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Professor updates ecosystem model

November 21, 2007

Anyone who has thrown a backyard barbecue knows that hot dogs are inexplicably packaged in different numbers than buns – eight hot dogs per pack versus 10 hot dog buns. Put in ecological terms, this means that weenie roasts are “hot-dog limited,” since the extra buns are worthless without hot dogs to fill them.

Such limiting factors are a cornerstone of natural ecology, where phosphorus or nitrogen limits plant production in most ecosystems. According to the customary model, the relative importance of these two key nutrients varies by ecosystem, but a group of researchers led by ASU professor James Elser has found that this view might need to be updated. Their paper, “Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems,” is highlighted in the “News and Views” section of the Oct. 25 edition of Nature.

This meta-analysis of more than 300 publications in the field of nutrient limitation in ecosystems – the most comprehensive study of its kind – recently was published online in the journal Ecology Letters.

Like all living things, plants require a number of chemical elements to flourish, including carbon, hydrogen and oxygen. They also need nitrogen, a building block of proteins, and phosphorus, which is used to make the nucleotides that compose DNA and RNA.
The interplay of these elements affects the growth of the food web’s foundational plants, and so understanding their interplay is of vital environmental and commercial concern.

Nitrogen and phosphorus, both widely used in fertilizers, must be in proper balance to be effective. Adding nitrogen alone to an ecosystem is helpful only up to a point, after which plants stop benefiting unless phosphorus also is added. If such a system responds positively to the initial nitrogen addition, it is said to be “nitrogen-limited,” because the availability of nitrogen instantaneously constrains the productivity of the ecosystem. The converse is true in “phosphorus-limited” systems.

Plant production in both cases is limited by the nutrient in shortest supply, a principle known as “von Liebig’s law of the minimum.”
Because of their differences in size, makeup, geology and other factors, different kinds of ecosystems have long been thought to differ widely in the strength and the nature of their nutrient limitation. For example, conventional wisdom has held that freshwater lakes are primarily phosphorus-limited, while oceans, forests and grasslands were believed to be nitrogen-limited.

But that is not what Elser’s group found. Rather, their data reveals that the three environments are surprisingly similar, and that the balance of nitrogen and phosphorus within each ecosystem conforms to a different pattern than previously expected.

“Our findings don’t support conventional views of ecosystem nutrient limitation,” says Elser, a professor of ecology, evolution and environmental science at ASU. “They don’t, for example, confirm the rule of thumb that, in freshwaters, phosphorus is more limiting than nitrogen.”

Instead, Elser’s group found that nitrogen and phosphorus are, in fact, equally important in freshwater systems, and that phosphorus is just as important as nitrogen in terrestrial ecosystems as well.

The determining factor, Elser says, is simplicity itself. Underlying all of the splendid diversity of the world’s ecosystems – whether soggy, arid, terrestrial, aquatic, arboreal or algal – is the simple unifying fact that all plants share a common core of biochemical machinery. That machinery is composed of proteins and nucleotides, meaning that all plants require nitrogen and phosphorus within a limited range of natural proportions.

Nicholas Gerbis,
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