Ammonia emissions from agriculture Brian Pain and Steve Jarvis Sources of ammonia 48 Factors affecting ammonia emission 48 Abatement options 5 Modelling emissions 51
Brian Pain and Steve Jarvis Ammonia is a highly reactive gas that has important effects on atmospheric chemistry and sensitive terrestrial or aquatic ecosystems. Deposition of ammonia as a gas or as ammonium salts, following reaction with acidic compounds in the atmosphere, can add nitrogen to nutrient poor soils e.g. heath land, and change the types of plants that grow there. Ammonium salts can be transported over long distances in the atmosphere in aerosols before being deposited to the surface and contributing to acid rain. Increase in soil acidity can result from nitrification to A B Poultry 19% Tillage Crops 5% Conserved grass 5% Pigs 12% Manure stores 8% Housing 39% Sheep 6% Fertilizer 1% Outdoor 13% Cattle 53% Manure spreading 3% Figure 8.1 The distribution of agricultural contributions to annual ammonia emissions in the UK (A) From different agricultural sectors (B) From components of livestock farming. nitrates. This affects the solubility of both essential and toxic elements which can be particularly damaging to woodlands on weakly buffered soils. Because of these environmental concerns, there are strong pressures within the EU to lower ammonia emissions from agriculture. Regulations are already in place in some countries and likely to follow in the UK within a few years. Sources of ammonia Agriculture is the major source of ammonia emissions to the atmosphere, mainly through the rapid hydrolysis of urea excreted by livestock. Although there are large uncertainties, a recent inventory compiled with colleagues from ADAS and the Silsoe Research Institute estimated that about 23kt ammonia-n per year arises from agricultural sources in the UK. In common with many other European countries, this accounts for about 8% of the total emission. The inventory was estimated by using agricultural statistics and survey data with measured emission factors, or rates of emission, for each source. Figure 8.1 shows the importance of manures, whether these are in animal houses, in storage or spread on land, and of cattle farming. Grazing animals and nitrogen fertilizers, especially urea, are smaller but significant sources. This information is used by policy makers, at a national and international level, in negotiating targets for reduction and assessing the impact of control measures. Factors affecting ammonia emission The process of ammonia volatilisation leading to emission is relatively simple and well understood. In practice, volatilisation from the surface of manures or soils in the field is influenced by a wide range of chemical, biological and physical factors that greatly increase complexity. When slurries are spread on land, rates of ammonia volatilisation are very high for the first few hours and then decrease to lower levels which may persist for several days. It is possible for all the ammonium - N in 48
Loss (% ammonium-n applied) 12 site 1 12 8 site 2 6 4 2 1 2 3 4 5 Wind speed (m/s) Figure 8.2 Effect of wind speed on ammonia loss from slurry applied to grassland. the slurry to be lost in this way. Slurry composition, especially ph, ammonium - N and dry matter content, soil type and conditions, crop and weather together with their interactions, all affect the extent and rate of loss. Figure 8.2 illustrates how wind speed, for example, has a large and consistent effect on rate, whereas the extent of loss varies between sites. This is largely due to differences between soils. For systems where stock are loose-housed on straw bedding, many factors affect ammonia emissions from the buildings, during storage, and/or composting of farm yard manure (FYM) and application to land. The amount of straw used for bedding, for example, has an impact not only on emissions from the buildings (Figure 8.3) but also on subsequent emissions during storage and spreading (Table 8.1). For grazed swards, the major source of ammonia is urine excreted directly onto the pasture. We have quantified losses from pastures when sheep (Figure 8.4) Table 8.1 Effects of the amount of straw bedding (at 1,.75 and 1.25 times the standard rate of use) on annual ammonia emissions from beef cattle systems Increasing straw usage.75 1. 1.25 kg NH 3 -N per 5kg liveweight Housing 2.7 1.6 1.2 Storage.9.9.4 Spreading 2.7 1.2.7 Total 6.3 3.7 2.3 and cattle graze and also examined the patterns of emission. As Figure 8.5 shows, there are complex patterns of emission and subsequent deposition and reemission from urine patches, which are superimposed on strong diurnal variation. Whilst this makes prediction of effects difficult it does indicate the importance of local effects in controlling ammonia fluxes from this source. Figure 8.3 Experimental housing for beef cattle designed for measuring ammonia emissions. 49
