Asignación de condiciones y límites para flujo de fluidos (Fluid Dynamic &

Nitrification inhibitors are chemical compounds that are used as a mitigation tool to reduce both NO3- leaching and N2O emissions. Nitrification inhibitors inhibit the first step of nitrification by

deactivating the active site of the AMO enzyme (McCarty & Bremner 1989; Di et al. 2009a) by interfering with the cytochrome oxidase in the respiratory electron transport system of ammonia oxidisers (O'Connor et al. 2012), slowing the conversion of NH3 to NO2-.Therefore

there is a greater concentration of NH4+ and lower concentration of NO3- present within the soil.

Higher soil NH4+ concentrations are desired because temperate region soils (such as those in

New Zealand) have an overall net negative charge, attracting the positively changed NH4+ which

is adsorbed onto the soil exchange surfaces. With larger quantities of NH4+ being adsorbed onto

the soil surface, greater plant uptake of NH4+ occurs as well as greater immobilisation into soil

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Nitrification inhibitors are an unusual example where there is the potential to both reduce pollutant loads and increase profit through promoting pasture production (Doole & Paragahawewa 2011). Subbarao et al. (2006) identified 64 different chemical compounds that have been tested as nitrification inhibitors. However, only a few have been selected for evaluation under field conditions. These nitrification inhibitors includes: nitrapyrin, 3,4- dimethylpyrazole phosphate (DMPP), and DCD (Zerulla et al. 2001; Di & Cameron 2002b; Subbarao et al. 2006).

2.6.2.1 Nitrapyrin

Nitrapyrin was developed by Dow Chemical Company. It has been marketed under the trade name of N-Serve® and has been sold as an N stabiliser (Subbarao et al. 2006) which has been extensively used in North America. Nitrapyrin can be added to any ammonium fertiliser as well as animal manure, however, due to its volatility, the incorporation of nitrapyrin into conventional N fertilisers is difficult. For this reason, in the United States, it has mainly been applied by injection into the soil in combination with anhydrous ammonia (Trenkel 2010).

When nitrapyrin was applied at the rate of 2 mg kg-1 soil to 87 different soils, 74 of these soils showed nitrapyrin was effective in reducing nitrification for six weeks (Goring 1962b, a). During cooler soil temperatures, nitrapyrin is very persistent and stable within the soil, thus reducing nitrification rates for longer periods of time during the autumn and winter months. However, at warmer soil temperatures, nitrapyrin normally completely decomposes within 30 days (Subbarao et al. 2006).

2.6.2.2 3,4-dimethylpyrazole phosphate (DMPP)

The nitrification inhibitor DMPP is relatively new compared to nitrapyrin and DCD. It was recently developed by BASF in Germany (Subbarao et al. 2006). DMPP has been shown to inhibit nitrification for a period of four to ten weeks when applied at a rate between 0.5 kg ha-1 and 1.0 kg ha-1 (Zerulla et al. 2001). However, its effectiveness is dependent on climatic conditions and site characteristics (Barth et al. 2001; Pasda et al. 2001). At a soil temperature of 5oC, after a 140 day period, DMPP was still present and ammonium sulphate had not yet been converted to NO3-

by nitrification. However, at a soil temperature of 20oC, the inhibitory effect of DMPP was reduced and nitrification occurred within 40 days (Zerulla et al. 2001).

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The use of DMPP has been shown to reduce both NO3- leaching and N2O emissions in a number

of agricultural systems. It has also been shown to be effective in cropping systems (Pasda et al. 2001; Zerulla et al. 2001). Coating N fertiliser in DMPP is the traditional way to apply DMPP to the soil but this form is not practical for all farming systems. Di and Cameron (2012) looked at the use of DMPP in a liquid form to reduce NO3- leaching and N2O emissions on soils that had

high N loading from animal urine depositions. They found that DMPP was just as effective as DCD and had the potential to be a mitigation tool. However, there are currently many unknowns surrounding DMPP. This makes DCD a safer choice due to the large knowledge base.

