The Phosphorus Cycle: _{Phosphates and fertilizer}

The importance of phosphorus to living things

Like carbon, oxygen, hydrogen, and nitrogen, phosphorus is a limiting nutrient for all forms of life, which means that the potential for an organism’s growth is limited by the availability of this vital nutrient. It forms part of the structure of DNA and RNA, is needed for energy transport in cells, provides structure to cellular membranes, and assists in giving bones and teeth their rigidity. In short, without phosphorus, we simply could not exist. And yet, for something so crucial, it is one of the most difficult elements for living things to access in nature.

Prior to the 1800s, very little was known about phosphorus or how it moved through the environment. Early chemists like Robert Boyle knew that the element was highly flammable and would phosphoresce, or glow, when exposed to oxygen. In fact, in 1680 Boyle took advantage of this flammability and developed the first matchstick by using phosphorus to ignite wooden sticks dipped in sulfur. But, like other elements, phosphorus’ contribution to the growth and health of organisms remained a mystery.

For almost a century, scientists believed Sir Francis Bacon’s hypothesis that water was the “principle of vegetation” – the essential nutrient for plant growth (Tindall & Krunkel, 1998). This idea was supported by experiments conducted by notable scientists like Jan Baptiste van Helmont, John Evelyn, and Robert Boyle. For example, in 1629, the Flemish alchemist van Helmont put Bacon’s theory to test with his famous willow tree experiment. Van Helmont’s experiment involved the growth of a willow tree in what he thought was a controlled environment. In his own words,

I took an Earthen vessel, in which I put 200 pounds of Earth that hadbeen [sic] dried in a Furnace, which I moystened with Rainwater, and I implanted therein the Trunk or Stem of a Willow Tree, weighing five pounds; and at length, five years being finished, the Tree sprung from thence, did weigh 169 pounds, and about three ounces: But I moystened the Earthen Vessel with Rain-water, or distilled water (alwayes when there was need) and it was large, and implanted into the Earth, and least the dust that flew about should be co-mingled with the Earth, I covered the lip or mouth of the Vessel with an Iron-Plate covered with Tin, and easily passable with many holes. I computed not the weight of the leaves that fell off in the four Autumnes. At length, I again dried the Earth of the Vessell, and there were found the same two hundred pounds, wanting about two ounces. Therefore 164 pounds of Wood, Barks, and Roots, arose out of water onely. (van Helmont, 1662)

We now know that there were flaws in van Helmont’s experiment, including his use of soil in an application meant to show that water alone was what nourished plants. However, his contributions to our knowledge of the role of elements in plant nutrition were significant. The willow tree experiment alone marked the beginning of experimental plant physiology, and is both one of the first quantitative experiments in biology and one of the first written accounts of the use of the scientific method (Hershey, 2003; Morton, 1981).

During the 17^th century, however, a German chemist named Johann Glauber argued that soil, not water, was the sole source of nourishment for plants. This sparked a debate that continued in various forms into the 18^th century. In 1775, Frances Home concluded that both arguments were correct. Home theorized that not one but many factors influence a plant’s growth – a conclusion that would open up new research in various fields.

Limitations for growth

In 1838, a competition was held by The Academy of Sciences in Göttingen, Germany (Tindall & Krunkel, 1998). The Academy asked members of the scientific community to determine whether the inorganic elements found in the ashes of plants are present in the living plant, and whether there was any evidence of these inorganic elements being necessary for plant growth and survival. Justus von Liebig, a German chemist, won the contest with his treatise Organic Chemistry and its Applications to Agriculture and Physiology (von Liebig, 1838).

Von Liebig explained that certain elements, like Carbon (C), Hydrogen (H), and Phosphorus (P), are vital to the growth and sustainability of plants. His work drew clear connections between crop yield and the amount of fertilizer offered during the growing season, and identified a hierarchy of minerals in these interactions. One of the most important moments in von Liebig’s work is a discussion of the “Law of Minimum.”

The Law of Minimum states that the growth and yield of a plant are limited by the nutrient in least abundance, regardless of which nutrient that might be. This law is often referred to as Liebig’s Law of Minimum, though it is understood now that the discovery actually belongs to Karl Sprengel, a German agronomist working at the same time. Because macronutrients like carbon, oxygen, hydrogen, and nitrogen are readily available in Earth’s atmosphere, more often than not the limiting nutrient for plant growth in natural ecosystems is phosphorus.

Understanding phosphate availability

Like many of Earth’s cycles, the phosphorus cycle involves movement through biological and geological systems, and this movement is driven by various chemical transformations. Unlike carbon or nitrogen, however, phosphorus moves only through the lithosphere, biosphere, and hydrosphere. It is one of the only biogeochemical cycles that does not involve a gaseous stage, meaning that it does not become part of Earth’s atmosphere in any significant way.

