J. Webb

A review of the effect of N fertilizer type on gaseous emissions

Abstract Between 10 and 20%of the N in fertilizers applied as urea is lost to the atmosphere as ammonia (NH 3 ). In contrast only small ( 3 have been measured following the application of ammonium nitrate (AN) fertilizer. In consequence the replacement of urea fertilizer with AN has been proposed as a cost-effective measure to reduce NH 3 emissions in Europe. However, because of the greater susceptibility of nitrate- (NO 3 − ) based fertilizers to denitrification, the replacement of urea by AN may lead to increased emissions of nitrous oxide (N 2 O). There was a need therefore to critically review the evidence for substantially greater emissions of NH 3 − from urea than from other N fertilizers and also to appraise the effect of fertilizer-N type on emissions of N 2 O. Ammonia emissions from N fertilizers are consistent with their known effects on soil chemistry. Those that increase soil solution pH, for example, by increasing HCO 3 concentration or by reducing the concentration of Ca 2+ , have the greatest potential for NH 3 emission. In consequence the greatest emissions of NH 3 are from urea applied to any soil and from ammonium sulfate (AS) applied to soils of pHs > 7.0. Losses of NH 3 from AN were confirmed to be consistently less than from urea. Emissions of NH 3 from solutions composed of urea and AN were found to be intermediate between the two fertilizers. Thus applying urea in solution will not reduce NH 3 emissions. However, NH 3 emissions from urea may be reduced by the use of urease inhibitors. Nitrous oxide emissions are crucially dependent on the interaction between timing of N fertilizer application and weather. Conditions in spring are more likely to be wet so that emissions are greater from NO 3 − -based fertilizers than from AS. In the summer conditions may be dry or wet; under dry conditions emissions are usually smaller than under wet conditions. For urea the effect of pH appears to be important. Generally greater emissions can take place from urea, except where temperature (controlling the rate of urea hydrolysis) and rainfall (controlling the dispersion of alkalinity) limit this. Thus, the substitution of AN for urea for spring applications is likely to increase emissions of N 2 O. For summer applications, the substitution of AN for urea is likely to decrease N 2 O emissions providing conditions are relatively dry; when conditions are wet large emissions may occur from both AN and urea. At this stage it is difficult to say with any certainty whether a strategy based on urea or AN would result in the smaller N 2 O emissions. Nitric oxide (NO) may also be released from soils following N fertilizer application. While soil emissions of NO are small in comparison with other sources of NO x , it is worth considering the effect of fertilizer type on this gas as well. Insufficient data is available to predict the effect of urea substitution on NO emissions, but since these are mainly a consequence of nitrification then replacing urea with AN should also reduce NO emissions.

Algorithms Determining Ammonia Emission from Buildings Housing Cattle and Pigs and from Manure Stores

Livestock excreta and manure stored in housing, in manure stores, in beef feedlots, or cattle hardstandings are the most important sources of ammonia (NH3) in the atmosphere. There is a need to quantify the emission, to assess the effect of emission on NH3 and ammonium (NH4+) deposition to ecosystems and on the health risks posed by NH4+-based particles in the air. To obtain a reliable estimate of the emission from these sources, the processes involved in the transfer of NH3 from the manure to the free atmosphere have to be described precisely. A detailed knowledge of the processes of NH3 transfer from the manure and transport to the free atmosphere will contribute to development of techniques and housing designs that will contribute to the reduction of NH3 emission to the atmosphere. For this reason, this review presents the processes and algorithms involved in NH3 emission from livestock manure in livestock buildings and manure stores for pigs and cattle. Emission from poultry buildings and following land application of manure, although significant sources of NH3, have been reported in earlier reviews and are not included here. A clear description of the features that contribute to the total NH3 emission from buildings will include information on stock class, diet and excreta composition, the distribution of emitting surfaces and knowledge of their mass transfer characteristics in relation to the building as a whole, as well as environmental variables. Other relevant information includes the quantity and composition of excreta produced by different classes of livestock and the influence of feeding regime; the influence of environmental variables on the production of NH3 from excreta; how excreta is distributed and managed in livestock buildings; and factors that affect mass transfer of NH3 in the building to the atmosphere outside. A major factor is the pH of the manure. There is a great need for algorithms that can predict pH as affected by feeding and management. This chapter brings together published estimates of NH3 emissions and abatement techniques, and relates these to the factors listed above (excreta, NH3 production, building, and mass transfer).

