Carbon footprints are an established means of communicating one aspect of a product's environmental impact. New research suggests additional 'nitrogen footprints' for food products could give consumers a more informed choice and help reduce eutrophication in water. However, it warns that there are environmental trade-offs for some food groups, for example, oil products have a small nitrogen footprint, but a fairly large carbon footprint.
Nitrogen and phosphorus pollution from food production is a major cause of eutrophication in water bodies, leading to low-oxygen conditions which damage fisheries, biodiversity and amenities. In tackling eutrophication, particularly via the 1991 Nitrates Directive1, the European Commission has concentrated largely on agricultural or environmental policy (e.g. the Water Framework Directive2), as opposed to consumer behaviour.
The researchers used life cycle analysis (LCA) methods to calculate approximate nitrogen and phosphorus footprints of different food products in the US. The footprint was considered to be the average nitrogen and phosphorus emissions to the aquatic environment resulting from the production, processing, packaging and distribution of cereal, fish, poultry, dairy, fruit and vegetable, sweets/condiments, oils and red meat food types. The total emissions were recalculated to provide a 'nitrogen equivalent' figure and expressed in terms of 'grams of nitrogen equivalent emissions per kilogram of food'.
For all food groups, at least 70 per cent of the footprint resulted from production, i.e. farming, largely owing to fertiliser application and manure emissions. The processing stage contributed significantly to the footprint for dairy products, and to a lesser extent for poultry and fish. The contributions of packaging and transport, where emissions are largely to the atmosphere, were minimal. At 150 grams of emissions per kilogram, red meat's nitrogen footprint was twice as big as dairy's, as both animal and feed production systems cause significant nutrient flows. Poultry and fish systems also had significant nitrogen footprints for the same reasons. The nitrogen footprints of other food groups were significantly smaller, with cereals producing only 2 grams of emissions per kilogram.
Of significance for policymaking, the data are also presented in terms of required daily calorie intake and cost to the consumer. Lowering the proportion of dietary calories obtained from dairy products in favour of higher cereal intake has the greatest potential to reduce footprints and therefore eutrophication. Replacing red meat has less impact, since it has a higher energy density per kilogram. Factoring in food costs, the same shift from dairy to cereals is also the most economically effective method of reducing the dietary eutrophication potential. In contrast, replacing red meat with vegetables, for example, may increase costs.
The researchers also compared the food group's nitrogen footprints with their carbon footprints. Both footprints were large for red meat, and both were small for cereals. However, this pattern was not found for other food groups. Dairy, poultry and fish have relatively low carbon footprints, but also relatively high nitrogen footprints. Conversely, fruits, sweets and oils have fairly high carbon footprints but very low nitrogen footprints. The researchers warn that policies based on just one aspect of a product's environmental impact, whether carbon footprint or nitrogen footprint, are only likely to shift environmental burdens from global warming to eutrophication (or vice versa).
The study reports results only for the US, so European patterns may differ due to different climatic zones, agricultural methods, diet and food. However, the results show that using environmental policy instruments to alter agricultural practices, directly or through consumer markets, will inevitably involve trade-offs between different environmental impacts.