2.6.2.3 Dicyandiamide (DCD)

DCD (C2N4H4) was developed by Showa Denko and is produced from calcium cyanamide (Frye

2005). DCD is at least 65% N (Boman et al. 1995; Trenkel 2010), making it suitable to be used as a slow release fertiliser (Di & Cameron 2002b; Trenkel 2010). Compared to other nitrification inhibitors, DCD is known to be very safe with: an LD50 > 10,000 mg kg-1, no evidence of

mutagenic activity when subject to the Ames test, no carcinogenic effects after a long-term study, and a short degradation time (Boman et al. 1995; Kelliher et al. 2008; O'Callaghan et al. 2010; Trenkel 2010). It has also been proven to be safe on soil organisms due to its bacteriostatic effect (Amberger 1989; Trenkel 2010) which suppresses their activity for a period of time rather than killing them (Trenkel 2010).

DCD is a versatile nitrification inhibitor. It can be incorporated into conventional ammonium fertilisers (Trenkel 2010) as well as being a suitable stabiliser for cattle manures and animal slurries (Amberger 1989; Dittert et al. 2001). DCD can also be applied as a fine particle suspension in solution (Di & Cameron 2005), resulting in an even coverage on the soil. Using DCD in solution results in DCD being suitable for a range of farming systems.

The use of DCD was extensive in Western Europe and Japan before it was introduced to America in 1984 (Trenkel 2010). It was not until the late 1990’s that the use of DCD was officially approved by the United States Environmental Protection Agency (USEPA) as a nitrification inhibitor (Association of American Plant Food Control Officials 2001; Frye 2005).

The use of DCD in combination with N based fertilisers (stabilised N fertilisers) has been studied in a number of systems including: winter wheat, sweet corn, grain sorghum, potatoes, cotton, sugar beets, citrus trees, and wheat (Amberger 1989; Frye et al. 1989; Malzer et al. 1989; Zerulla

39

& Knittel 1991a, b; Boman et al. 1995; Serna et al. 1995). Stabilised N fertiliser use has led to a greater nutrient use efficiency resulting in a 20 - 30% reduction in N fertiliser usage (Trenkel 2010). However, different degrees of N efficiency have been observed between different crops. Frye et al.(1989) identified an increase in maximum yield for sweet corn, grain sorghum, and potatoes when DCD was applied alongside N fertiliser. In contrast, the results for cotton varied. When DCD was applied alongside N fertiliser to a wheat crop no advantages were observed. Not all results were as encouraging as Frye et al.(1989). Malzeret al. (1989) observed varying results with DCD when applied to corn, wheat, and potato crops.

In Western Europe, the practice of stabilising ammonium fertilisers with DCD has been replaced by using DCD in combination with other nitrification inhibitors thus reducing the application rate while maintaining full activity. Such new combinations include DCD and Triazole (10:1 ratio) and DCD and 1H-1, 2,4-triazole (Weber et al. 2004) which is commercially available as Alzon® 46

(Khalil et al. 2009).

In America, the use of DCD has not been widely adopted by growers and farm managers. Several factors are believed to have contributed to the low interest in DCD usage, including (Frye 2005):

 Availability of inexpensive N fertiliser;

 The requirement for delayed and split applications;

 Inconsistent field responses;

 Economic rather than environmental drivers; and

 US Environmental Protection Agency restrictions.

Overall, the use of DCD has a number of key advantages over other nitrification inhibitors. These advantages include: it is cheap to produce, it has a high water solubility therefore is able to be applied in liquid form, it is less volatile making it being more suitable for use with solid fertilisers, and it decomposes completely in the soil to NH4+ and carbon dioxide (CO2) (McCarty & Bremner

1989; Di & Cameron 2012).

2.6.2.4 The effect of DCD on nitrification rates

DCD applications significantly reduce nitrification rates, in turn reducing NO3- leaching losses

(Figure 2.11) (Williamson et al. 1996; Di & Cameron 2002b, 2004a; Di et al. 2009a; Moir et al. 2010).

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Figure 2.11 Effect of the nitrification inhibitor DCD on nitrate leaching losses from urine and urea treated lysimeters. Vertical bars are the standard errors of the mean (Di & Cameron

2004a). New Zealand pastoral farming

The use of DCD has been shown to substantially reduce NO3- leaching and N2O emissions from

both dairy and sheep farming systems within New Zealand (Table 2.5). Di and Cameron (2002b) recorded reductions in NO3--N leaching of 76% during autumn and 42% during the spring under

high N loaded dairy grazed pastures, giving an annual NO3--N leaching reduction of

approximately 60% when 15 kg DCD ha-1 was applied. Table 2.5 shows that numerous trials have been undertaken within New Zealand under different conditions to see what effect DCD has on NO3- leaching from animal urine patches.