Dispersion, deposition and impacts of atmospheric ammonia: quantifying local budgets and spatial variability

Ammonia is a reactive pollutant emitted primarily by agricultural sources near ground level in the rural environment. The consequence of these factors is that, in addition to the effects of long-range pollutant transport, ammonia has major effects at a local scale, with emission and receptor areas often closely located in the rural landscape. There is a substantial local spatial variability that needs to be considered in effects assessments, while variations in local deposition may affect the amount of ammonia available for impacts further afield. The wide-ranging UK programme ADEPT (Ammonia Distribution and Effects ProjecT) has addressed these issues through a combination of measurement and modelling activities concerning the distribution of emissions, atmospheric transport, deposition and effects assessment. The results are illustrated here by summarizing the findings of a joint experiment at Burrington Moor, Devon, and wider modelling contrasting the variability at a field scale with 5 km resolution estimates for the UK. The fraction of emitted NH3 deposited locally is shown to depend critically on the downwind land-cover, with fluxes being dependent on interactions with the ammonia compensation point. This will restrict deposition back to agricultural land, but may mean that non-conservation woodlands could be of benefit to recapture a significant fraction of emissions. The generalized models demonstrate the high spatial variability of ammonia impacts, with a case study being used to show the consequences at a field scale. In source regions substantial variability occurs at sub-1 km levels and this will have major consequences for the emission reduction targets needed to protect ecosystems.

Estimating the potential for ammonia emissions from livestock excreta and manures

This paper reports a desk study to quantify the total-nitrogen (N) and ammoniacal-N contents of livestock excreta, and to compare them with estimates of N losses to the environment from that excreta. Inventories of ammonia (NH3), nitrous oxide (N2O), dinitrogen (N2), and nitric oxide emissions (NO), together with estimates of nitrate (NO3-) leaching and crop N uptake were collated. A balance sheet was constructed to determine whether our estimates of N in livestock excreta were consistent with current estimates of N losses and crop N uptake from that N, or whether emissions of N compounds from livestock excreta may have been underestimated. Total N excretion by livestock in England and Wales (E&W) was estimated as 767-816 x 10(3) t of which 487-518 x 10(3) t was estimated to be total ammoniacal-N (TAN). Estimates of NH3 and N2O losses during housing and storage were derived from the difference between the total amount of TAN in excreta deposited in and around buildings, and the total amount of TAN in manure (i.e. the excreta deposited in and around buildings after collection and storage) prior to spreading and were ca. 64-88 x 10(3) t. The NH3-N emission from livestock buildings and manure storage in E&W quoted in the UK Emission Inventory (Pain et al., 1999. Inventory of Ammonia Emission from UK Agriculture, 1977. Report of MAFF contract WAO630, IGER, North Wyke) is ca. 80 x 10(3) t. Losses from NO3- leaching in the season after manure application and grazing were estimated as 73 and 32 x 10(3) t, respectively. Other gaseous losses of N were estimated as ca. 54 x 10(3) t. Crop uptake of manure N was estimated to be between 7 and 24 x 10(3) t. For manures, estimated N losses, immobilization and crop uptake total 326 x 10(3) t compared with estimates of 293-319 x 10(3) t TAN in excreta. Total N losses and crop uptake from TAN deposited at grazing were estimated to be 179-199 x 10(3) t compared with ca. 224 x 10(3) t TAN excreted. Thus all the TAN in manures appears to be accounted for, but ca. 25-45 x 10(3) t of TAN in urine deposited at grazing were not, and could be an underestimated source of gaseous emission or nitrate leaching.