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· collected 2026-09-03 · by TOI Science Desk
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.
asserted
nutrients → restore → 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.
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researchers → spread → Hampshire
The treatment was meant to replace important soil nutrients that had been washed away by decades of industrial pollution.
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that → mean → pollution
It worked at first, improving tree growth and leaf health over the next decade.
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It → work → 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.
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valley → reveal → 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.
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ecosystems → publish → 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.
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it → expect → 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.
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research → carry → 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.
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deposition → show → soil
The loss of calcium weakened important tree species such as sugar maple and red spruce and slowed forest growth across the area.
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loss → weaken → area
To see whether replacing the lost calcium could help, scientists launched a whole-ecosystem restoration project.
uncertain
scientists → see → project
Using a helicopter, they spread 45 tons of pelletised wollastonite, a natural calcium silicate mineral, across the drainage basin.
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they → use → 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.
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treatment → add → pollution
The early results matched what researchers expected.
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researchers → match → what
Soil acidity fell, the stream became better able to neutralise acids, and forest plants recovered.
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plants → fall → acids
Sugar maples developed larger leaf canopies, while native red spruce trees became more tolerant of cold winter conditions.
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trees → develop → conditions
Unintended changes in stream chemistry
The experiment took an unexpected turn about a decade after the mineral was added.
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mineral → take → turn
Long-term monitoring of the streams showed a sharp rise in dissolved inorganic nitrogen, mainly nitrate.
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monitoring → show → nitrogen
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.
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Watershed → continue → direction
By 2013, annual nitrate losses from the treated catchment had risen to levels 30 times higher than those in neighbouring untreated watersheds.
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losses → treat → 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.
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amount → release → clearing
The discovery challenged a basic idea in ecosystem science.
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discovery → challenge → science
Healthy forests with strong plant growth are generally expected to act as nutrient sinks because growing trees need large amounts of nitrogen.
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trees → expect → nitrogen
Instead, Watershed 1 changed from holding onto nitrogen to releasing large amounts of it.
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Watershed → change → 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.
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what → point → soil
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.
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microbes → accord → floor
The extra calcium increased microbial activity and sped up the breakdown of old leaves and other organic material.
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calcium → increase → leaves
This released inorganic nitrogen faster than the growing trees could take it up.
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trees → release → it
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.
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which → reduce → streams
The researchers said the experiment showed how forest restoration can cause unexpected changes throughout an ecosystem, including in soil microbes and nutrient cycles.
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restoration → say → microbes
Lessons for environmental management
The findings offer important lessons for land managers considering calcium additions, liming or other chemical treatments to repair damaged ecosystems.
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findings → offer → 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.
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restoration → add → ecosystems
The Hubbard Brook experiment also shows why long-term monitoring is important.
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monitoring → show → ?
If researchers had stopped studying the site after five years, the project might have been considered a complete success.
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project → stop → years
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.
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ecosystems → reveal → intervention
