THE PHOSPHRUS CYCLE

In all biogeochemical cycles other than those involving water, carbon, oxygen, and nitrogen, the reservoir of the nutrient exists in mineral form, rather than in the atmosphere. The phosphorus cycle is presented as a representative example of all other mineral cycles because of the critical role phosphorus plays in plant nutrition worldwide.
Phosphorus is, more than any of the other required plant nutrients except nitrogen, apt to be so scarce that it limits plant growth. Phosphates-phosphorus an-ions-exist in the soil only in small amounts, because they are relatively insoluble and are present only in certain kinds of rocks. As phosphates weather out of soils, they are transported by rivers and streams to the oceans, where they are precipitated. They are naturally brought up again only by the uplift of lands, such as along the Pacific coast of North and South America, or by marine animals. Such animals are often consumed by sea birds, which deposit enormous amounts of guano (feces) rich in phosphorus along certain coasts;

these deposits have traditionally been used for fertilizer. Crushed phosphate-rich rocks, found in certain regions, are also used in this way, but the seas are the only inexhaustible source of phosphorus, one of the reasons that deep-seabed mining now looks so commercially attractive.
Every year, millions of tons of phosphate are added to agricultural lands in ihi belief that it becomes fixed to and enriches the soil. In general, four times as much phosphate as a crop requires is added each year, usually in the form of superphosphate, which is soluble calcium dihydrogen phosphate, CatH^PC^)^, derived by treat ing bones or apatite, the mineral form of calcium phosphate, with sulfuric acid. Bin the enormous quantities of phosphates that are being added annually to the world'1 agricultural lands are not leading to proportionate gains in crops; plants can apparently use only so much of the phosphorus that is added to the soil. Agricultural sci entists are actively seeking new ways to approach the problem of phosphate supply,
Symbiotic associations occur between the roots of most plants and fungi. Called mycorrhizae, they play a major role in absorbing phosphorus from the soil. The ap-plication of suitable fungi to agricultural soils and the fostering of conditions suitable to their growth is one strategy that might be used to increase the supply of phosphorus available for plant growth. The recycling of animal and human wastes to recover phosphates would also make an important contribution to the availability ofphospho rus in industrial countries, where such practices are not normally employed. Human sewage, in fact, represents a potential source of about 1 kilogram of phosphorus per hectare of cultivated land worldwide per year. The world's rivers carry 17 million metric tons of phosphorus to the sea every year, half from natural erosion and half from domestic and industrial use and from sewage.
Phosphates are relatively insoluble and are present in most soils only in small amounts. They often are so scarce that their absence limits plant growth.
Biogeochemical Cycles Illustrated: Recycling in a Forested Ecosystem
The overall recycling pattern of some nutrients has been revealed in impressive detail by an ongoing series of studies conducted at the Hubbard Brook Experimental Forest in New Hampshire. The way in which this ecosystem functions, and especially the way in which nutrients cycle within it, has been the subject of study since 1963 by Herbert Bormann of the Yale School of Forestry and Environmental Studies, Gene Likens of the New York Botanical Garden's Institute for Ecosystem Research, and their colleagues. These studies have yielded much of the information that we now have about the cycling of nutrients in forest ecosystems. They have also pro-tidfd [he basis for the development of much of the experimental methodology that is bang applied successfully to the study of other ecosystems.
Hubbard Brook is the central stream of a large watershed that drains a region of cmperate deciduous forest. For measurement of the flow of water and nutrients within the Hubbard Brook ecosystem, concrete weirs with V-shaped notches were buill across six tributary streams that were selected for study. All of the water that flowed out of those valleys had to pass through the notch, since the vvdrs were anchored in bedrock. The precipitation that fell in the six valleys was measured, and the amounts of nutrients that were present in the water flowing in the six streams were also determined. By these methods, it was demonstrated that the undis-;urbed forests in this area were very efficient at retaining nutrients. The small amounts of nutrients that precipitated from the atmosphere with the rain and snow were approximately equal to the amounts of nutrients that ran out of the valleys. These quantities were very low in relation to the total amount of nutrients in the system. There was a small net loss of calcium-about 0.3% of the total calcium in the system per year-and small net gains of nitrogen and potassium.
In 1965 to 1966, the investigators felled all of the trees and shrubs in one of the six watersheds and then prevented their regrowth by spraying the area with herbicides. The effects of these activities were dramatic. The amount of water running out of that valley was increased by some 40%, indicating that water that normally would have evaporated into the atmosphere from the leaves of the trees and shrubs was now running off. For the 4-month period of June to September 1966, the runoff was actually four times higher than it had been in comparable periods during the preceding years, The amounts of nutrients running out of the system also increased greatly. For example, the loss of calcium was 10 times higher than it had been previously. Phosphorus, on the other hand, did not increase in the stream water; it apparently was locked up in the soil. A great deal of the available phosphorus may have reached deeper levels in the soil and thus become less available for plant growth.
The change in the status of nitrogen in the disturbed valley was especially striking. The undisturbed ecosystem in this valley had been accumulating nitrogen at a rate of about 2 kilograms per hectare per year, but the cut-down ecosystem lost it at a iaie of about 120 kilograms per hectare per year! The nitrate level of the water rapidly increased to a level exceeding that judged safe for human consumption, and the stream that drained the area generated massive blooms of cyanobacteria and algae. In other words, the fertility of this logged-over valley decreased rapidly, while at The same time the danger of flooding greatly increased. This experiment is particularly instructive in the 1990s, as large areas of tropical rain forest are being destroyed to make way for cropland.