Figure 8.4 Measurement of ammonia emissions from grazing sheep using mass balance micrometereology. Sheep are grazing within an enclosed circle and profiles of ammonia concentration entering and leaving the circle are determined. Abatement options In addition to the impact on the wider environment, ammonia emission is a major source of loss from the N- cycle on livestock farms so the potential benefits of abatement are twofold. There are few practical or low cost opportunities for decreasing ammonia losses from livestock housing or grazing. Making more efficient use of fertilizer and reducing application rates has some effects, but this has only a small impact on the overall farm loss. Urea fertilizer can have a high ammonia emission rate when applied to grass swards under warm moist conditions, but when there is insufficient rain to µg NH 3 -N / m 2 / h 4 3 2 1-1 -2 LD SE SD LE 1 2 3 4 5 Time (days) wash the ferilizer into the soil, so there is potential to make more careful use of this product. Because of the relatively low losses from grazing compared with losses from the housed phase, one suggestion has been to extend the grazing season so that the amount of excreta produced indoors is reduced. The covering of slurry stores may be required by future regulations, but the focus is likely to be on decreasing the large emissions that occur when slurries or solid manures, such as FYM, are spread on the land. We have shown how shallow injectors (Figure 8.6) and band spreaders can decrease ammonia emissions under UK conditions by 5-8% compared with more conventional broadcast application of slurry. The effectiveness of these machines relies on reducing the surface area of slurry exposed to the air, increasing the rate of infiltration into the soil so that ammonium-n becomes bound to clay particles, or reducing air flow over the slurry surface by placement beneath a crop or grass canopy. Entrapment or uptake of ammonia by plant leaves may also be important for the latter. Incorporation into the soil by ploughing soon after spreading is an option for FYM. Figure 8.5 Rates of ammonia exchange (emission (E) and deposition (D)) from short (S) and long (L) swards after deposition of urine. However, measures designed to reduce losses of N as ammonia can increase losses through other pathways. Although the risk of increasing nitrate leaching appears 5
Figure 8.6 Shallow injectors cut 4-5 cm deep slots in grassland soils which are filled with slurry. Micrometeorological techniques are used for measuring ammonia emissions. to be small, denitrification losses and release of the greenhouse gas nitrous oxide are sometimes greater from injected slurry (Figure 8.7). Modelling emissions Modelling is needed at a range of scales to aid understanding of the complex processes and interactions involved and to assess the impacts of abatement measures on total emissions from farms and larger land areas. As part of an EU project, a mechanistic model of emissions from manures is being developed to enable prediction of losses under a wide range of circumstances. Other models are also being developed to integrate with existing grassland nitrogen cycling models (NCYCLE) so that prediction of overall effects on all the nitrogen losses can be made and effects of management changes examined. We have already used existing information in a desk study to examine effects on ammonia loss over a whole dairy farm, and to quantify the impact of improving the nitrogen use efficiency within the whole system on N emitted as nitrous oxide (mg/m 2 per hour Kg N / Ha 5 4.5 4 3.5 3 2.5 2 1.5 1.5 Figure 8.7 Nitrous oxide emissions following broadcast or shallow injection of slurry to grassland and from the untreated sward (control) 5 4 3 2 1 Control Broadcast Injected 27-Jul 1-Aug 6-Aug 11-Aug 16-Aug 21-Aug Total losses Grazing & fertilizers House + slurry store Slurry application A B C D B+D Figure 8.8 Ammonia losses from a typical dairy farm ( A: current practice) and effects of management changes involving improved fertilizer and slurry use (B) converting to mixed grass/clover swards (C), substituting a proportion of the grass silage with maize silage (D) and combining B and D. emissions from all components of the farm. As can be seen in Figure 8.8, it is possible, using methods which are already at hand, to reduce substantially the losses of ammonia by manipulating farm practice to make better use of the nitrogen within the system. Contact: steve.jarvis@bbsrc.ac.uk or brian.pain@bbsrc.ac.uk 51