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.