Bottom-Up Changes in Ecosystems

Bottom-up changes at continental and/or regional
scales have implications for future survival of
hundreds and perhaps thousands of plant and
animal species. Bottom up changes in the
relationships between soils, carbon and water are
among the expected changes, and researchers have
been testing increasingly pointed questions about
these relationships.

Can soils store CO2 well enough to counteract
rising levels of CO2 in the atmosphere? High
hopes for that scenario don’t hold up when
precipitation starts arriving at a different
season.  See #1 of the two releases posted below.

But, apart from emergent seasonal shifts of
precipitation,  how well do we understand even
the most basic relationships between soil and
water at the continental or regional scale?
According to #2 of the 2 releases posted below, ”
… large-scale simulations of water dynamics in
soil may be imprecise to completely wrong.”

Lance

#1——————————————————————————–
” In fact, organic matter levels may have even been lower than
in plots not exposed to elevated carbon dioxide levels.”
———————————————

University of Illinois at Urbana-Champaign

Increased carbon dioxide in atmosphere linked to decreased soil organic matter

Contact: Debra Levey Larson
dlarson@uiuc.edu
217-244-2880

University of Illinois at Urbana-Champaign

URBANA – A recent study at the University of
Illinois created a bit of a mystery for soil
scientist Michelle Wander – increased carbon
dioxide in the atmosphere was expected to
increase plant growth, increase plant biomass and
ultimately beef up the organic matter in the soil
— but it didn’t. What researchers found instead
was that organic matter decay increased along
with residue inputs when carbon dioxide levels
were increased and they think the accelerated
decay was due to increased moisture in the soil.

“Going into the study, the assumption was that
higher levels of carbon dioxide in the atmosphere
will increase crop yield and soil organic
matter,” said Wander. “We did see a 30 percent
increase in above- and below- ground soybean
biomass so we expected that to be mirrored in
soil organic matter, but there wasn’t an
increase. In fact, organic matter levels may have
even been lower than in plots not exposed to
elevated carbon dioxide levels.”

The study was conducted at U of I’s SoyFACE
facility – an open air laboratory in which rings
of pipes surround corn and soybean crops and can
be exposed to various levels of carbon dioxide,
ozone or both pumped through the pipes. The
findings from the study are published in the
February issue of Plant and Soil.

“My student Adriane Peralta and I were looking at
younger soil organic matter that would be most
influenced by today’s practices and we were
expecting a big change — a 30 percent increase
in soil organic matter, reflecting the changes we
saw above ground.

“The source of carbon is plant biomass, so we
would expect increased yield, increased biomass,
increased soil organic matter in the soil. This
kind of positive feedback would be good because
it could offset the increases in decay that will
result from rising temperature,” said Wander. She
explained that the increases in carbon dioxide
levels in the atmosphere insulate the earth and
contribute to global warming. Average annual air
and soil temperatures are increasing while
winters are getting shorter. By the end of the
century, maximum daily temperatures could rise by
5 to 12 degrees Fahrenheit in winter and 5 to 20
degrees Fahrenheit in summer.

“We know that microbial activity is directly
influenced by an increase in temperature if other
factors, like moisture aren’t limiting their
growth,” she said. “Increased decomposition of
organic matter is undesirable from a soil quality
and climate perspective; microbial degradation of
organic stocks releases carbon and nitrogen and
over the long term this reduces soil’s
productivity and ability to resist erosion, plus
it returns the carbon dioxide to the atmosphere.”
All of this talk about using agricultural lands
to mitigate climate change depends upon our
ability to keep the carbon in soil reserves.

Wander said that carbon dioxide is rising every
year in the atmosphere because of human use of
fossil fuel and deforestation. “We attribute the
higher soybean yields over the past several
decades to the rising carbon dioxide levels in
the Earth’s atmosphere – some attribute a 10
percent increase in soybean yields already due to
this carbon dioxide fertilization effect.

“Most models or projections of the future assume
the carbon dioxide fertilization effect would be
a good thing for agriculture and the world’s food
supply and have a benefit to soil organic matter,
but more and more we are finding things are a
little more complicated. What our study shows is
that in this system, rising carbon dioxide levels
are not contributing to soil health after all.

“So, we had a bit of a mystery to solve. Where
did the organic carbon that was added by
increased plant growth go” We know for certain
that soil organic matter stocks result from the
balance of inputs and decay so we had to look at
factors influence decomposition. Nutrient levels
soil pH and available N were all high in this
fertile field and so we ruled these factors out.”

