Animal- and feed-based interventions
Ruminants excrete between 75 per cent and 95 per cent of the N they ingest; excess dietary N is mainly excreted in the urine, whereas dung N excretion remains relatively constant (Castillo et al, 2000; Eckard et al, 2007). Of the dietary N consumed by ruminants, <30 per cent is utilized for production, with >60 per cent lost from the grazing system (Whitehead, 1995). The effective N application rate within a urine patch from a dairy cow is commonly between 800 and 1300kg N/ha (Eckard and Chapman, 2006), and N is deposited at concentrations that are orders of magnitude greater than what soil-plant systems can efficiently utilize. Therefore, strategies for reducing N2O emissions should also focus on improving the efficiency of N cycling through the soil-plant-animal system.
Conceptually, if animal urine in grazing systems was spread more evenly across the paddock, the effective N application rate would be reduced, potentially reducing N2O emissions. Although no specific animal technologies have currently been developed, this may require some physical intervention with the animal, yet be practically and ethically conceived, that will cause the urine to be distributed more evenly (de Klein and Eckard, 2008).
Genetic manipulation or animal breeding may improve the N conversion efficiency within the rumen, or produce animals that urinate more frequently or walk while urinating, all leading to lower N concentrations or greater spread of urine (de Klein and Eckard, 2008). Coffey (1996) reported that an improvement in the feed conversion efficiency ratio of 0.01kg dry matter (DM) intake per kg live weight gain could result in a 3.3 per cent reduction in nutrient excretion, assuming a similar growth rate and nutrient retention. Therefore, breeding for more efficient feed conversion should produce animals that partition more of their intake into production and less into N excretion, thereby reducing potential N2O losses.
Ruminants on lush spring pasture commonly ingest protein in excess of their requirements, but are usually energy-limited. The excess protein is used as an energy source and a relatively larger proportion of the ammonia produced in the rumen is excreted as urea (Whitehead, 1995). Therefore, balancing the protein-to-energy ratios in the diets of ruminants is important for minimizing the N2O emissions resulting from excess urinary N excretion. Misselbrook et al (2005) showed that dairy cows fed a 14 per cent crude protein (CP) diet excreted 45 per cent less urinary N than did dairy cows fed a 19 per cent CP diet. Similarly, van Vuuren et al (1993) showed that supplementing cows on a perennial ryegrass diet with low protein/high sugar supplements reduced the amount of total N and urinary N excreted by 6-9 per cent and 10-20 per cent, respectively, compared with those of cows fed an allgrass diet. More recently, Miller et al (2001) reported that dairy cows on a novel 'high-sugar' variety of perennial ryegrass excreted 18 per cent less N in total and 29 per cent less urinary N.
Improving N efficiency and reducing excess urinary N can be achieved through either breeding animals for more efficient feed conversion, breeding forage crops that use N more efficiently and have a higher energy-to-protein ratio, or balancing high-protein forages with high-energy supplements.
Condensed tannins (CT) form complexes with proteins in the rumen, protecting them from microbial digestion, resulting in either the more efficient digestion of amino acids in the abomasum and lower intestine, or the excretion of the CT-protein complex in the dung (Min et al, 2003; de Klein and Eckard, 2008). Carulla et al (2005) showed that sheep fed a CT extract from Acacia mearnsii (black wattle) increased their partitioning of N from the urine to the faeces, reducing urinary N by 9.3 per cent as a proportion of the total N excreted. Grainger et al (2009) added a CT extract from black wattle to the diet of lactating dairy cattle and observed a 45-59 per cent reduction in urinary N excretion, with 18-21 per cent more N in the faeces. Similarly, Misselbrook et al (2005) showed that dairy cows on a 3.5 per cent CT diet excreted 25 per cent less urinary N, 60 per cent more dung N, and 8 per cent more N overall compared with cows on a 1 per cent CT diet.