As Brand discovered, elemental phosphorus is a highly reactive substance. Simply exposing it to air will stimulate a chemical reaction with oxygen. This means that in nature the element is typically found as a phosphate (\(PO_4^{-3}\)). Phosphates, in their most basic form, contain one phosphorus atom bound to four oxygen atoms, with one of those oxygen atoms being bonded to another atom, like hydrogen (Figure 2). A very common phosphate found in nature, for example, is \(HPO_4^{-2}\). There are a wide variety of combinations that take place with this simple \(PO_4^{-3}\) anion: bonding with carbon, nitrogen, and hydrogen to create the energy storage compound ATP, for example, or with calcium (and occasionally hydrogen) to create calcium phosphate (Figure 3). DNA, our genetic blueprint, relies on phosphate groups to provide the backbone to its double-helix structure (see our DNA II: The Structure of DNA module for more information), and cell membranes rely on phospholipids to give them structure (see our Membranes I: Introduction to Biological Membranes module).

In the environment, phosphates can be found in both organic and inorganic forms. Organic phosphates are mainly created through biological processes and include a bonded carbon, such as in plant or animal tissues. Inorganic phosphates on the other hand, are not associated with carbon. They are produced through natural processes like chemical weathering of phosphorous-containing rocks, or man-made processes like the chemical manufacturing of fertilizers. While animals are able to use either of these forms, plants are able to use only the inorganic form.

Phosphorus’ movement through different spheres

The phosphorus cycle is similar to other elemental cycles and is often described in an overly-simplified way: As Earth’s tectonic plates shift, volcanic action, earthquakes, and movement at plate boundaries expose buried sediments and rock to the surface of the planet (to learn more, read our Plates, Plate Boundaries, and Driving Forces and The Rock Cycle: Uniformitarianism and Recycling modules). When exposed to elements like wind and water, mechanical and chemical weathering of these rocks take place. These transformations release phosphates that have been bound in these reservoirs to the environment, where they become available in soil and water. After passing through biological systems via the food chain, phosphorus is eventually returned to the soil and then into aquatic systems, where it ultimately becomes sediment and can move back into the geological part of the cycle (Figure 4).

Like all of Earth’s cycles, there is no start or finish to the phosphorus cycle, and certainly no single direction of movement. Earth’s cycles are complex webs where resources move in multiple directions. In fact, it might be even easier to think of the phosphorus cycle as being a process made up of a series of smaller processes that may or may not ever interact – processes that take place over a time frame as short as weeks and as long as millennia. To get a better sense of the movement of phosphorus through the lithosphere, biosphere, and hydrosphere, it helps to view it in terms of its movement on a shorter time-scale and through a specific ecosystem.

The Experimental Lakes Area: A look at the phosphorus cycle in one ecosystem

During the months of September and October 1967, scientists T.R. Cleugh and B.W. Hauser began a helicopter survey on 463 lakes in the Precambrian Shield in northwestern Ontario, Canada (Figure 5). Lakes were numbered in the order they were sampled, and data on maximum depth, visibility, dissolved solids, and conductivity were recorded to create lake profiles (Cleugh & Hauser, 1971). This was the first step in what would become one of the most well-known examples of extreme science: the creation of the Experimental Lakes Area (ELA), a project run by the Freshwater Institute to manipulate whole-lake ecosystems.

The Experimental Lakes Project had its beginnings in 1965 when the US and Canadian governments were asked by the International Joint Commission (IJC), a commission that helps Canada and the US prevent disputes over boundary waters, to devote resources to understanding pollution in the lower Great Lakes/St. Lawrence Plain. This unique region – spanning the southern part of Ontario to areas of central New York, Vermont, Pennsylvania, and Ohio – had been transformed during the early 20^th century from forests rich in oak, hemlock, and mixed-conifers to land primarily devoted to agriculture. Housing developments throughout the region also increased significantly over this time. The water bodies of interest to the IJC were beginning to show effects of eutrophication – a condition of excessive plant and algae growth that can kill fish and other wildlife in the water – and little to no information existed on the causes or controls.

As a result, the Experimental Lakes Area was created to study these questions. It consisted of isolated, pristine land containing 58 lakes and watersheds, free from cultural or industrial influence, where researchers could actively manipulate whole ecosystems. The first experiments involved researchers directly controlling nutrient influxes to isolate the factors that might influence eutrophication in the water bodies. One of the experiments – that on Lake 227 – was conducted over the course of 44 years. It and a shorter experiment in Lake 226 were the first of their kind to clearly identify phosphorus as a driving factor in eutrophication.

