A strategy scientists tested nearly
20 years ago to neutralize acidic forest soils, and which was found to boost
tree growth, has resulted in an unexpected spike in nitrogen export a decade
later, confounding the researchers’ efforts to explain why.
The study launched in 1999 at the
Hubbard Brook experimental forest in New Hampshire was designed to mitigate the
acidification of soils caused by acid rain. Scientists from the Cary Institute of Ecosystem Studies and other institutions dropped 2,600 pounds of calcium
silicate by helicopter over a 30-acre forested watershed and then waited to see
what would happen.
They expected that the addition of
calcium would compensate for the calcium that acidic precipitation had depleted
from the soil and reverse the negative effect the acidic soils had on the
health of red spruce, sugar maple and other tree species. It appeared to work,
as the calcium dissolved and worked its way into the soil profile, decreasing
soil acidity and boosting tree growth.
“For nearly 10 years, it looked like
our predictions were correct,” explained Gene Likens, president emeritus of the Cary Institute and the leader of the study. “The calcium was largely retained
and the forest was growing. Then, in 2010, we noticed streams draining the
treated site had elevated nitrogen levels. By 2013, yearly inorganic nitrogen
losses were 30 times what we expected, an increase we had only seen after
forest clear-cutting experiments.”
Nitrogen is a vital nutrient in a
healthy forest ecosystem. Growing forests typically absorb and retain nitrogen,
so the depletion of nitrogen a decade after the addition of calcium to the
ecosystem is worrisome to the researchers.
“The rules of conventional ecology
suggested that after the calcium addition, forest growth would lead to even
more nitrogen retention,” said co-author Emma Rosi-Marshall. “Yet the treated
watershed is shedding nitrogen,” which could slow forest growth.
The reason for the depletion of
nitrogen is largely a mystery that the scientists are just beginning to
investigate. They speculate that when the addition of calcium lowered the
acidity of the forest soils, it stimulated microbial processes in the soil
organic matter, which then released nitrogen stored in the forest floor.
Likens said that the unexpected –
and delayed – results of the study raise numerous questions for those
responsible for managing forests. He
said it is especially concerning in mountainous regions because nitrogen loss
in stream water will be carried downhill and never find its way back to the
mountains.
But Likens also took a philosophical
perspective on the results of his study.
“It often takes a long time to see a
response or to get an insight into what that response might mean,” he said. “If
we’re going to try to understand how these systems work, then we have to be
aware that if we manipulate them – through acid rain or the treatment of acid
rain – then it’s likely going to have an effect that we don’t expect.”
This article first appeared in Northern Woodlands magazine on December 1, 2017.
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