Nitrogen study may improve climate change predictions

The pattern of nitrogen release from decaying plant material is remarkably similar and predictable across the planet, researchers have concluded in a new study, which should make it easier to understand nutrient dynamics, vegetation growth, estimate carbon release and sequestration, and better predict the impacts of climate change.

The findings, to be published in the journal Science, are the results of one of the largest and longest studies ever done on nitrogen release during plant decomposition, involving dozens of researchers working for 10 years in 27 sites, ranging from Arctic tundra to tropical forests of North and Central America.

"The availability of nitrogen is one of the key factors limiting vegetation growth around the world, but its release from plant litter can be very slow," says Mark Harmon, a professor of forest science at Oregon State University and the coordinator of the study. "For the first time, we studied this process at enough sites and over a long enough time period to really understand what's happening."

The surprise, researchers said, is that the basic pattern of nitrogen release is pretty much the same wherever it occurs, and is driven primarily by the initial concentration of nitrogen present in the decaying plant material. It has little to do with location, soil types, microbes present, or other factors. The speed of the process is affected by climate, particularly temperature and precipitation, the study concluded. But the overall pattern, or "trajectory" of nitrogen release remains much the same regardless of the site.

There is significant interest in the way that nitrogen recycles in the ecosystem, scientists say, because it plays such a critical role in the growth of almost all vegetation – grasses, shrubs, trees and agricultural crops. The presence or absence of adequate amounts of nitrogen can often dictate what types of vegetation are able to survive in a certain area, and how quickly it grows. Very little of this nutrient is made available from geological sources (illustration: the nitrogen cycle - click to enlarge).

Plant growth, in turn, is one of the main factors that affects the input or removal of carbon from the atmosphere – an issue of growing importance during an era of global warming. Plant decomposition releases more carbon each year than all of the fossil fuel combustion produced by humans, the researchers note in their study:
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"If we hope to better predict carbon dynamics, climate change and other issues, we first must understand these basic ecological processes," Harmon said.

In plant decomposition, it's not unusual for the microbes which are decomposing the plant matter to first retain nitrogen from the dying plants and other sources, until they have all they need for the decomposition process, Harmon said. This "immobilization" of nitrogen can actually cause a reduction in available soil nitrogen for an extended period of years, until at some point the plant material is sufficiently broken down that nitrogen in excess of decomposer needs becomes available. It had been thought that this process might be highly variable, depending on several interacting factors. In fact, the study found that it is pretty predictable, affected primarily just by the initial nitrogen concentration in the plant material which is decaying.

"It was really surprising to see how similar these processes were across wide geographic and climatic scales," Harmon said. "The basic trajectory is much the same regardless of many variables. A fairly simple model can accurately predict it."

The overall decomposition process, he said, does speed up in warmer or wetter conditions, which many anticipate as a result of climate change and global warming. In that event, nitrogen should more rapidly be made available to plants, at least initially spurring increased vegetation growth and offsetting carbon losses from increased decomposition.

Less clear is the overall long-term impact on carbon sequestration and storage, Harmon said. That may depend on whether the growth that occurs is in the form of vegetation parts that quickly die, such as leaves, or in wood that lives much longer. So whether increased vegetation growth on a global basis will increase enough to offset global warming is still uncertain, he said, and requires further study.

The research, called the Long-Term Inter-site Decomposition Experiment, or LIDET study, was funded by the Long Term Ecological Studies program of the National Science Foundation. Participants included OSU, Colorado State University, University of California/Berkeley, LSI Logic, University of Michigan, University of Minnesota, Northern Arizona University, and 23 other institutions that conducted the field work.

A wide range of "biomes," or general types of ecosystems, were included in the research to increase its applicability on a global scale. Among the sites was the H.J. Andrews Experimental Forest in the Cascade Range of Oregon, one of the state's leading programs of long term ecological research.