We need to talk about Nitrogen

Plantlife, together with the Centre for Ecology & Hydrology and other organisations (RSPB, Stockholm Environment Institute, Woodland Trust, and British Lichen Society) have been talking about nitrogen. In a recent report and workshop we have highlighted the need to raise the profile of the nitrogen problem.

Why nitrogen?

Reactive nitrogen (Nr) is defined here as all other nitrogen forms apart from N2, for example oxidized nitrogen (NO, NO2, NO3) and reduced forms of nitrogen (eg NH3 , NH4+)

Nitrogen (N) in the form of reactive nitrogen (Nr) had until the last century been a limited natural source to plants provided mainly by biological fixation. Nitrogen is a major growth nutrient - all plants need N in order to grow. It is a major constituent of structural tissue, facilitating conversion of CO2 to carbohydrate. However, this limited availability of N has made many plants and habitats very efficient and effective at surviving at low nutrient levels.

With the necessity to sustain a growing population improvements in crop yields have paved the way to increase production of synthetic fertiliser using the Haber-Bosch process – the process of producing ammonia from hydrogen and nitrogen in air (N2) under high temperature and pressure. This additional availability of reactive N has increased agricultural intensification, increased livestock production and boosted crop yields. Furthermore, the increased use of fossil fuels has intensified the expansion in the use of N around the globe. However, with the excessive use of nitrogen has come the unintended losses and leakages in the system leading to environmental issues like eutrophication of soils and freshwaters, acidification, poor air quality, and greenhouse gas emissions.

Sources

Nitrogen is emitted into the atmosphere in a number of ways – as nitrogen dioxide (NOx) from power station stacks, and other fossil-fuel combustion processes including transport, and as ammonia (NH3) predominantly from agricultural sources like livestock manure and fertiliser application to crops.

Nitrogen deposition describes the input of reactive nitrogen from the atmosphere to the biosphere both as gases, dry deposition and in precipitation as wet deposition. And while essential for the cultivation of arable crops, nitrogen deposition from the atmosphere has a negative impact on sensitive plants and habitats that are used to low nitrogen inputs.

NOx sources with % contribution to UK emissions

Vehicle exhaust fumes  Gas powered generator
Transport ~50% (above left) and Power generation eg gas powered generator ~25% (above right)

Production areas of ammonia (NH3) emissions with % contribution to UK

Chickens inside a poultry farm Fertilizer application
Housing and hard standings 33% (above left) and Fertilizer application 17% (above right)

Farmyard manure Slurry spreading
Manure/slurry storage 13% (above left) and Manure/slurry spreading 24% (above right)

Ecosystem Impacts and current health status

Current scientific evidence demonstrates clear impacts of nitrogen deposition in low nutrient ecosystems including:

  • Loss of species richness

A number of studies have shown that nitrogen deposition is an important driver of plant community change (Field et al., 2014; Stevens et al., 2010). As more nitrogen is added from the atmosphere to semi-natural habitats the diversity of sensitive lichens, mosses and forbs declines, while the percentage of the cover of grasses increases.

Graph showing relationship between N deposition and percentage of maximum number of species in each habitat
Figure: Relationship between N deposition and the percentage of maximum number of species in each habitat (Field et al., 2014)

  • Loss of sensitive species like lichens and bryophytes (mosses)

Lichens obtain almost all their nutrients from the atmosphere. They have no means of controlling nutrient uptake, unlike vascular plants, with free exchange of both gases and solutions. This makes them very sensitive to increases in N and as a consequence lichens are equivalent to an early warning ‘canary’ for sensitive plants. Similarly mosses are among the most sensitive components of the vegetation with respect to air pollutant deposition, and can be sensitive to both acidity and N.

Mosses and algae on two tree trunks

  • Changes in soil chemistry leading to acidification and
  • Habitat degradation

In 2014, the nitrogen critical load was exceeded for 62% of the area of sensitive habitats in the UK (excluding Scotland, the area is more than 90%). For calcareous grassland and many types of woodland, the area exceeded in the UK was 86% - 100%. Critical loads for nitrogen can be seen as an environmental limit set for habitats where below a certain exposure to a pollutant harmful effect do not occur– for example the critical load for a raised bog is between 5-10 kg N/hectare/year.

Emission Trends

Levels of atmospheric reactive nitrogen have tripled in Europe in the last century. Since 1990, there have been substantial declines in NOx emissions, with continuing projected declines, but ammonia emissions have seen little decrease and predicted to remain stable. Furthermore, ammonia emissions from agricultural sources are the dominant source of nitrogen deposition across the Natura 2000 network. This means that future options for reducing critical load exceedance need to focus on reducing ammonia emissions.