Faecal N is mainly present in a complexed organic form and is thus less volatile than urinary N, which is largely urea that is mineralized to NH4+and to NO3—. As well as being vulnerable to leaching, this urinary N accounts for about 60 per cent of N2O emissions from pasture (de Klein and Ledgard, 2005). The CT-protein complex in dung is also more recalcitrant to breakdown in the soil, because mineralization of the complex is inhibited and the faeces decompose more slowly than faeces that do not contain CT (Fox et al, 1990; Palm and Sanchez, 1991; Somda and Powell, 1998; Niezen et al, 2002). By reducing N excretion in the urine, the risk of subsequent N2O emission from this highly concentrated N source is reduced (de Klein and Eckard, 2008). Currently, CT extracts are expensive because there is no large commercial demand for their production in agriculture. Because many forage plants contain CT, plant breeding may be a way to introduce CT into the diet of animals when daily supplementation is not practical or economic. However, further research is required to identify suitable and cost-effective high-tannin forages and tannin extracts with which to supplement the diets of ruminants.
Salt supplementation increases water intake in ruminants, both reducing their urinary N concentration and inducing more frequent urination events, thus spreading urinary N more evenly across grazed pasture (Ledgard et al, 2007b). In a laboratory study, van Groenigen et al (2005) found that reducing the N concentration of urine tended to reduce N2O emissions from incubated soil cores by 5-10 per cent. However, no field measurements of actual N2O emissions have been reported in the context of breeding or salt supplementation, and this area requires further research.
Soil/management interventions Fertilizer and effluent inputs
The rate, source and timing of applications of N fertilizer are important management factors affecting the efficiency of pasture growth responses, and thus potential N2O losses. When conditions are suitable for denitrification, N2O emissions increase exponentially with the rate of N applied in any single application (Mosier et al, 1983; Whitehead, 1995; Eckard and Chapman 2006). In a modelling study, Eckard and Chapman (2006) predicted that annual N2O emissions will increase exponentially as the annual rate of N fertilizer is increased, with the rate of increase being faster for nitrate than for urea fertilizer. Galbally (2005) reported N2O emissions of 1.0-1.2kg N2O-N ha"1 yr"1 from unfertilized irrigated dairy pastures in temperate Australia, increasing to 2.4kg N2O-N ha"1 yr"1 after three applications of 50kg N/ha per year.
Nitrate-based N fertilizer generally produces high N2O emissions relative to ammoniated N sources when applied to actively growing pasture. In a review, de Klein et al (2001) cited N2O emission factors for N fertilizers of <0.1-1.9 per cent (median 0.5 per cent) when N was applied as urea fertilizer, and <0.1-12 per cent (median 3.2 per cent) when N was applied as calcium nitrate. Similarly, Eckard et al (2003) reported that average N2O losses were higher with ammonium nitrate (12.9 per cent) than with urea. In South Africa, Australia and New Zealand, urea or diammonium phosphate is the main source of N applied to intensive pastures, with recommended rates not exceeding 50-60kg of fertilizer-N per hectare in any single application per grazing rotation (Ledgard, 1986; Eckard et al, 1995; Eckard and Franks, 1998). Although the total amount of N fertilizer used can be reduced and the timing of its application can be optimized to the soil moisture conditions (Luo et al, 2007), any further N2O abatement potential in the rate of application and the source of N fertilizer may be limited in pasture-based grazing systems.
The rate, timing and placement of animal effluent applied to soils all affect potential N2O emissions (for example Luo et al, 2008c). The N2O emissions from effluent applied to soils are generally lower than those from urine patches, if the effluent is applied at the recommended rates and at the appropriate times of the year (Chadwick, 1997; de Klein et al, 2001; Saggar et al, 2004). Saggar et al (2004) demonstrated that N2O emissions from effluent are higher when applied to wet soil than when applied to drier soil, and that emission peaks generally occur within 24 hours of application. The timing of the application of effluent relative to the application of N fertilizer can also affect N2O emissions; N2O emissions are lower when the N fertilizer is applied at least three days after the effluent, rather than together with the effluent (Stevens and Laughlin, 2002).
Effluent application techniques can also affect N2O emissions. For example, the injection or incorporation of effluent into the soil can increase direct N2O emissions but reduce ammonia (NH3) volatilization (Chadwick, 1997; Saggar et al, 2004), resulting in lower indirect N2O emissions. Effluent injection is also likely to increase the overall efficiency of the use of effluent N and could thus reduce the N fertilizer requirement and the associated N2O emissions (Chadwick, 1997).
