Of mice, men, trees and the global carbon cycle

A team led by a University of Minnesota researcher has found a universal rule that regulates the metabolism of plants of all kinds and sizes and that may also offer a key to calculating their carbon dioxide emissions, a number that must be known precisely in order to construct valid models of global carbon dioxide cycling. Emissions of the gas occur in both plants and animals through the process of respiration; Peter Reich, a professor of forest resources, and his colleagues have found that plant emissions can be deduced from the nitrogen content of any plant. The study also reveals that the respiration, or metabolic, rates of plants and animals follow different laws of scaling with respect to body size. The work will be published in the Jan. 26 issue of the journal Nature.

In revealing nitrogen content as the key to plant metabolic rates, the work uncovered a fundamental difference between plants and animals in how their metabolism varies with size. The larger an animal, the slower its metabolism on a per-weight basis. Thus, although an elephant burns many more calories per hour than a mouse, the mouse has a much higher rate per pound of body weight. An elephant with the same rate per pound as a mouse would generate so much heat it would have serious problems maintaining body temperature and eating fast enough to keep up. Instead of a one-to-one ratio between body size and metabolic rate, as an animal’s body weight quadruples, its respiration rate only triples.

In contrast, when Reich and his colleagues studied 500 plants from 43 species, they found that within a wide range of plant sizes, a quadrupling of weight leads to a quadrupling of respiration rate. The important variable was nitrogen content: The more nitrogen in a plant, the more it respired and the more carbon dioxide the plant emitted. Similarly, if two plants were the same size but had different concentrations of nitrogen in their tissues, the one with the higher nitrogen concentration had a higher respiration rate. Conversely, a big plant and a small plant with the same total nitrogen content would put out equivalent amounts of carbon dioxide over the same time period.

The universal rule linking plant metabolism to nitrogen can also assist efforts to measure the global carbon cycle. Through the process of photosynthesis, plants absorb and store more carbon dioxide than they emit through respiration. But global plant respiration is a huge variable that must be taken into account.

“If we estimate the nitrogen content of plants, we can model their metabolic rates, helping us to better assess the global plant metabolic rate,” said Reich, a professor in the university’s College of Natural Resources. “The amounts of carbon dioxide given off by plants is one of the weak spots in models of global carbon cycling.”

To predict how fast atmospheric carbon dioxide will rise in the future, it is important to know all the sources that emit the gas and all the sources that soak it up. The amount of carbon dioxide in the atmosphere is well known, as is the rate of emissions from fossil fuel burning. The rate of photosynthesis, in which carbon dioxide is absorbed and stored as plant tissue, is difficult to measure but can be estimated globally from satellites, based on the visible plant cover. The plant cover indicates how much light the plants will intercept. Even harder to calculate are the global amounts of carbon dioxide released by living, respiring plants; the amounts released as plants are decomposed by microbes; and the amounts being absorbed and emitted by oceans.

“If all the carbon dioxide emitted from fossil fuel burning were to stay in the atmosphere, its rate of accumulation in the atmosphere would be two-and-a-half times as fast as it actually is and climate would change two-and-a-half times faster,” said Reich. “Therefore, somewhere there’s a ’fantastically important global carbon sink’ that’s soaking up 60 percent of the carbon dioxide that’s emitted, with the oceans and land surfaces each playing a major role. However, researchers have estimated that plant respiration releases five to 10 times as much carbon dioxide as fossil fuel burning. It’s crucial, therefore, to know the amount of plant emissions more accurately because that number makes a huge difference in calculating how much of the gas is being absorbed from the atmosphere and staying in the biosphere. This in turn will help scientists figure out what the carbon sink is and what its capacity might be.”