NSF PR 03-13 - January 31, 2003
A Ferrous Wheel in the Forest
Scientists develop new hypothesis on the fate of acid rain
A new hypothesis is being presented by scientists from The Woods Hole Research Center, the University of Arizona, and the University of Maine that may help explain how acid rain reacts chemically in forest soils.
"Acids of nitrogen and sulfur in rainwater, often called 'acid rain,' can have both positive and negative effects on the health of forests, streams, and lakes," explained Eric Davidson of The Woods Hole Research Center, lead scientist of the study. "Acid rain is caused by air pollution, such as emissions from cars and industrial smoke stacks. It is known to cause nutrient imbalances in soil and in water that are harmful to trees, fish, and other plants and animals."
The nitrogen in acid rain could also, potentially, act as a fertilizer, thus enhancing the growth of forest trees. However, most of the nitrogen that falls on forests as acid rain tends to remain in the soil rather than being taken up by the trees.
In a paper published in the February 2003 issue of the journal Global Change Biology, the scientists present evidence of a complex set of reactions of iron minerals, organic matter, and nitrate in the soil that bind nitrogen present in acid rain to organic matter in forest soil. Laboratory studies on soils taken from the National Science Foundation (NSF)'s Harvard Forest Long-Term Ecological Research (LTER) site in central Massachusetts showed that nitrate reacts with iron and organic matter under sterile conditions. The products of these reactions are forms of nitrogen that may be relatively stable in soils, thus preventing much of the nitrogen from being used by the trees.
The scientific name for one form of iron is "ferrous," and, in a play on words, the scientists have dubbed the reaction cycle as the "ferrous wheel hypothesis." A novel aspect of this hypothesis is that part of the chemical reaction is abiological - that is, without the participation of living organisms. "Davidson's team has developed an elegant model that could explain how nitrate is rapidly incorporated into organic matter without being directly
metabolized by soil organisms," said Knute Nadelhoffer, program director in NSF's division of environmental biology, which funded the research.
Until now, ecologists and soil scientists have not understood how this nitrogen gets bound to the soil and why most of it tends to stay there. This study needs to be confirmed in other forest soils, and the exact chemical pathway is still not entirely clear. However, this hypothesis provides a new avenue for directing future research on the effects of acid rain on forests.