Twenty years ago, scientists experimented with adding calcium to forest soils and found that it boosted tree growth. Today, they’re seeing an unexpected spike in nitrogen loss in these same forests.
The study, launched in 1999 at the Hubbard Brook Experimental Forest in New Hampshire, was designed to mitigate soil acidification. 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 were interested in finding out whether the addition of calcium would compensate for the calcium that acidic precipitation had depleted from the soil, thereby reversing the negative effect that acidic soils were having 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, soil acidity decreased and tree growth got a boost.
“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 clearcutting 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 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, eventually releasing 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 a concern in mountainous regions, because lost nitrogen will be carried downhill by streams 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.”