Nitrification inhibitors are chemical compounds that inhibit the oxidation of NH4+ to NO3" in soil and thus reduce N2O emissions from NH4+-based fertilizers and from urine (Di and Cameron, 2002). The most widely used are nitrapyrin and DCD (de Klein and Eckard, 2008). Fertilizers coated with nitrification inhibitors have been shown to be effective in reducing nitrification and N2O emissions by up to 80 per cent, as reviewed by de Klein et al (2001). Applied as a spray or as a granular formulation, nitrification inhibitors can also effectively reduce N2O emissions from animal urine by 61-91 per cent, with pasture yield increases of 0-36 per cent (Di et al, 2007; Kelly et al, 2008; Smith L. C. et al, 2008; Mon-aghan et al, 2009). However, some of these studies have been conducted under optimal conditions for N2O production and over short periods, so the potential on-farm abatement is likely to be more conservative in a grazing system than the published data suggest (de Klein and Eckard, 2008). Nitrification inhibitor can also reduce nitrate leaching emissions from urine patches by 17-68 per cent (for example Di and Cameron, 2002,2005) or from grazed pasture by 21-56 per cent (Monaghan et al, 2009), thus reducing indirect N2O emissions.
Novel approaches to placing nitrification inhibitors where they are most effective could include either feeding the inhibitors to animals, with the inhibitor excreted in the urinary stream, or breeding plants that exude natural inhibitors from their roots. Ledgard et al (2007a) demonstrated that ruminants supplemented with a nitrification inhibitor (DCD) excreted the inhibitor unaltered in the urine. Further research is required to quantify the N2O abatement potential of this approach, including a slow-release delivery mechanism, because this has great potential for the abatement of N2O from urine in grazing systems. Recently, Subbarao et al (2006) reported the release of a natural nitrification inhibitor from the roots of Brachiaria humidicola, raising the possibility of breeding plants that synthesize their own inhibitors.
Apart from directly reducing N input into grazing systems, nitrification inhibitors are currently the only well-published technology available for reducing the loss of N from soils. Although their use has historically been limited, predominantly by cost, with future constraints on emissions envisaged in many countries they are likely to form a significant part of any comprehensive abatement strategy to reduce N2O emissions from both urinary and N fertilizer inputs to pasture systems.
Restricting grazing on seasonally wet soils not only reduces urinary N returns, but also reduces soil anaerobicity caused by hoof compaction. De Klein et al (2006) and Luo et al (2008b) reported a total reduction in direct and indirect N2O emissions of 7-11 per cent from farm systems under restricted grazing regimes during relatively wet months, and following the application of effluent from feed pods or stand-off pads. Schils et al (2006) reported that the combined effect of reduced N fertilizer use and reduced grazing time on case study farms in The Netherlands was a reduction in N2O emissions of 10 per cent for the total farm system. These studies show that increased N utilization increases N efficiency and reduces N losses while increasing production.
Nitrous oxide fluxes from flood-irrigated dairy pasture rose rapidly two to three days after irrigation, when the soil WFPS was >95 per cent (Phillips et al,
2007). The emissions remained high for a further one to two days before gradually subsiding to background levels as the soil moisture decreased. The N2O emissions remained low immediately following irrigation, which was most likely the result of complete denitrification, producing mainly N2 emissions (Phillips et al, 2007). Irrigation through extended dry seasons may in fact reduce N2O emissions in the later wetter season by reducing the build-up of unutilized soil NO3~ through increased plant uptake (Jordan and Smith, 1985).
As denitrification is enhanced under conditions of low soil aeration, reducing the waterlogging of pastures will reduce potential N2O emissions. A common practice in the management of seasonally wet soils has been to introduce surface or subsurface drains, but the impact of this management practice on N2O emissions is not straightforward. Waterlogged soils will denitrify more efficiently than well-drained soils but improved drainage will increase N loss through the drains, only to denitrify in a wetland or elsewhere in the landscape (de Klein and Eckard, 2008). However, if the improved drainage merely reduces the WFPS of the soil to below saturation (about 80 per cent), but it remains above wilting point (40 per cent), this may actually increase N2O emissions (Granli and Bock-man, 1994) and could lead to increased nitrate leaching. In some cases, stimulating denitrification has been recommended as a way of reducing nitrate leaching in nitrate-sensitive areas (Russelle et al, 2005). These data highlight the need for further research into the compromise between managing irrigation for efficient plant growth and N2O emissions, and the compromise between improved drainage and enhanced NO3 leaching, for a range of soils, systems and environmental objectives.
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