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January
18, 2007 Over the past
decade, in numerous
field sites throughout the world, mesh bags of leaf and root litter sat
exposed
to the elements, day and night, throughout the four seasons, gradually
rotting
away. Now, those bags of
decomposing
organic matter have allowed a research team led by scientists at the 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 January 19 issue 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 "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 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."
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