Monday, August 12, 2002

Nature V: The problem with fertilizer
Further increases in nitrogen and phosphorus application are unlikely to be as effective at increasing yields (Fig. 2a) because of diminishing returns (Fig. 2b). All else being equal, the highest efficiency of nitrogen fertilizer is achieved with the first increments of added nitrogen; efficiency declines at higher levels of addition. Today, only 30�50% of applied nitrogen fertilizer and 45% of phosphorus fertilizer is taken up by crops. A significant amount of the applied nitrogen and a smaller portion of the applied phosphorus is lost from agricultural fields. This nitrogen contributes to riverine input into the North Atlantic that is 2- to 20-fold larger than in pre-industrial times. Such non-point nutrient losses harm off-site ecosystems, water quality and aquatic ecosystems, and contribute to changes in atmospheric composition. Nitrogen loading to estuaries and coastal waters and phosphorus loading to lakes, rivers and streams are responsible for over-enrichment, eutrophication and low-oxygen conditions that endanger fisheries.

Nitrogen fertilization can increase emission of gases that have critical roles in tropospheric and stratospheric chemistry and air pollution. Nitrogen oxides (NOx), emitted from agricultural soils and through combustion, increase tropospheric ozone, a component of smog that impacts human health, agricultural crops and natural ecosystems. As much as 35% of cereal crops worldwide are exposed to damaging levels of ozone. NOx from agroecosystems can be transported atmospherically over long distances and deposited in terrestrial and aquatic ecosystems. This inadvertent fertilization can cause eutrophication, loss of diversity, dominance by weedy species and increased nitrate leaching or NOx fluxes. Finally, nitrogen inputs to agricultural systems contribute to emissions of the greenhouse gas nitrous oxide. Rice paddy agriculture and livestock production are the most important anthropogenic sources of the greenhouse gas methane.

David Tilman, et al., "Agricultural sustainability and intensive production practices" Nature 418, 671-677 (2002).

0 Comments:

Post a Comment

Links to this post:

Create a Link

<< Home

©2002-2005 by the author