Effects of the different rates of urease and nitrification inhibitors on gaseous emissions of ammonia and nitrous oxide, nitrate leaching and pasture production from urine patches in an intensive grazed pasture system

Abstract

Minimizing nitrogen (N) losses via ammonia (NH₃) and nitrous oxide (N₂O) emissions into the atmosphere and nitrate (NO₃⁻) leaching into surface and ground waters from intensively grazed pastures is essential for environmental protection worldwide. Applying urease inhibitor such as N-(n-butyl) thiophosphoric triamide (nBPT) or (Agrotain) and nitrification inhibitor dicyandiamide (DCD) to grazed pastures has the potential to mitigate such N losses. A lysimeter/mini plot experiment, using Paparua silt loam soil near Lincoln, Canterbury New Zealand, was conducted to quantify these N losses during May 2007 to July 2008. The nine treatments were: cow urine only applied at an equivalent rate of 600 kg N ha⁻¹, urine + DCD at 5 kg ha⁻¹, urine + DCD at 7 kg ha⁻¹, urine + DCD at 10 kg ha⁻¹, urine + double inhibitor (DI), i.e. both Agrotain and DCD applied at 1 L ha⁻¹ and 7 kg ha⁻¹, respectively (or 1:7 of v/w basis), urine + DI (1:10), urine + DI (2:7), urine + DI (2:10) and the control (no urine). These treatments were randomly applied to one set of lysimeters or mini plots in May as autumn and then to another set of lysimeters or mini plots in August as spring applications. Additional nine lysimeters received DCD only at rates equivalent to 5, 7 and 10 kg ha⁻¹ in autumn to see if DCD has any effect on NO₃⁻ leaching and pasture production and N uptake from non-urine patches in autumn. Gaseous emissions of NH₃ and N₂O, NO₃⁻ leaching and pasture production and N uptake varied with the types and rates of the applied inhibitors during the two seasons. DCD applied at 7 and 10 kg ha⁻¹ rates with urine was more effective than its lower rate of 5 kg ha⁻¹ and reduced N₂O emissions by 37–53% (autumn) and 47% (spring), NO₃⁻ leaching losses by 57–55% (autumn) and 26–10% (spring) compared with urine alone. However DCD increased NH₃ emissions by 41% and 18% compared with urine alone treatment after autumn and spring, respectively. DCD applied at higher rates also increased pasture dry matter by 9% and 12% and N uptake by 12% and 6% after autumn and spring applications, respectively. However DCD applied at different rates without urine in autumn had no such effect on either NO₃⁻ leaching or pasture dry matter yield or N uptake. The DI at 1:7 ratio was more effective than the higher rates of DI and DCD in reducing losses of NH₃ (48% and 51%), N₂O (55% and 63%) and NO₃⁻ leaching (56% and 42%) as well as increasing pasture production (13% and 17%) and N uptake (7% and 18%) compared with urine alone treatment in autumn and spring, respectively. These results suggest that applying Agrotain + DCD at a ratio of 1:7 (v/w) may provide the best option for both mitigating N losses and improving pasture production in intensively grazed systems.

Introduction

Nitrogen is an essential plant nutrient and key to the sustainability and economical viability of agricultural systems. The highly dynamic nature of N makes its efficient use and management a challenging task, especially in intensive agricultural systems where huge N inputs are likely to result in significant N losses via surface runoff of NH₄⁺, NO₃⁻ leaching into surface and ground waters and gaseous emissions of NH₃ and N₂O into the atmosphere. Such N losses have economical implications and pose a major threat to environmental quality worldwide, as 90% of NH₃ (Boyer et al., 2002), 70% of N₂O emissions (Janzen et al., 1998) and a significant amount of NO₃⁻ leaching (Jarvis et al., 1995) are reported to be from land-use activities. In New Zealand, the major land-use is legume-based pastures, which is predominantly comprised of ryegrass (Lolium perenne L.) and some white clover (Trifolium repens L.). These pastures are regularly grazed by animals (approximately 5.2 m dairy cows, 4.4 m beef cattle and 40 m sheep) throughout the year. Animal excreta (urine + dung) of grazing animals are reported to make up 50% (1.58 m tonnes) of the total 3 m tonnes of annual N inputs these grazed pastures receive (Saggar, 2004), therefore they have been identified as the major source of N losses.

