Determinación y cálculo de los parámetros de validación

Agricultural sector emissions of ammonia represent a major nitrogen loss in the New Zealand pastoral system with a number of associated downside effects via soil acidification and eutrophication of aquatic systems (Saggar et al. 2004a). Ammonia volatilisation is commonly used to refer to the process by which gaseous NH3 is emitted from the soil surface to the atmosphere

and is a complex process involving physical, chemical and biological factors. A supply of free ammonia (i.e. NH3 (g) + NH3 (aq)) near the soil surface is a prerequisite to gaseous loss and is

favoured by high pH via the reaction between ammonium and hydroxide ions (2.4-1).

𝑁𝐻₄+_{+ 𝑂𝐻}− _{⇄ 𝑁𝐻}

3↑ +𝐻2𝑂

2.4-1 Major ammonium sources include animal urine and faeces, farm effluent, organic residues and native SOM but also ammonium (NH4+)-incorporated fertilisers (e.g. NH4NO3, NH4Cl, (NH4)2SO4,

and (NH4)2HPO4,) and urea ((NH2)2CO). Urea (fertiliser or urine) undergoes hydrolysis readily in

soils (2.4-2), catalysed by the enzyme urease to form ammonium carbonate (NH4)2CO3:

(𝑁𝐻₂)₂𝐶𝑂 + 2𝐻2𝑂 → (𝑁𝐻4)2𝐶𝑂3

2.4-2 The hydrolysis reaction in soil is often rapid and zero order (Vlek & Carter 1983), with half-lives of urine-urea measured commonly in hours. Sherlock and Goh (1984) calculated half-lives for urea in urine of 3.0 and 4.7 hours for summer and autumn in New Zealand, respectively, the higher value attributable to lower soil temperatures. The release of ammonium ions after hydrolysis allows interaction with those on the cation exchange complex, resulting in electrostatic binding to soil colloids and equilibrium reactions with NH4+ ions in soil solution. The cation exchange capacity

(CEC) of the soil buffers to some extent, this increase, and any associated rise in soil pH. Consequently, high cation exchange capacity (CEC) soils (e.g. clay loam) tend to have lower NH4+

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Soil pH has a significant influence on ammonia volatilisation and soils with naturally high pH (>7; e.g. calcareous soils) are capable of losing significant amounts of ammonia gas (Bishop & Manning 2011). However, neutral or acid soils can also lose significant amounts via the reaction shown in equation 2.4-3 due to the sharp increase in pH from the reaction of the carbonate ion with water. Localised zones of high pH (>8) form close to the site of hydrolysis and these favour production of ammonia gas (Figure 2.4-1) and thus, increased volatilisation (Sherlock & Goh 1984; Black et al. 1987; Sherlock et al. 1995; Sommer et al. 2004). Indeed, Black et al. (1985b) found there was a strong positive relationship between volatilisation and maximum surface pH achieved by the fertiliser used. With the onset of nitrification processes, soil pH eventually decreases, reducing the volatilisation rate.

(𝑁𝐻₄)₂𝐶𝑂3+𝐻2𝑂 ⇄ 2𝑁𝐻4++ 𝐻𝐶𝑂3−+ 𝑂𝐻−⇄ 𝑁𝐻4++ 𝑁𝐻3↑ +𝐻2𝑂 + 𝐶𝑂2

2.4-3

Figure 2.4-1. Ammonia volatilisation loss and soil pH around a broadcast urea granule (McLaren & Cameron 1990a).

The conversion of NH4+ to NH3 regulates the potential loss of NH3 to the atmosphere through the

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solution, release into the soil atmosphere, and diffusion away from the soil surface. Generally, the higher the concentration of ammonium in soil solution, the greater the potential ammonia emission rate. Rising air and soil temperature, windspeed, soil moisture (rapid hydrolysis), roughness (increased turbulence), and porosity (gas diffusion rate) are all environmental and soil factors shown to increase ammonia volatilisation rates (Vlek & Carter 1983; Black et al. 1987; Sommer & Olesen 1991; Sherlock et al. 1995; Sommer et al. 2004). Conversely, mitigation methods to reduce ammonia losses include transporting urea to depth via rainfall or irrigation soon after fertiliser application (Sherlock & Goh 1984), incorporation below the surface (Sommer et al. 2004), and the use of urease inhibitors such as N-(n-butyl) thiophosphoric triamide (NBPT) to coat urea granules (Watson et al. 1994). The latter has been shown to reduce NH3 volatilisation by up to 95%.

Although dry soils slow the initial hydrolysis of solid urea fertilisers over the short-term, losses over the long-term may be similar if the NH4+ ions remain largely on or near the soil surface (Sherlock

et al. 1995).

Volatilisation losses from N fertilisers or animal excreta applied to soils can vary widely depending on the amount, form, concentration and degree of incorporation. Bishop and Manning (2011) recently summarised published results on urea fertiliser volatilisation losses for NZ, the most commonly used N fertiliser, for both pastoral and arable cropping. Mean N volatilisation losses for acid soils were 20±10% for grasslands (15-500 kg Nha-1 _{applied) and 14±7% (46-200 kg N ha}-1

applied) for arable cropping. These are comparable with losses reported in New Zealand of typically 5-15% of the N applied where urea application rates are <50 kg N ha-1 _{(Black et al. 1985b; Haynes}

& Williams 1993; Di & Cameron 2004b). Volatilisation losses from dung deposited whilst grazing are regarded as less significant than those from urine due to the fewer applications and smaller area covered by dung, their slower decomposition and the majority of N occurring in less mineralisable forms (Haynes & Williams 1993). Where excreta storage is required (common in European systems), volatilisation losses of the ammoniacal-N present can be significant (19-100%) (Sommer & Olesen 1991) but these same wastes applied to pastures can also incur large losses, although this is very dependent on the waste type and ammoniacal-N present (Bolan et al. 2004).

In the pastoral system the grazing animal is estimated to return up to 85-90% of the ingested N in urine and dung (Cameron et al. 2013). Of this amount, it has been observed that sheep and dairy

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cattle excrete 70–75% and 60–65% of N in urine, respectively, when grazing N-rich grass/legume pastures (Oenema et al. 1998). Whilst the majority of the N present in both dung and urine is in organic forms, typically 70% of the N in urine is present as easily hydrolysable urea and the rate of N application may be as high as 1000 kg N ha-1 _{in a urine spot (Oenema et al. 1998; Di & Cameron}

2002a). Ammonia volatilization losses from urine, consequently, can range from 2-46% and commonly 15-25% of the N present, with losses typically greatest in the first few weeks after application (Haynes & Williams 1993). Losses are favoured under hot, dry summer conditions but minimised under cooler, moist winter conditions (Haynes & Williams 1993; Di et al. 2002). Sherlock and Goh (1984) reported losses of 22-25% over summer/autumn in New Zealand but only 12% in winter, whilst Di et al. (2002) found volatilisation losses from an autumn urine application of only 2% of total-N. Total volatilisation losses for pastoral grazing systems in New Zealand have been shown to range from 15-68 kg N ha-1 _{for intensive dairying systems (Ledgard et al. 1999) and}

around ~13 kg N ha-1 _{for hill country sheep pastures (Haynes & Williams 1993).}