An introduction to the global nitrogen cycle

Current estimates are tabulated for the quantities of nitrogen circulating in the global nitrogen cycle. Five gases, NH 3 , N 2 O, NO, NO 2 and N 2 , dominate the movement of nitrogen between the earth's surface and the atmosphere. The input of combined nitrogen to the land surface of the earth is tentatively estimated at 290 million tonnes per year, a total which includes 74 million tonnes from fertilizers. Known outputs from land (as gaseous NH 3 , N 2 O and NO X , and as inorganic nitrogen carried to the sea by rivers) are much less, totalling 130 million tonnes per year. Emissions of N 2 gas probably account for most of the difference. There has been an increase in the use of nitrogen of about 5% per year over the last ten years. The demand for fertilizer nitrogen is likely to continue to grow if the population of the world continues to increase.