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Key policy issues

  • How will the increasing use of fertilisers affect terrestrial and marine ecosystems, with possible consequences for human health?
  • To what extent can the negative impacts be reduced by more efficient nutrient management and wastewater treatment, while retaining the positive effects on food production and land productivity?


Human activity has accelerated the Earth’s biogeochemical nitrogen (N) and phosphorus (P) cycles through increasing fertiliser use in agriculture (Bouwman et al., 2013c). Increased use of N and P fertilisers has raised food production to support the rapidly growing world population, and increasing per capita consumption particularly of meat and milk (Galloway et al., 2004).

The side effect is that significant proportions of the mobilised N are lost through ambient emissions of ammonia (NH3), nitrous oxide (N2O) and nitric oxide (NO). Ammonia contributes to eutrophication and acidification when deposited on land. Nitric oxide plays a role in tropospheric ozone chemistry, and nitrous oxide is a potent greenhouse gas. Moreover, large proportions of mobilised N and P in watersheds enter the groundwater through leaching, and are released to surface waters through groundwater transport and surface runoff. Subsequently, nutrients in streams and rivers are transported to coastal marine systems, reduced by retention but augmented by releases from point sources, such as sewerage systems and industrial facilities.

This has resulted in negative impacts on human health and the environment, such as groundwater pollution, loss of habitat and biodiversity, an increases in the frequency and severity of harmful algal blooms, eutrophication, hypoxia and fish kills (Diaz and Rosenberg, 2008; Zhang et al., 2010). The harmful effects of eutrophication have spread rapidly around the world, with large-scale implications for biodiversity, water quality, fisheries and recreation, in both industrialised and developing regions (UNEP, 2002). Input of nutrients in freshwater and coastal marine ecosystems, also disturbs the stoichiometric balance of N, P and Si (silicon) (Rabalais, 2002) affecting total plant production and the species composition in ecosystems.

To assess eutrophication as a consequence of increasing population, and economic and technological development, IMAGE 3.2 includes a nutrient model (Beusen, 2014; Beusen et al., 2015; Beusen et al., 2016), which comprises three sub-models:

  1. Wastewater module calculating nutrient flows in wastewater discharges (Figure Flowchart, top);
  2. Soil nutrient budget module describing all input and output of N and P in soil compartments (Figure Flowchart, middle);
  3. Nutrient environmental fate describing the fate of soil nutrient surpluses and wastewater nutrients in the aquatic environment (Figure Flowchart, bottom).

Input/Output Table

Input Nutrients component

IMAGE model drivers and variablesDescriptionSource
Fertiliser use efficiency Ratio of fertiliser uptake by a crop to fertiliser applied. Drivers
GDP per capita - grid Scaled down GDP per capita from country to grid level, based on population density. Drivers
Manure spreading fraction Fraction of manure produced in staples that is spread on agricultural areas. Drivers
Population - grid Number of people per gridcell (using downscaling). Drivers
Production system mix Livestock production is distributed over two systems for dairy and beef production (intensive: mixed and industrial; extensive: pastoral grazing), and to three systems for pigs (backyard, intermediate, intensive) and poultry (backyard, boilers, laying hens) with specific intensities, rations and feed conversion ratios. Drivers
Livestock rations Determines the feed requirements per feed type (food crops; crop residues; grass and fodder; animal products; scavenging), specified per animal type and production system (extensive/intensive/backyard/intermediate/intensive/broiler/laying hens). Drivers
Animal stocks Number of animals per category: non-dairy cattle; dairy cattle; pigs; sheep and goats; poultry. Livestock systems
Land cover, land use - grid Multi-dimensional map describing all aspects of land cover and land use per grid cell, such as type of natural vegetation, crop and grass fraction, crop management, fertiliser and manure input, livestock density. Land cover and land use
Nitrogen deposition - grid Deposition of nitrogen. Emissions
Actual crop and grass production - grid Actual crop and grass production on agricultural land, based on potential yield and management intensity Crops and grass

Output Nutrients component

IMAGE model variablesDescriptionUse
N and P discharge to surface water - grid N and P discharge to surface water.
N and P in wastewater discharge - grid Discharge of N and P to surface water from wastewater. Final output
NH3 emissions - grid Ammonia emissions from applied nitrogen fertiliser and manure. Final output
Soil N budget - grid N budget in the soil, used to calculate fate of nitrogen in the soil-hydrology system and for determining emissions to the atmosphere. Final output
Soil P budget - grid P budget in the soil, used to calculate fate of nitrogen in the soil-hydrology system (residual soil P or surface runoff). Final output