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Leaf litter falls to the ground where it is
decomposed by microbes. As it decays it releases nitrogen, a key
nutrient for plants and animals.
Photo by William S. Currie
Leaf and root litter were placed
in mesh bags on the ground and left for up to 10 years. Subsets of
bags were collected every year to determine the rate at which the
leaves and roots decayed. The nitrogen content of the litter was
also measured.
Photo by Jay M. Sexton |
The researchers found that the dominant
drivers of nitrogen release were the initial concentration of nitrogen
and the remaining mass of the leaf and root litter. The equations and
how the researchers developed them are described in the Jan. 19 issue
(2007)
of the journal Science.
"In the world of complex biogeochemistry, we've
discovered that this fundamental process of nutrient cycling by plants
and microbes turns out to be relatively straightforward," said Whendee
Silver, professor of ecosystem sciences at UC Berkeley's College of
Natural Resources and co-lead author of the study. "Whether it's hot
or cold, wet or dry, the equations work. This study highlights the
fact that, for microbes, there is a fundamental physiological
constraint controlling nitrogen release during decomposition. It took
a large-scale, long term study like this to help us see how simple
these processes can be."
The project, known as the Long-Term Inter-site
Decomposition Experiment, involved 21 field sites. The sites represent
seven biomes, from the tundra to tropical forests, encompassing the
array of climatic conditions around the world. Many of the sites were
part of the Long Term Ecological Research Network sponsored by the
National Science Foundation.
Each mesh bag included leaf or root litter, such as
pine needles, wheat straw, sugar maple leaves or grass roots. The
samples were chosen to represent a wide range of chemical composition.
At each site, dozens of bags were staked to the
ground and left to rot. Every year, researchers at the sites would
remove a subset of bags so their contents could be dried, weighed and
sent to a central lab at Oregon State University for analysis. At the
central lab, the contents were weighed a second time and analyzed for
their chemical composition.
"The most important component of this study is that
we've developed a generic global law that can predict large-scale
patterns in litter mass decay rates and nitrogen release from litter,"
said William Parton, senior research scientist at the Natural Resource
Ecology Laboratory at Colorado State University and co-lead author of
the study. "There are a lot of global nutrient cycling models out
there, but the model we've developed is based on only two parameters,
and thus is more scientifically elegant and more widely applicable
than the models currently being used."
More than three-fourths of the air we breathe
contains nitrogen, an essential element found in all amino acids, the
building blocks of protein. In the soil, organic forms of nitrogen are
converted by bacteria into the inorganic forms of ammonium and nitrate,
primary nutrients plants need for growth. Lack of nitrogen limits
plant growth in most regions of the world.
The researchers point out that the cycling of
nutrients and carbon in the ecosystem is a key variable in climate
change models. "As people try to construct computer models and predict
future climate changes, being able to accurately predict carbon and
nitrogen cycling will play a key role," said Silver.
While the study improves the ability of scientists
to predict the rate of nitrogen release in climate models, the
researchers point out that the findings could also improve predictions
for the amount of carbon released into the atmosphere from decomposing
litter.
"The debate is whether the enhanced litter decay
rate from warming will also increase the release of carbon from
ecosystems," said Parton. "When you increase litter decomposition
rates, you are enhancing carbon dioxide release to the atmosphere. Our
study provides algorithms to better predict the rates of these
processes under a wide range of conditions."
The researchers found that the rate of
decomposition, not nitrogen release, was affected by the two key
variables of temperature and moisture. The slowest rates of
decomposition were in cold regions, such as boreal forests and tundra,
and the fastest in the warm, moist, tropical forests. The only places
where litter decomposed completely were the humid tropical sites where,
over the course of five years, only 10 percent of the initial litter
material remained.
Notably, there was one exception where the model
did not apply. "Arid grasslands didn't fit the model because the
nitrogen release in those environments is likely to be controlled by
exposure to UV radiation," said Silver. "The leaves decomposed faster
than they should have based upon the climate alone, and released
nitrogen faster than the model predicted based on the initial nitrogen
concentrations. The most probable explanation is that systems exposed
to high UV radiation circumvent the biological processes in other
ecosystems." |
