At the root of nutrient limitation, ecosystems are not as different as they seem

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 Arizona State University 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 October 25 edition of Nature. The most comprehensive study of its kind, this meta-analysis of more than 300 publications in the field of nutrient limitation in ecosystems was recently published online in the journal Ecology Letters.

Like all living things, plants require a number of chemical elements in order to flourish, including carbon, hydrogen and oxygen. They also need nitrogen, a building block of proteins; and phosphorus, 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 characteristic 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 along with terrestrial forests and grasslands were believed to be nitrogen-limited.

Yet 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,” said 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.

“This is in contradiction to conventional wisdom, which seems to emphasize N on land while disregarding P,” Elser said.

The determining factor, according to Elser, is simplicity. 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.

“Thus, N and P both play a major role in limiting production, no matter where you look,” Elser said.