Mitigation actions, technologies and policy drivers

Nitrogen oxides - NOx

Mitigation technologies are available for NOx emissions from transport, power stations and other sources using Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) where the levels of NOx can be reduced up to 90%. Other technologies to reduce NOx also include the use of renewable energies and electric vehicles:

Two graphs showing UK NOx and NH3 emissions 1990 to 2020

Ammonia - NH3

In agriculture, nitrogen management plans can decrease the N surplus and increase N use efficiency, leading to reduced ammonia emissions. Examples of management measures include (with corresponding potential ammonia emissions reductions):

  • Housing floor systems to remove cattle/pig manure (20 – 46% reduction)
  • Slurry spreading systems – trailing shoe (30 – 60% reduction) or disc injector (70 – 90% reduction)
  • Covering slurry stores with tight lid (80% reduction) or floating cover (60% reduction)
  • Acidifying slurry with sulphuric acid in bunded tanks (60% reduction)
  • Drying of poultry manure (35 – 45% reduction)
  • Acid scrubbers in housing can remove ammonia removal by up to 90%, can also reduce odour and particulate matter by 75% and 70%;
  • Anaerobic Digestion can potentially increases ammonia emissions when spreading the digestate, unless an acid is added to reduce the pH or it is further processed to recover its N content.
  • Tree planting as shelter and grazing areas to capture N and increase dispersion (20 – 25% reduction) to protect semi-natural areas downwind of ammonia sources.

Discs cutting into soil to place slurry Trailing shoe slurry spreader, bomech.nl
Above left: Discs are used to cut vertical slots in the soil up to 50mm deep into which slurry is placed. Photo: bomech.nl
Above right: Trailing shoe slurry spreader enables plants to absorb nutrients from slurry faster and reduces leaf contamination. Photo: bomech.nl

Tight lid covers on slurry tanks Hens ranging under trees
Above left: Tight lid cover is the best proven and most practicable method to reduce emissions from slurry stored in tanks or silos. They can be retro-fitted. Photo: cunninghamcovers.co.uk
Above right: Hens ranging under trees. Tree planting can be used to capture ammonia and provide suitable shelter and ranging areas.

Importantly, options for reducing ammonia at the housing, storage and spreading stages are interdependent, and combinations of measures are not simply additive in terms of their combined emission reduction. For example, controlling emissions from spreading is very important as it is the last stage of manure/slurry handling, and much of the benefit of abating during housing and storage may be lost. Likewise, controlling emissions from land application will have less benefit if large losses occur at the housing and storage stages.

Finally, ammonia emissions not only have negative effects on the environment but also represent an inefficient use of manure-N as a fertilizer. Therefore there is a potential for farmers to save money by reducing ammonia emissions.

Bill Bealey

Further information

RoTAP (2012) Review of Transboundary Air Pollution: Acidification, Eutrophication, Ground Level Ozone and Heavy Metals in the UK. Contract Report to the Department for Environment, Food and Rural Affairs. Centre for Ecology & Hydrology.

Field, C, Dise, N, Payne, R, Britton, A, Emmett, B, Helliwell, R, Hughes, S, Jones, L, Lees, S, Leake, J, Leith, I, Phoenix, G, Power, S, Sheppard, L, Southon, G, Stevens, C and Caporn, S M (2014). The Role of Nitrogen Deposition in Widespread Plant Community Change Across Semi-natural Habitats. Ecosystems 17(5): 864-877. DOI: 10.1007/s10021-014-9765-5

Stevens, C, Dupre, C, Dorland, E, Gaudnik, C, Gowing, D J G, Bleeker, A, Diekmann, M, Alard, D, Bobbink, R, Fowler, D, Corcket, E, Mountford, J O, Vandvik, V, Aarrestad, P A, Muller, S & Dise, N B (2010). Nitrogen deposition threatens species richness of grasslands across Europe. Environmental Pollution, vol 158, no. 9, pp. 2940-2945. DOI: 10.1016/j.envpol.2010.06.006

Bealey W.J., Loubet B., Braban C.F., Famulari D., Theobald M.R., Reis, S., Reay, D.S. and Sutton M.A. (2014) Modelling agro-forestry scenarios for ammonia abatement in the landscape. Environmental Research Letters, 9 (12), doi:10.1088/1748-9326/9/12/125001. 

Bittman, S., Dedina, M., Howard C.M., Oenema, O., Sutton, M.A., (eds), 2014, Options for Ammonia Mitigation: Guidance from the UNECE Task Force on Reactive Nitrogen, Centre for Ecology and Hydrology, Edinburgh, UK 

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Bill Bealey is an environmental scientist at CEH Edinburgh and was a keynote speaker at the ‘We need to talk about Nitrogen’ workshop hosted by Plantlife and the BES Plants, Soils and Ecosystems Group in London on 19 January 2017.

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