From the successful results in reducing NO3- leaching, Hoogendoorn et al. (2008) looked into the

use of DCD on hill country farming as a mitigation tool to reduce N2O emissions from sheep urine

patches. It was concluded that, on the lower North Island trial site, reduction in N2O emissions

was 60 - 80%, while the South Island trial site showed a reduction in N2O emissions of 40%. Di

and Cameron extended the research on NO3- leaching reductions when using DCD to see if

similar reductions could be made in reducing N2O emissions (Table 2.6). 0 20 40 60 80 100 120

Urea 200/Urine 1000 Urea 200/Urine

1000/DCD (May) Urea 200/Urine 1000/DCD (May + Aug.) NO 3 --N l ea chin g loss (kg N h a -1 yr -1) LSD (P< 0.05) = 57

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Table 2.5 The use of DCD in pastoral systems to mitigate nitrate-N leaching. DCD Rate

(kg DCD ha-1)

Urine Rate (kg N ha-1)

Soil Type Reduction (%)

Season Sock Type

Reference

10 1000 Stony 45 Winter Dairy (Di & Cameron 2007) 15 1000 Deep sandy 74 - 76 Autumn Dairy (Di & Cameron

2004a) 15 1000 Stony silt

loam

76 Autumn Dairy (Di & Cameron 2002b) 15 1000 Stony silt

loam

42 Spring Dairy (Di & Cameron 2002b) 10 1000 Silt loam 61 Winter Dairy (Shepherd et al.

2010) 10 1000 Clay 36 Winter Dairy (Shepherd et al.

2010) 10 700 Stony 60 Winter Beef (Di & Cameron

2007) 10 300 Stony 83 Winter Sheep (Di & Cameron

2007) 15 580 Silt loam 39 Winter Beef (McDowell &

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Table 2.6 The use of DCD in pastoral systems to mitigate nitrous oxide emissions. DCD Rate

(kg DCD ha-1)

Urine Rate (kg N ha-1)

Soil Type Reduction (%)

Season Stock Type

Reference

15 1000 Silt loam 76 Autumn Dairy (Di & Cameron 2003) 15 1000 Silt loam 78 Spring Dairy (Di & Cameron

2003) 7.5 1000 Silt loam 65 Winter Dairy (Di & Cameron

2006) 10 1000 Silt loam 70 Winter Dairy (Di & Cameron

2006) 15 1000 Silt loam 73 Winter Dairy (Di & Cameron

2006) 10 1000 Sandy loam 61 Winter Dairy (Di & Cameron

2006)

Arable and horticultural use of DCD

As shown above, DCD can have a significant impact on reducing NO3- leaching and N2O emissions

within a pastoral farming system. Table 2.7 shows that DCD can also reduce NO3- leaching in

various non-pastoral systems, but the reduction varies between the different studies from 39% to 80%.

Table 2.7 The use of DCD in non-pastoral systems to mitigate nitrate-N leaching. DCD Rate

(kg DCD ha-1)

N Rate (kg N ha-1)

Soil Type Reduction (%)

Crop Type Extra Info Reference

10 650 Silt/clay loam

59 Vegetables N Fertiliser (Cui et al. 2011) 10 650 Clay loam 39 Vegetables N Fertiliser (Cui et al.

2011)

13 90 Sandy

loam

77 Lettuce N – urea (Asing et al. 2008) 13 90 Sandy loam 80 Lettuce N - organic manure (Asing et al. 2008)

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It is clear that the nitrification inhibitor DCD has been widely studied and used to decrease the environmental impacts of NO3- leaching and N2O emissions while increasing yields within the

agricultural and horticultural industries for a number of years (Moir et al. 2007; Cui et al. 2011). Extensive research has been undertaken on the use of DCD in pastoral farming systems including: dairy grazed pastures, hill country sheep farms, and beef farming systems (Di & Cameron 2007; Hoogendoorn et al. 2008; Shepherd et al. 2010). DCD has also been used on a range of horticultural and arable systems (Asing et al. 2008; Khalil et al. 2009; Cui et al. 2011). To date, no work has been undertaken on the use of DCD on dairy winter forage grazing systems, where the soil is continuously bare, wet, cold, anaerobic, and draining, with a higher stocking rate. Thus, there are substantial gaps in our knowledge.

