GROUND COVER: The decay of leaves and other plant detritus determines the nutrients available to forest trees for growth--thus playing a key role in how an ecosystem responds to climate change. Image: © WILLIAM S. CURRIE
Plants thrive off the nutrient remnants of their ancestors and peers. Trees in a temperate forest draw nitrogen from the soil that is largely derived from the breakdown of the leaves that carpet the ground. From the Arctic tundra to the tropical jungles of Costa Rica, this cycle of decay follows roughly the same rules, according to a 10-year study reported in the January 19 Science.
Ecologist William Parton of Colorado State University and his colleagues set out metal mesh containers filled with leaf litter at 21 sites throughout the western hemisphere. Every site had such packets filled with the detritus typically found there as well as others containing standard plant matter, such as oak leaves or grass straw. "We wanted to find out the long-term fate of carbon and nitrogen in litter, how that contributed to soil organic matter and what are the controls on this litter decay rate and nitrogen release," Parton says. "We wanted things with a lot of lignin and a little [lignin] as well as things with differing amounts of nitrogen in them."
The mesh containers allowed in light, water and, most importantly, the microbes and other organisms responsible in that ecosystem for plant matter decay. Over the decade, more litter covered the tacked-down containers as well, mimicking natural conditions. The researchers found that most ecosystems—whether the icy Arctic tundra or the warm, moist tropical jungle—followed the same set of rules, where the amount of nitrogen released by decay was determined by the initial concentration of nitrogen in the litter itself. "In the world of complex biogeochemistry, we've discovered this fundamental process of nutrient cycling by plants and microbes turns out to be relatively straightforward," noted study co-author Whendee Silver, an ecologist at the University of California, Berkeley, in a statement. "For microbes, there is a fundamental physiological constraint controlling nitrogen release."
By reducing such litter decay to a simple mathematical equation, researchers can better understand how carbon and nitrogen cycle through terrestrial systems—and thus improve climate change models. "If you enhance decay rates with higher temperatures, then there is more nitrogen released and it might enhance productivity. If you enhance production, you have more carbon in the system and in forest systems that might result in carbon sequestration," Parton says. "This model will help us better predict how the system is going to behave under changes."
Of course, not every system fit the model: the amount of UV light—rather than the initial nitrogen levels—determined nitrogen release in dry grasslands, the only outlier in this set of sites. Parton also plans to use litter tagged with specific carbon isotopes to better determine the fate of carbon and nitrogen in natural ecosystems over time. But 10 years spent watching leaves rot in metal bags has added immeasurably to the scientific understanding of global cycles. "The most surprising thing is that they were all the same," Parton adds. "It's a really different set of organisms in all these systems so you might see a different pattern but the similarity is the thing."