Physical and Chemical Manipulation of Urea Fertilizer To Limit the

Chapter 9

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Physical and Chemical Manipulation of Urea Fertilizer To Limit the Emission of Reactive Nitrogen Species M. I. Khalil*,1,2 1School

of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Republic of Ireland 2Climate Change Research Programme, Environmental Protection Agency, Johnstown Castle Estate, Wexford, Republic of Ireland *E-mail: [email protected]; [email protected]

Due to well established global warming concerns, technological attempts have been made to decrease reactive nitrogen (N) species emitted from the application of urea fertilizer to agricultural soils. This chapter summarizes previous work which investigates the mitigation potential for ammonia (NH3), nitrogen oxides (NOx) and nitrous oxide (N2O), which is a potent greenhouse gas, from arable lands. Specifically, the studies examined urea granule size, depth of placement, and contribution of chemical inhibitors. Relatively large urea granule (referred to as Urea Super Granule, USG; ~10 mm) inhibited nitrification up to 7 weeks and reduced both NH3 and NOx emissions up to 94%, compared to normal urea size. Under cropped conditions, the USG point-placed at 7.5 cm depth showed similar N2O emissions as urea prills (0.20-0.21% of the N applied) but increased to 0.51% under relatively higher soil moisture conditions. Surface application of urease inhibitor (phosphoric acid diamide-amended urea) decreased NH3 volatilization up to 50%. Compared to surface application, a modified version of the inhibitor (substituted phosphoric acid triamide) mixed with the soil reduced N2O emissions by 47%. Nitrification inhibitor (Dicyanamide plus triazole) inhibited nitrification up to 5 weeks and reduced N2O emissions up to 60%. A combination of the urease and nitrification inhibitors

© 2011 American Chemical Society In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

has the potential to limit NH3 emissions and decrease N2O losses up to 52%. Both deeper placement of USG and the combination of urease and nitrification inhibitors can mitigate the urea-induced emissions of reactive gaseous N species substantially.


Keywords: Urea fertilizer; mitigations; arable crop

nitrification;

trace gases;

Introduction: Perspectives and Progress of Urea Forms in Reducing Reactive N Species Reactive gaseous N species [NH3, nitrogen oxides (NOx = NO + NO2) and N2O] have great potential to cause environmental damage through processes such as global warming, stratospheric ozone layer depletion and acid deposition. Ammonia volatilization occurs through hydrolysis by reacting with enzyme urease, and NOx and N2O form in soils mainly through nitrification and denitrification, both direct and indirect. These chemical and biological reactions release gaseous N species into atmosphere under favorable soil and environmental conditions (1–4). Technological options are being researched globally to reduce gaseous N emissions from agricultural soils. Of particular concern are emissions associated with urea fertilizer, which is the cheapest and most widely used synthetic N fertilizer (>50% of global total nitrogenous fertilizers). Urea is an alkaline-hydrolyzing N fertilizer and is commonly available in the forms of small granules (10 mm) is gaining popularity in rice-growing areas, both irrigated or rainfed, due to greater agroeconomic benefits and less N losses (5). However, the potential of USG in reducing gaseous N losses in dry land crop production has got little attention. Under aerobic conditions, highly localized urea/NH4+, NO2- levels and specific soil pHs develop through enzyme-catalyzed urea hydrolysis in the placement zone of USG and diffuse slowly outward. This results in either little or no immobilization initially and inhibition of both urease and nitrification activity (6) compared to reduced gaseous N emissions (3, 7). Both surface-applied urea prills (PU) and shallow-placed USG in coarse-textured soils increase NH3 volatilization (1, 8, 9). High soil NH4+ concentrations can also result in high NOx emissions under aerobic conditions (10). Mixing urea with soils, banded/deep placement, immediate rainfall/irrigation and rapid drying of the surface soil after application might reduce N losses (11, 12). Reduction of N losses in deep or point-placed urea (13), USG in a clay soil (14), an alfisol (15), and medium to fine textured soils under upland rice and barley (16, 17) has been observed. In previous laboratory studies (3, 11), it was reported that deeper placement of USG can reduce NH3 and NOx emissions substantially compared to broadcasting/mixing PU with soils. The USG-induced 150 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


