Nitrogen is one of the most important elements for life on earth. It is a key ingredient in DNA and RNA, photosynthesis and amino acids – the building blocks of proteins. Nitrogen is essential for the growth of plants and crops on which humans and livestock depend. Approximately half of the world’s population rely on nitrogen fertilisers for their food consumption, making nitrogen fundamental to global food security.

Nitrogen is often the limiting nutrient for the growth of plants and crops on which animals and humans feed. The rapid growth in the use of fertilisers has been one factor contributing to increased crop yields. Around half the world’s population depends on nitrogen fertilisers for their food consumption, making nitrogen essential to global food security, and it will be increasingly so as population grows to an estimated 9.7 billion by 2050.

This chapter explains why the nitrogen cycle is an important issue for environmental policy. It provides an overview of the main sources of nitrogen, the pathways normally used by nitrogen once released into the environment, and the health and environmental risks associated with excess nitrogen in the receiving ecosystems. The chapter introduces the concept of "nitrogen cascade", which translates an unpredictable sequence of nitrogen cycle effects.

This chapter proposes a three-pronged approach to cost-effectively respond to nitrogen pollution. First, to better manage the risks of air, soil and water pollution and associated ecosystems through a detailed analysis of the nitrogen pathways, the so-called “spatially targeted risk approach". Second, address the steady increase in nitrous oxide concentrations in the atmosphere through a "global risk approach". Third, take into account the uncertainty of cascading effects and anticipate potentially significant long-term impacts through a "precautionary approach”.

This chapter provides examples of impact-pathway analysis to improve risk management of air pollution and water pollution. The examples illustrate the management of urban air pollution (Paris) and nitrogen deposition on forest ecosystems (in Germany). Other examples are the management of dead zones in Chesapeake Bay (eastern United States), the risk of lake pollution (in New Zealand) and groundwater pollution (western United States).

This chapter warns against the possible unintended effects of nitrogen pollution management measures by providing a case study on agriculture. The chapter reviews the various practices implemented in the United States to manage agricultural nitrogen pollution. The possible unintended effects of each measure are detailed and general lessons are learned.

This chapter provides a framework for analysing the merits of nitrogen management policy instruments. It establishes a typology of the different types of instruments available to decision makers and proposes three criteria to evaluate them (effectiveness, cost-efficiency and feasibility). It stresses the importance of strengthening the coherence between nitrogen pollution management policy and other policies, both environmental and sectoral.

This chapter evaluates, generically, the pros and cons of different policy instruments for nitrogen management, and their combinations, with respect to the criteria of effectiveness, cost-efficiency and feasibility. It provides examples of evaluation of effectiveness, cost-efficiency and feasibility for a number of instruments implemented in Australia, France, Japan, Sweden and the United States.

Conversion of the highly stable (“inert”) dinitrogen (N2) molecule to biologically available (“reactive”) nitrogen, a process called “fixation”, is difficult. Fixation is achieved in soil and water by specialised bacteria which can reduce atmospheric dinitrogen to ammonia (NH3) or ammonium (NH4+) (Figure A.1). Converting dinitrogen into NH3/NH4+ is the role of “nitrogen-fixing bacteria”. Microorganisms early on in the Earth's history developed the ability to use enzymes to produce or "fix" NH4+ from dinitrogen, possibly because the availability of nitrogen through abiotic routes were biologically limiting (McRose et al., 2017). Nitrogen-fixing prokaryotes (bacteria and archaea) live in water (e.g. cyanobacteria), in soil (e.g. Azotobacter), in association with plants (e.g. Azospirillum), or in symbiosis with leguminous plants such as peas, clover and soyabeans (e.g. Rhizobium). In the latter case, the prokaryote shares the nitrogen with the plant; in exchange, the plant supplies the prokaryote with a source of carbon and energy for growth.