A large-scale effort to restore an American forest damaged by acid rain had an unexpected environmental effect when added nutrients caused a huge increase in nitrogen flowing into nearby streams. In October and November 1999, researchers spread about 45 short tons (40 metric tonnes) of a calcium-rich mineral over a 29-acre (11.8-hectare) catchment in New Hampshire. The treatment was meant to replace important soil nutrients that had been washed away by decades of industrial pollution. It worked at first, improving tree growth and leaf health over the next decade. But long-term monitoring later revealed a surprising change: by 2013, the treated valley was sending 30 times more inorganic nitrogen into its streams than nearby untreated forest catchments. The findings, published in the journal Proceedings of the National Academy of Sciences (PNAS), challenged long-standing ideas about how forest ecosystems handle nutrients. Scientists had expected a healthier, faster-growing forest to take in and store more nitrogen instead of allowing it to escape into nearby waterways.
An experimental intervention in the White Mountains
The research was carried out at Watershed 1 in the Hubbard Brook Experimental Forest, in the White Mountains of New Hampshire. Records from the Hubbard Brook Ecosystem Study show that acid deposition during the mid-to-late 20th century removed large amounts of base cations, especially calcium, from the soil. The loss of calcium weakened important tree species such as sugar maple and red spruce and slowed forest growth across the area. To see whether replacing the lost calcium could help, scientists launched a whole-ecosystem restoration project. Using a helicopter, they spread 45 tons of pelletised wollastonite, a natural calcium silicate mineral, across the drainage basin. The treatment added about 1,189 kilogrammes of calcium per hectare, with the goal of bringing soil base saturation back to levels estimated before industrial pollution. The early results matched what researchers expected. Soil acidity fell, the stream became better able to neutralise acids, and forest plants recovered. Sugar maples developed larger leaf canopies, while native red spruce trees became more tolerant of cold winter conditions.
Unintended changes in stream chemistry
The experiment took an unexpected turn about a decade after the mineral was added. Long-term monitoring of the streams showed a sharp rise in dissolved inorganic nitrogen, mainly nitrate. While nearby reference forests at Hubbard Brook continued to show a long-term decline in nitrogen exports, Watershed 1 began moving in the opposite direction. By 2013, annual nitrate losses from the treated catchment had risen to levels 30 times higher than those in neighbouring untreated watersheds. The amount of nitrogen being released was similar to levels normally seen after major environmental disturbances, including complete forest clearing or severe ice storms. The discovery challenged a basic idea in ecosystem science. Healthy forests with strong plant growth are generally expected to act as nutrient sinks because growing trees need large amounts of nitrogen. Instead, Watershed 1 changed from holding onto nitrogen to releasing large amounts of it. The shift pointed to an important difference between what was happening above ground in the trees and what was happening below ground in the soil.
Soil microbes and carbon dynamics
According to the PNAS study, which is also indexed in PubMed (PMID: 27335456), adding calcium changed the way soil microbes processed organic matter on the forest floor. The extra calcium increased microbial activity and sped up the breakdown of old leaves and other organic material. This released inorganic nitrogen faster than the growing trees could take it up. Changes in soil chemistry may also have reduced the soil’s ability to store organic carbon, which could have added to the release of nitrate into streams. The researchers said the experiment showed how forest restoration can cause unexpected changes throughout an ecosystem, including in soil microbes and nutrient cycles. Such effects may not be obvious in smaller laboratory studies.
Lessons for environmental management
The findings offer important lessons for land managers considering calcium additions, liming or other chemical treatments to repair damaged ecosystems. Although adding calcium successfully reversed signs of forest decline and improved tree health, the later increase in nitrogen runoff showed that restoration can also create problems downstream, including poorer water quality and excess nutrients in aquatic ecosystems. The Hubbard Brook experiment also shows why long-term monitoring is important. If researchers had stopped studying the site after five years, the project might have been considered a complete success. Continued monitoring more than a decade later revealed the delayed release of nitrogen and gave scientists a clearer picture of how complex forest ecosystems respond to human intervention.