N2O emissions were found to be lower from coarse to medium textured soils (0.53-0.59%) than from a clay soil (2.61%; 3), though emissions (1.24%) from a silt loam was also reported (18). Studies on various urease and nitrification inhibitors as well as the combination of both with different chemistry have shown substantial reduction of gaseous N emissions from agricultural soils. The urease inhibitors, N-(n-butyl) thiophosphoric triamide (NBPT, commercially available as Agrotain®) and hydroquinone (HQ) reduced NH3 loss efficiently (19, 20). The nitrification inhibitor dicyandiamide (DCD) amended urea reduced N2O emissions but its efficacy in limiting NH4+ oxidation was short-term compared to 3,4-dimethypyrazole phosphate (DMPP; (21, 22)). The NBPT, DCD, and NBPT + DCD-amended urea applied to a maize field were found to reduce N2O emission by 37.7%, 39.0%, and 46.8%, respectively, over urea alone (23). The effects of urease inhibitor (phospohoric acid diamide; PAD; P204/98; 0.2%) on NH3 volatilization and of nitrification inhibitor (DCD plus 1H-1,2,4-triazole, DCD/TZ; commercially available as Alzon ® 46), both developed by the SKWP GmbH in Germany, on N2O emissions were tested and found to have capacity to decrease N losses substantially (24–26). A single compound, like DCD, demonstrated lower efficacy, higher volatility, greater instability, and a higher decomposition rate. Consequently, the need to apply larger doses of that compound may enhance NH3 volatilization loss. The combination of two nitrification inhibitors (e.g., DCD/TZ) resulted in a synergistic effect, which enhanced nitrification inhibition efficiency by prolonging the conversion duration of the fertilizer and allowed the dosage to be reduced (27). In recent times, SKWP introduced a new urease inhibitor at a lower concentration (substituted phosphoric acid triamide, sPAT; P101/04; 0.06%) than (PAD, P204/98; 0.2%), and a combination of urease and nitrification inhibitor (PAD + DCD/TZ refers to UNI). Urea produced with mixing either inhibitor is available in granular form, which is larger (2-3 mm) than the PU form. This chapter evaluates the comparative efficacy of the above-mentioned physical and chemical manipulation of urea fertilizer in limiting nitrification and decreasing the emission of reactive N species (NH3, NOx and N2O) from upland crop fields (3, 24–26, 28, 29).

Inhibiting Nitrification: Influence of Urea Granule Size and Inhibitors Nitrification is one of the key processes in the soil-plant system with particular reference to soil fertility and productivity but makes available the substrate for subsequent formation of gaseous N species. This process is universal and rapid in aerobic soil systems, occurring mainly through biological oxidation of NH4+ to NO2- and NO3-. The extent however depends on the availability of substrate as NH4+ (either added or mineralized), soil-environmental conditions and the associated microorganisms. Any attempts to depress the activities of nitrifiers result in limited/retarded oxidation of NH4+ and thereby decrease 151 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


leaching losses of NO3- and the formation of reactive gaseous N compounds (NO2, NO and N2O) during both nitrification and denitrification from anaerobic microsites. Other than management options, there are few reported works on limiting nitrification particularly using large urea granules applied to upland crops. There are numerous chemical compounds that can limit nitrification but only a few have been examined to find their affectivity under field conditions, for example, Nitrapyrin, DCD and a recently released DMPP (21, 22). However, a single chemical compound may be incompatible in limiting nitrification (27) and thereby reducing gaseous N losses across soil and land use types. To assess the potential of nitrification inhibitor (DCD/TZ) compared to urea granule size in delaying nitrification, Khalil et al. (29) carried out a greenhouse experiment in a loess (silt loam) soil cropped to spring wheat experiencing all natural weather conditions except rainfall, which was substituted by watering. The treatments consisted of three urea sizes: prills (PU

Physical and Chemical Manipulation of Urea Fertilizer To Limit the

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