A nitrogen footprint model to help consumers understand their role in nitrogen losses to the environment

Abstract

The human use of reactive nitrogen (N_r) in the environment has profound beneficial and detrimental impacts on all people. Its beneficial impacts result from food production and industrial application. The detrimental impacts occur because most of the N_r used in food production and the entire amount of N_r formed during fossil fuel combustion are lost to the environment where it causes a cascade of environmental changes that negatively impact both people and ecosystems.

We developed a tool called N-Calculator, a nitrogen footprint model that provides information on how individual and collective action can result in the loss of N_r to the environment. The N-Calculator focuses on food and energy consumption, using average per capita data for a country. When an individual uses the N-Calculator, the country average is scaled based on the individual's answers to questions about resource consumption.

N footprints were calculated for the United States and the Netherlands, which were found to be 41 kg N/capita/yr and 24 kg N/capita/yr, respectively. For both countries, the food portion of the footprint is the largest, and the food production N footprints are greater than the food consumption N footprints.

The overarching message from the N-Calculator is that our lifestyle choices, and especially our food consumption, have major impacts on the N_r losses to the environment. Communicating this message to all of the stakeholders (the public, policymakers, and governments) through tools like the N-Calculator will help reduce N_r losses to the environment.

Introduction

The human use of reactive nitrogen (N_r, all nitrogen species except N₂) in the environment has profound beneficial and detrimental impacts on all people. Agricultural uses, including both food production and consumption, contribute the most N_r to the global environment; the burning of fossil fuels is the next biggest contributor. The beneficial impacts of the agricultural use of N_r are related to food production using nitrogen fertilizer and human-enhanced biological nitrogen fixation. These two processes provide the N_r to sustain about half of the world's population (Erisman et al., 2008). The detrimental impacts result because most of the N_r used in food production and the entire amount of N_r formed during fossil fuel combustion are lost to the environment.

Once lost to the environment, the nitrogen moves through the Earth's atmosphere, forests, grasslands, and waters causing a cascade of environmental changes that negatively impact both people and ecosystems. These changes include smog, acid rain, forest dieback, coastal ‘dead zones’, biodiversity loss, stratospheric ozone depletion, and an enhanced greenhouse effect (Galloway et al., 2003, Galloway et al., 2008). The disruption of the N cycle and its impacts necessitate measures to optimize food production and energy use while minimizing the effects. Therefore policies, measures, and behavioral changes are essential. Here we describe tools that can support policymakers, stakeholders, producers, and consumers in that process.

Of all the chemical elements, nitrogen is one whose abundance has been increased the most by human activity (Erisman et al., 2008). Globally, humans contribute about twice the amount of nitrogen to the environment as do all the natural terrestrial processes; on a regional basis it can be many-fold more (e.g., US, 5-fold (United States Environmental Protection Agency, 2011); Europe, 4-fold (Sutton et al., 2011)). In contrast, human activity contributes 5–10% of CO₂ emissions; natural processes contribute the remainder (Le Quéré, 2010).

Over the past decade, there have been significant advances in our scientific understanding of the magnitude and consequences of the human alteration of the nitrogen cycle. With these scientific advances, and a much better understanding of how to decrease the negative impacts of nitrogen without impacting the ability to produce food and use energy, we feel it is appropriate to address the challenge of communication and to help consumers and policymakers minimize their role in the disruption of the N cycle and the resulting environmental consequences. We plan to do this by means of a collection of tools brought together in a system called N-PRINT (Fig. 1). Ultimately, N-PRINT will be able to describe how N_r is lost to the environment and its resulting impacts due to individual (consumer) and collective (producers and society) consumption behavior and the ways in which policy can have an effect on these losses.

In this paper we will focus on the N-Calculator, which is a per capita N footprint model that defines an N footprint as the total amount of N_r that is lost to the environment due to individual's consumption of food and energy. The N-Calculator provides an integrated approach that takes into account not only the food and energy consumed by individuals, but also the “up-stream” processes that release N_r to the environment by virtue of the production of the food, energy, goods, and services that individuals use.

The ‘footprint’ concept has been developed over the last decade to serve as a metric of the single or collective impacts of people on the environment, relative to the capacity of the planet to support those people. There are a number of existing footprint calculators, with The Ecological Footprint being one of the earliest and the most well known with components like energy, food and fiber, timber and paper, and built-up land (Wackernagel et al., 1999, Wackernagel et al., 2002, Global Footprint Network, 2011).

Several groups have examined N_r flows associated with the use of food and energy by people and regions (e.g., cities, nations). For example, the Chesapeake Bay Foundation in the US has a calculator (Chesapeake Bay Foundation: Your Bay Footprint, 2011) that estimates a household's contribution to N_r inputs to the Bay resulting from energy use, sewage production, and lawn fertilizer usage. A much more specific calculator has been prepared to estimate N, C, and P fluxes in 360 household ecosystems in the Minneapolis-Saint Paul, Minnesota (USA) urban region. Fissore et al. (2011) found that N_r fluxes into the households were dominated by human diet, lawn fertilizer applications and surface transportation, which together accounted for ∼85% of total household N_r inputs.

In a life-cycle analysis Xue and Landis (2010) examined the N and P eutrophication potential of food consumption patterns. They showed that different food groups exhibit a highly variable nitrogen-intensity. On average, red meat and dairy products require much more nitrogen than cereals/carbohydrates. An important point they make is that the ranking of foods' nitrogen footprints is not consistent with their carbon footprints. For example, dairy products and chicken/eggs have high nitrogen footprints but low carbon footprints.

Most recently, Galli et al. (2011) have clustered a suite of indicators (ecological, carbon, water; the “Footprint Family”) to track human pressure on the planet. The work was developed under the One Planet Economy Network: Europe (OPEN:EU) project, funded by the 7th Framework Program in the European Commission, and builds on the premise that no single indicator per se is able to comprehensively monitor human impact on the environment, but indicators rather need to be used and interpreted jointly. As they note in the paper, to better track the environmental impacts of production and consumption activities and assess tradeoffs, the Footprint Family could thus benefit from the inclusion in the suite of additional footprint-type of indicators such as ‘nitrogen’. In that regard, one of the objectives of this paper is to provide the methodology behind the nitrogen footprint to make it easier to integrate into the Footprint Family.

The objectives of this paper are (1) to provide background on the scientific approaches in model development and explain the methodology of the N-Calculator and (2) to present the nitrogen footprint of the United States and the Netherlands.