Lakes 227 and 226: Testing limiting nutrients

Lake 227 is small by most lake standards and offered limnologist David W. Schindler and his research team an ideal subject on which to test their ideas about eutrophication. In June 1969, Schindler and his team began to intentionally fertilize Lake 227 on a weekly basis, using a fertilizer with a 12:1 ratio by weight of nitrogen to phosphorus (Schindler, 2008). At the time, they were interested in testing a hypothesis popular in North America that the supply of carbon could limit the growth of phytoplankton in lakes. They chose Lake 227 specifically because it had a low concentration of dissolved inorganic carbon (Schindler, 2009). For the first five years of the experiment, the researchers added phosphorus and nitrogen to the lake to ensure phytoplankton had adequate amounts for growth and sustainability, but limited access to carbon.

After nutrient-loading Lake 227 for the first time, Schindler and his team noticed that algae growth increased significantly (or “bloomed”) despite the low concentration of carbon. Further, they saw that the blooms had a direct correlation to the amount of phosphorus they added to the water (Figure 6). While Schindler began to suspect phosphorous as the culprit, he needed further evidence. Importantly, the soap and detergent industry, whose products contained phosphorous, worked to take the focus off of the phosphorous-containing products by arguing that nitrogen was as influential in aquatic systems as phosphorus. So, the team began to test the effects of nitrogen separately by adding nitrogen and carbon to Lake 226.

Lake 226, shaped like an hourglass, had two basins that could be isolated from one another at the narrows with a heavy nylon curtain (Figure 7). Schindler’s group added nitrogen and carbon to both of the basins, but in the north basin they also added phosphorus. Again, algal blooms were in direct relation to the amount of phosphorus added – the south basin remained pristine, while the north basin bloomed within weeks (Schindler, 1977).

Schindler’s research began to show clearly that phosphorus, and not carbon or nitrogen, is the nutrient that has the greatest effect on plant growth in aquatic ecosystems. As Schindler notes in his personal recount of the history of the ELA, the aerial photograph of the two basins shown in Figure 7

…had more impact on policy makers than hours of testimony based on scientific data, helping to convince them that controlling phosphorus was the key to controlling eutrophication problems in lakes. (Schindler, 2009)

The reason for phosphorus’s impact is simple. Along with carbon, nitrogen, oxygen, and potassium, phosphorus is a macronutrient that determines whether an organism will grow and survive, or wither and die. Without it, living beings cannot grow, reproduce, move, or do much of anything. But because other macronutrients are readily available, more often than not the limiting nutrient for plant growth in natural ecosystems is phosphorus. This is partly because the largest reservoir of phosphorus is locked up in sedimentary rock and unavailable, and partly because its chemistry in the environment limits its availability.

Human impacts on the phosphorus cycle

Since humans began to walk the Earth, we have interacted with – and influenced – many natural processes, and the phosphorus cycle is no exception. Because phosphates are quite limited in soil naturally, modern agricultural practices frequently involve the application of fertilizers heavy in inorganic phosphates. When phosphorus is added to an ecosystem through non-natural or excessive means – run-off from farms (both fertilizers and animal excrement), sewerage, or phosphate-containing detergents – the sudden increase in nutrient availability can have a dramatic effect on plant growth.

Soil has a saturation point with respect to how much phosphate it can hold, and plants have a limit as to how fast they can take it up, so the application of too much phosphate results in both leaching into the water supply and run-off into lakes, streams, and oceans. Since aquatic ecosystems have very low phosphate concentrations naturally, whenever phosphate enters the water column, phytoplankton like algae quickly consume it.

As Schindler and his team showed with the ELA, if the influx of phosphate steadily continues for a period of time, the algae and other aquatic phytoplankton are able to reproduce so quickly and efficiently that they literally form a mat on the surface of the water, blocking out light for other plants and organisms living below (Figure 8). This reduces the ability of bottom-dwelling plants to photosynthesize, reducing the amount of oxygen being released into the water.

As the algae die, they fall to the bottom where they are decomposed by bacteria – a process that uses a large amount of dissolved oxygen. As this dissolved oxygen is depleted, fish and other organisms living in the water body slowly suffocate and die.

Though we have learned better and made many efforts to change, the effects of these practices still linger. The over-application of fertilizers with high concentrations of phosphate is still a problem, and bodies of water in places with heavy agricultural communities suffer the greatest. Fortunately, as we learn more about the impacts our actions have on our environment, we can consciously make choices that will benefit rather than harm our surroundings.

The body of research on phosphorus conducted at the Experimental Lakes Area was a seminal contribution to environmental science. While the phosphorous cycle can be simplified, as we did above, to a cycle that includes a geological component and a biological component, the cycle is actually far more detailed than this.