Eutrophication of estuarine and coastal zones has emerged as an immense and growing problem in recent years. Over 40 million tonnes of nitrogen, in dissolved and particulate form, are transported by the world's rivers into estuaries and coastal waters each year - double the pre-industrial rate. Unlike freshwater systems, where phosphorous is usually the limiting growth factor, nitrogen is usually the limiting growth factor in saline waters. Additional
nitrogen can therefore promote huge algal blooms and significant oxygen depletion (hypoxia) in lower-depth waters. Some of the best-documented examples of coastal eutrophication come from the United States. According to a recent survey, 52 percent of the nation's estuaries suffer from some degree of oxygen depletion.
The worst affected area is the Gulf of Mexico, where 85 percent of estuaries are affected. In the most dramatic example, a so-called "dead zone" of 16,000-18,000 km2 has developed where the Mississippi River discharges into the Gulf. Fish and shrimp have disappeared from the area, threatening the local fishing industry, while less mobile life-forms, such as starfish and clams, have died. Scientists have linked the growth of the dead zone to nitrogen fertilizers and livestock manure from farms hundreds of miles upstream. More than half the 11 million tonnes of nitrogen added to the Mississippi Basin annually come from fertilizer, and only about 50 percent is taken up by plants. Nearly 2 million tonnes of nitrogen flow down the Mississippi each year, more than triple the amount 40 years ago and the dead zone has ebbed and flowed consistently with peak river discharges and the associated nutrient flux. The Mississippi dead zone is one of more than 50 similar oxygen-starved coastal regions which now exist world-wide, a threefold increase over the past 30 years.
Coastal zones are among the world's richest fishing grounds, unchecked agricultural run-off poses a serious threat to commercially important fish stocks. Nitrogen pollution is blamed in part for the collapse of the Baltic Sea cod fisheries in the early 1990s, as well as major fish kills (and associated human illness) following outbreaks of Pfiesteria, such as that affecting the Chesapeake Bay, in the United States, in the summer of 1997. Toxic algal blooms, known as "red tides" or "brown tides" are growing world-wide in frequency and severity, damaging offshore fisheries and causing losses to aquaculture enterprises.
The world's population is projected to grow to about 7.3 billion by 2020, with over 90 percent of the increase occurring in developing countries. More people will eat more food, and more protein since, as already described, people almost invariably choose diets which are richer in meat and dairy products as their incomes rise. This will require more cereal to be grown per capita: nearly 40 percent of total grain production is already fed to livestock and the grain-to-protein conversion efficiency is low, lying between 2:1 (chickens) and 7:1 (feedlot cattle). The International Food Policy Research Institute has recently projected that global demand for cereals will increase by 41 percent between 1993 and 2020, and that meat demand will rise 63 percent to 306 million tons.
Analysts at the International Fertilizer Development Center (IFDC) have made detailed projections of future fertilizer demand, and come to a problematic conclusion. Their "real world" projection takes into account various economic and non-economic variables, such as foreign exchange availability, crop and fertilizer prices, the development of irrigation and other infrastructure, and the impact of policy reforms. On this basis, global consumption of
fertilizer is projected to rise from 134 million tons today to 208 million tons in 2020.
This substantial increase might not be enough, however. The IFDC's second projection is based on food production needs, that is, the amount of fertilizer needed to grow 2.5 billion tons of cereal or more. This leads to global fertilizer consumption in 2020 of 263 million tons, and nitrogen consumption of some 160 million tonnes (assuming today's NPK ratio does not change). The fertilizer "shortfall" in developing countries might be as much as 64 million tons. A third projection is based on "sustainable farming" needs, or the amount of fertilizer needed to maintain nutrient reserves in the soil at their initial levels. The projection is based on assumptions about nutrient uptake efficiency for various crops, expected improvements by the year 2020 in different world regions and the proportion of crop residues which are returned to the soil. Under this scenario, producing enough grain to feed the world, without mining the soil of its nutrients, would require application of 366 million tonnes of fertilizer; the proportion of nitrogen would probably decrease but would still be substantial. The fertilizer shortfall in developing countries is estimated at 130 million tonnes.
from the 2007 UN FAO report, and IFDC report