permaculture

["Lawrence F. London, Jr." <lflj@intrex.net>]

March 14 issue of Science Daily
http://www.sciencedaily.com/releases/2008/03/080311123413.htm

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Increased Carbon Dioxide In Atmosphere Linked To Decreased Soil Organic Matter

ScienceDaily (Mar. 14, 2008) - 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.

[snip]

"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.