It has been estimated that 60–90% of the N ingested by a dairy cow is not efficiently metabolized (Whitehead, 1995) and returned to the grazing paddocks in the form of animal excreta of which over 70% is urine. A single urination event deposits N in the range of 600–1000 kg N ha⁻¹ (Haynes and Williams, 1993, Jarvis et al., 1995) which is well above the N requirement of pastures (Blennerhassett et al., 2006). Urea–N constitutes the major fraction (80%) of urine N (Zaman et al., 2007, Zaman et al., 2009), while the rest is a mixture of readily mineralizable amino acids, peptides and ammonium–N (NH₄⁺) (Bolan et al., 2004). The deposition of a huge amount of urinary N in a small area accelerates N cycling. First urea–N starts to be hydrolyzed quickly within 1–2 days (Zaman et al., 2009) and produces NH₃, hydroxyl (OH⁻) ions and carbon dioxide (CO₂) (Mulvaney and Bremner, 1981). The large production of NH₄⁺ along with a temporary rise in soil pH by OH⁻ as a result of fast urea hydrolysis in a urine patch thus provides ideal hotspots for NH₃ volatilization (Zaman et al., 2009). Ammonia itself is not a greenhouse gas; however after deposition on land, NH₃ produces N₂O through the microbial processes of nitrification and denitrification and thus contributes to global warming and ozone (O₃) depletion (Martikainen, 1985). Ammonia emissions also represent a major loss of nutrient N (Rochette et al., 2009) and also result in degradation of air and water quality (Galloway et al., 2003). Ammonium ions produced as a result of urine–N hydrolysis or present in cow urine are largely protected from leaching losses because of NH₄⁺ adsorption onto exchange sites on clay particles. However NH₄⁺ is likely to be nitrified within 2–4 weeks and during that time significant amount of N₂O is produced as byproduct (Firestone and Davidson, 1989). Nitrate produced as a result of NH₄⁺ oxidation, is repelled from soil exchange sites because of its negative charge and is prone to both leaching losses if drainage occurs, and emission to the atmosphere as N₂O via denitrification. Nitrate leaching from intensive agricultural land-use also represents a major N loss and contributes to water pollution (i.e. increased NO₃⁻ level in drinking water; eutrophication and fish poisoning) (Howarth, 1988) and N₂O production by providing substrate for denitrification. Nitrous oxide, a long lasting (150 years life time) greenhouse gas, accounts for 7% of the current anthropogenic greenhouse effect (Duxbury, 1994), and is predominantly produced by soil microbial processes of nitrification (Inubushi et al., 1996) and denitrification (Tiedje, 1988, Firestone and Davidson, 1989). Nitrous oxide also depletes O₃, a substance that protects the biosphere from harmful ultraviolet (UV) radiation, by oxidizing into N-oxide (NO) in the stratosphere (Crutzen, 1981). About half (49.4%) of total greenhouse gas emissions in New Zealand come from the agriculture sector, of which N₂O accounts for one third (Mfe, 2008). As a signatory of the Kyoto Protocol, New Zealand must reduce its greenhouse gas emissions back to 1990 level by 2008–2012. However, the rapid expansion of dairy farming, high stocking rate per ha, increased use of urea fertilizer, and high return from dairy products are making it difficult to achieve the 1990 level of greenhouse emission and minimize other N losses such as NH₃ emissions and NO₃⁻ leaching. Therefore developing mitigation tools for N losses is critical not only to avoid the purchase of carbon (C) credits but also to preserve our clean and green image by protecting our surface and ground water reserves.

Among the different management options proposed by various researchers to minimize these N losses from intensive grazed pastures (Monaghan et al., 2005, de Klein et al., 2006, Lehmann et al., 2006, Zaman et al., 2007, Zaman et al., 2008a, Zaman et al., 2008b, Zaman et al., 2008c); the use of nitrification inhibitors to treat urine patches has received the most attention recently. Several lysimeter studies (Di and Cameron, 2002a, Di and Cameron, 2002b, Di and Cameron, 2003, Di and Cameron, 2004, Di and Cameron, 2005) and glasshouse and field studies (Hoogendoorn et al., 2008, Smith et al., 2008) have reported that DCD can reduce N₂O emissions and NO₃⁻ leaching from pasture soils treated with either urine or urea. However, none of those studies assessed and reported the effect of DCD on increasing NH₃ emission which is reported to contribute to greenhouse gas emission indirectly as discussed earlier. A number of studies have reported increased NH₃ emissions after application of DCD to soil treated with urea or urine (Davies and Williams, 1995, Dobbie and Smith, 2003, Asing et al., 2008, Singh et al., 2008, Zaman et al., 2009). It is therefore important to assess the complete N system rather than focusing on some aspects and leaving others while developing mitigation tools for N losses. One such approach is to slow down hydrolysis of urea (added as chemical fertilizer or cow urine) through urease inhibitor and then retain N in relatively less mobile form (NH₄⁺) through the use of nitrification inhibitor. Zaman et al. (2009) recently reported that the combination of both Agrotain and DCD applied at 3:7 (v/w) to Tokomaru silt loam soil at Massey University, Palmerston North, New Zealand was more effective in reducing both NH₃ and N₂O emissions, improving pasture production, controlling urea hydrolysis and retaining N in NH₄⁺ form than applying DCD or Agrotain alone. However, the application rate of Agrotain in the combined inhibitor was beyond the level required for the product to be cost effective for farmers to use. Therefore the objectives of our study were to identify the best rate of Agrotain plus DCD inhibitor which could offer hope to minimize the three key N losses from urine patches and improve pasture production in a cost effective manner. These treatments were also compared with the N losses and pastoral production from different rates of DCD alone.