2.6.2.5 Factors affecting the effectiveness of DCD

The performance and effectiveness of DCD in reducing nitrification rates is not constant (Di & Cameron 2004b). The rate of degradation is dependent on: soil temperature, moisture content, organic matter, soil pH, and the number of applications (Amberger 1989; Rajbanshi et al. 1992; Kelliher et al. 2008; Shi et al. 2011). DCD is also easily leached out of the root zone reducing its effectiveness (McCarty & Bremner 1989; Shepherd et al. 2012a).

Soil temperature

The rate at which DCD degradation occurs at is highly dependent on the soil temperature. As the soil temperature increases, the rate at which DCD degrades also increases. Thus, when the soil temperature is low DCD is effective for longer. Di and Cameron (2004b) identified that, at a soil temperature of 8oC, the DCD half-life is between 110 - 115 days. This is compared to a half-life of only 18 - 25 days at a soil temperature of 20oC. Similar to these results, in a review on the effect that temperature has on DCD degradation, Kelliher et al. (2008) concluded that, at a soil temperature of 22oC, the half-life of DCD was only 39 days. This is compared to a soil temperature of 10oC which had a DCD half-life of 100 days (Kelliher et al. 2008). Under New Zealand soil conditions, where the average soil temperature is about 13oC it is expected that DCD will have a half-life of 50 days (Cookson & Cornforth 2002). Based on published data from controlled environment studies, Kelliher et al. (2008) quantified the relationship between temperature (T) and the time it took the DCD concentration in soils to decline to half its application rate (t½) (Figure 2.12) as shown in Equation 2.12.

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Figure 2.12 Relationship between the half-life (t1/2, d) of DCD mixed into soil samples,

incubated under controlled conditions, and the corresponding temperature (T oC) (adapted from Kelliher et al. 2008).

Soil pH

The effectiveness of DCD reduces as the pH becomes more alkaline (Shi et al. 2011). Over a 60 day period, a soil which had a pH between 4 and 4.3 showed a 4% degradation of DCD. However, at a pH of 6.8 a 48% of DCD degradation was observed over the same 60 day period (Rodgers et al. 1985). Another example has been shown by Shi et al. (2011). When the pH was lower than 5.4, nitrification rates were reduced for 60 days. In comparison, nitrification rates were reduced for only 30 days when the soil pH was 6.2. Puttanna et al. (1999) identified a 35% reduction in soil ammonium-N (NH4+-N) content as the result of the soil pH increasing from 5.4 to 8.3 through

liming.

The effect of repeat applications of DCD

Currently, in New Zealand, the best management practice for the application of DCD is to use two applications a year. The first application is applied in late autumn within seven days of grazing when the soil temperature is less than 15oC. Approximately 60 days later, a second application of DCD occurs. Both applications of DCD use the recommended application rate of 10 kg DCD ha-1 (Watkins et al. 2013). Given the requirement (and pressure) to reduce the environmental impacts of NO3- leaching and N2O emissions there may be a long-term effect on a

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It has been identified that, when DCD is applied to a soil repeatedly, the degradation rate of DCD is greater compared to a DCD free soil (Rajbanshi et al. 1992). However, more recent studies have shown that both short-term and long-term repeat applications of DCD do not alter the DCD effectiveness (de Klein et al. 2011; Watkins et al. 2013). Watkins et al. (2013) identified that reapplication of DCD 57 days after the initial application resulted in no decline in DCD efficacy and the effectiveness of DCD was similar to that of the initial DCD application. In comparison, de Klein et al. (2011) identified that the long-term use of DCD over five consecutive years did not alter the effectiveness of DCD in suppressing N2O emissions. There was also no evidence for the

development of a DCD tolerance within the AOB community. However, it cannot be conclusively stated whether or not a tolerance to DCD will occur (Watkins et al. 2013).

Soil moisture content

The application of DCD is affected by the soil moisture content. A soil at field capacity has a lower DCD effectiveness, thus increasing nitrification. The opposite effect occurs as the soil moisture content decreases. At a soil moisture content of 36% of field capacity, DCD was effective in inhibiting the rate of nitrification for 90 days. In comparison, when the soil moisture content increased to 72% of field capacity, DCD effectiveness reduced to 60 days (Shi et al. 2011). This is due to the soil moisture status significantly affecting the movement of DCD within the soil profile. As the soil moisture content increases drainage increases thus DCD is leached out of the soil profile due to it being mobile (Shepherd et al. 2012a).