Environmental challenges associated with needed increases in global nitrogen fixation

Abstract

Human-induced input of fixed nitrogen (N) into the earth biosphere, primarily through combustion of fossil fuels, crop biological N-fixation and N-fertilizer use, has provided many human benefits. These benefits have not come, however, without significant cost. According to data compiled by the Food and Agriculture Organization of the United Nations, synthetic N fertilizer input into global agricultural systems increased from by approximately 430% (∼19 to ∼82 Tg N) from 1965 to 1998. During this period, global grain production, human population and global fossil fuel consumption increased about 250%, 190% and 240%, respectively. Although fuel consumption increased faster than population growth globally, land used to produce grain decreased from 0.2 to 0.12 ha/person over this 30-yr period. Grain production, however, increased 16%/person. Agricultural production increase has come through the use of new crop varieties which respond to increased N-fertilization, pesticide use, irrigation and mechanization. Even though agricultural production has increased dramatically, fertilizer N use efficiency remains relatively low. Globally fertilizer N use efficiency was approximately 50% in 1996. Since fertilizer N is not used efficiently in most parts of the world, N use in excess of crop potential utilization leads to losses to the environment through volatilization and leaching. These N losses result in N fertilization of pristine terrestrial and aquatic systems through NH_x and NO_ydeposition and contribute to global greenhouse gases through N₂O production and local elevated ozone concentrations due to NO_x emission. Inefficient use of N and energy is exacerbated by the global inequity of use distribution. Some areas don't have enough while others use too much. Additionally, dietary patterns of food consumption which tend to be more inefficient, i.e. cereal-based diets compared to animal-based diets, are changing in global terms. The resulting increasing inefficiencies in N utilization in food production and in energy use lead to large-scale input of N into down wind and down stream terrestrial and aquatic systems. Increasing N-use-efficiency remains a clear goal by which to maintain food production while decreasing excessive N use and unwanted distribution in the environment.