Wander and Peralta suspect soil moisture plays a
role. Wander points out that changes in rainfall
are another important aspect of climate change
and notes that we are already seeing shifts in
the distribution of rainfall with increases in
winter and spring rains with drier summers. Dry
conditions can constrain plant growth and
microbial decay rates. So, what they saw in the
SoyFACE plots, was evidence of an important
feedback — where crops exposed to elevated
carbon dioxide became more water use efficient.
“When plants take up moisture they open their
stomata — the pores through which they transport
both carbon dioxide and water and when plants
satisfy their need for carbon dioxide they can
close those stomata and conserve water. This
appears to have happened at SoyFACE in both corn
and soybean crops. So, moisture feedbacks that
increased microbial activity might solve the
mystery”. Wander said it’s a little tricky to
project the future with these findings, because
they are manipulating carbon dioxide but not
rainfall in the SoyFACE test plots.

“We have learned that we can’t say ‘yield equals
organic matter.’ We have to understand the
nuances of the time and place. SoyFACE is giving
us early clues about what could happen in the
future and where to direct our research
attentions.” The frontier of science right now
includes anticipation of these interactions
-reality might be stranger than the fiction that
we create in the laboratory- even in an open
field study like SoyFACE.

————————————————————————————————————————————–

#2—————————————————-
“This study implies that large-scale simulations of water
dynamics in soil may be imprecise to completely wrong.”
——————————————

Soil Science Society of America
677 South Segoe Road * Madison WI 53711-1086 *
Tel. 608-273-8080 * Fax 608-273-2021
www.soils.org * email: headquartersatsoils.org

NEWS RELEASE
Contact: Sara Uttech, Soil Science Society of
America, 608-268-4948, suttechatsoils.org

Are existing large-scale simulations of water dynamics wrong?

In the February issue of Vadose Zone Journal,
researchers find that a much smaller spatial
resolution should be used for modeling soil water.

MADISON, WI, March 3, 2008 — Soils are
complicated porous media that are highly relevant
for the sustainable use of water resources. Not
only the essential basis for agriculture, soils
also act as a filter for clean drinking water,
and, depending on soil properties, they dampen or
intensify surface runoff and thus susceptibility
to floods. Moreover, the interaction of soil
water with the atmosphere and the related energy
flux is an important part of modern weather and
climate models.

An accurate modeling of soil water dynamics thus
has been an important research challenge for
decades, but the prediction of water movement,
especially at large spatial scales, is
complicated by the heterogeneity of soils and the
sometimes complicated topography.

Simulation models are typically based on
Richards’ equation, a nonlinear partial
differential equation, which can be solved using
numerical solution methods. A prerequisite of
most solution algorithms is the partitioning of
the simulated region into discrete grid cells.
For any fixed region, such as a soil profile, a
hill slope, or an entire watershed, the grid
resolution is usually limited by the available
computer power. But how does this grid resolution
affect the quality of the solution?

This problem was explored by Hans-Joerg Vogel
from the UFZ – Helmholtz Center of Environmental
Research in Leipzig, Germany and Olaf Ippisch
from the Institute for Parallel and Distributed
Systems of the University of Stuttgart, Germany.
The results are published in the article
“Estimation of a Critical Spatial Discretization
Limit for Solving Richards’ Equation at Large
Scales,” Vadose Zone J. Vol. 7, p. 112-114, in
the February 2008 issue of Vadose Zone Journal.

Vogel and Ippisch found that the critical limit
for the spatial resolution can be estimated based
on more easily available soil properties: the
soil water retention characteristic. Most
importantly, this limit came out to be on the
order of decimeters for loamy soils, and is even
lower, on the order of millimeters, for sandy
soils. This is much smaller than the resolution
used in many practical applications.

This study implies that large-scale simulations
of water dynamics in soil may be imprecise to
completely wrong. But, it also opens new options
for a specific refinement of simulation
techniques using locally adaptive grids. The
derived critical limit could serve as an
indicator that shows where a refinement is
necessary. These findings should be transferable
to applications such as the simulation of oil
reservoirs or models for soil remediation
techniques.

The full article is available for no charge for
30 days following the date of this summary. View
the abstract at:
http://vzj.scijournals.org/cgi/content/full/7/1/112

Vadose Zone Journal,
http:/www.vadosezonejournal.org/ is a unique
publication outlet for interdisciplinary research
and assessment of the biosphere, with a focus on
the vadose zone. VZJ is a peer-reviewed,
international, online journal publishing reviews,
original research and special sections on across
a wide range of disciplines that involve the
vadose zone, including those that address broad
scientific and societal issues. VZJ is published
by Soil Science Society of America, with
Geological Society of America as a cooperator.

The Soil Science Society of America (SSSA)
www.soils.org is an educational organization
based in Madison, Wisconsin, which helps its
6,000+ members advance the disciplines and
practices of soil science by supporting
professional growth and science policy
initiatives, and by providing quality,
research-based publications and a variety of
member services.

© Copyright 2008

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