Farms in northern China use six times more nitrogen fertilizer per acre than farms in the midwestern United States but produce just about the same yield of corn. Meanwhile, farms in western Kenya uses 1/13th what the U.S. farms uses, and corn yields remain paltry.
The comparison, published in the Policy Forum piece of this week's Science journal (324:5934) by a group of 17 researchers, including Laurie Drinkwater, Cornell associate professor of horticulture, shows how fertilizer use varies dramatically around the world, from inadequate to excessive, with both extremes having substantial human and environmental costs. The article, whose first author is P.M. Vitousek of Stanford University, compares the nutrient balances of the three very different agricultural systems that grow maize as a major grain.
Farms in the Midwest, the article notes, overfertilized their crops through the 1970s, but by the mid-1990s, better farming methods increased corn yields and at the same time improved the efficiency of synthetic nitrogen fertilizer use. By 2005 corn yields in the Midwest and China were about the same, but Chinese farmers were using about 525 pounds of nitrogen fertilizer per acre compared with 83 pounds in the Midwest, and farms in northern China generated nearly 23 times the amount of excess nitrogen than those in the Midwest.
Yet, the environmental consequences of using inorganic fertilizers have persisted, says Drinkwater, an agroecologist who studies the Mississippi River Basin. Nitrogen runoff from farms has drained into the basin and then into the Gulf of Mexico for years, creating huge "dead zones," including one that grows to an area the size of New Jersey in the summer. The dead zones are due to the runoff nutrients that fuel massive algal blooms, which, in turn, consume most of the water's oxygen, making it uninhabitable to fish. In addition, ammonia from fertilized cropland has become a major source of air pollution, while emissions of nitrous oxide form a potent greenhouse gas.
"As we see from the situation in the Mississippi River Basin, reducing the nitrogen fertilizer surplus does not resolve the environmental consequences of using these fertilizers," said Drinkwater. "We know we need to apply more of our basic understanding of biogeochemical processes to succeed in that; we need to think about diversifying rotations and using practices that solve some of the root causes of nutrient loss from agriculture."
Places like Kenya and China need very different solutions that should be tailored to particular locations, environments, climates and needs of the population.
This work is based on discussions at the Aspen Global Change Institute supported by NASA, the William and Flora Hewlett Foundation, and the David and Lucille Packard Foundation, and at a meeting of the International Nitrogen Initiative sponsored by the Scientific Committee on Problems of the Environment.