soils with a high pH and low buffering capacity (Watson et al. 1 994a). As these were the soil conditions leading to high NH3 loss from unamended urea, NB PT has the potential to improve the efficiency of urea for temperate grassland. There is little evidence of any long-tenn adverse effect on grass production or reduced efficacy with repeated applications of NBPT amended urea over a period of 3 years (Watson et al. 1 998). Results of more recent studies on the effect of Uls on NH3 volatilisation are summarised in Table 2.2.
Urease inhibitors have little effect on nitrification. Although NB PT has been shown to decrease N02- and N03- accumulation in soil (Bremner & Chai 1 989; Watson
et al. 1 994a), this is probably due to the slow fonnation of exchangeable NH/ caused by the inhibition of urea hydrolysis (Vittori-Antisari et al. 1 996)_
In New Zealand, there has recently been increasing interest in the use of NIs to mitigate environmental impacts of N losses through leaching and gaseous emissions from animal excreta and effluent application (Table 2 .3). Di & Cameron (2002b) found the application of DCD following two urine applications ( l OOOkg Nlha) reduced N20 emissions by 82%. Williamson & Jarvis ( 1 997) obtained 74% reduction in N20 emissions in a short-term study (37 days) where DCD was applied to urine (60 kg Nlha). The new inhibitor DMPP ( 1 kg ha-I ) reduced N20 emissions by 60% in autumn and by 48% in spring when applied to a grassland after slurry application (Merino et al.
2005).
About 60% reduction in N03 - leaching from grazed pasture soils, including animal urine patches with DCD, has been reported with soil lysimeters (Di & Cameron 2004c) using a free-draining shallow stony soil. As N03- leaching is accompanied by counter cations, e.g., calcium, potassium, and magnesium, the leaching of these cations was also reduced by the NI (Di & Cameron 2004b).
There is some evidence that both UIs and NIs may have detrimental effects on plant leaves, e.g., transient leaf tip scorch with UIs, and DCD phytoxicity under certain weather condition (Bremner 1995; Prasad & Power 1 995; Watson 2000; Belastegui
Macadam et al. 2003). However, the benefits of inhibitors in reducing N losses and
increasing pasture production would appear to outweigh these short-term detrimental effects. These same trials showed a wide range of economic returns, depending upon soil type, drainage, time of application and environmental conditions. The greatest likelihood of N losses is from coarse-textured or poorly drained soils; it is in these situations that the use of inhibitors would be most economical. However, inhibitors do not work as well in coarse-textured soils as in these soils urea and N� + ions have a tendency to move away from the inhibitor with rainfall or irrigation (University of Illinois).
Studies on the effect of nitrification inhibitors on N economy (Table 2.3) have shown that the inhibitory action of these chemicals depends on their persistence and bioactivity in soils, which in turn are affected by the intrinsic properties of the compound, soil properties and climatic conditions. The half-lives of inhibitors may vary from a few days to several weeks, depending on the nature of the compound, rate of application, soil type, pH and season (soil temperature). The ideal inhibitor for use in agriculture should:
• specifically b lock an enzymatic reaction (e.g., NIs should block
ammonium oxidation to nitrite, but not nitrite oxidation to nitrate, during the nitrification process)
• remain in close contact with N compounds (e.g., UIs must move with
urea molecules that are not readily absorbed by soil; whereas NIs must be close to NHt + ions that are readily retained by soil)
• not adversely affect other beneficial soil organisms arid higher plants
• remain effective in the soil for several weeks after N input through
fertiliser addition and excretal deposition
• not to be toxic to animals and humans at the levels used to inhibit
nitrification effectively
• cost effective to use.
The ultimate goal of any inhibitor is to increase the efficiency of N use. For an economic benefit to occur, the N saved from leaching and gaseous losses by using the inhibitors would have to result either in an increase in pasture production, with a value greater than the cost of the inhibitors, or in a reduction in fertiliser input. The economic benefits of reduced environmental pollution and future damage to our environment from leaching and gaseous emissions are of higher significance over the long-term than the productivity gains. The value of inhibitors in reducing N losses from N fertilisers and increasing crop yields is well established in arable soils. The inhibitors are also reported to increase pasture production. The increase in stocking rates needed to utilise this extra pasture may, however, enhance emissions of other greenhouse gases. Results of a recent desktop study (de Klein & Monaghan 2005), demonstrated the use of NIs had a limited effect on total greenhouse gas emissions reduction, and compared with the reduction in N20 emissions, due to an increase in both CH4 and CO2 emissions from the farm system.
Lysimeter studies by (Di & Cameron 2002a,b, 2003, 2004b, c), showed DCD reduced N03 - leaching and N20 emissions from urine and urea applications. Understanding the soil and plant processes controlling DCD decomposition, the variable response in different soils, and the impact DCD has in causing changes in the N transformations and N cycle is the focus of our current research. This will allow us to develop simple assays and models to monitor and simulate the degradation of these
inhibitors in various soil types, and enhance our understanding of the impact of these inhibitors on the bioavailability ofN in managed grassland ecosystems.