Nitrogen-Fixing Trees Could Speed Tropical Forest Regrowth and Carbon Storage

New research indicates that a carefully targeted nitrogen boost - supplied not through bags of fertiliser, but by selecting the right tree species - could substantially accelerate regrowth. At a pivotal time for the climate, this approach may also enable tropical forests to store considerably more carbon.

Young forests, major benefits from additional nitrogen

A research team in Panama has found that supplying extra nitrogen can nearly double the growth of young tropical forests regenerating after agricultural use. This greatly raises the amount of carbon dioxide they take up for at least 10 years.

Researchers from the Smithsonian Tropical Research Institute and collaborating organisations conducted a four-year field trial across plots in the Panama Canal watershed. These sites covered a range of forest ages and land-use histories:

• cattle pasture abandoned less than a year earlier
• regenerating forest aged 10 years
• secondary forest aged 30 years
• old-growth forest approximately 600 years old

For three months of every year, field teams walked into the plots to apply fertilisers containing nitrogen, phosphorus, both nutrients or neither. They then measured tree trunks carefully to monitor biomass and growth.

In the youngest forests, extra nitrogen increased tree biomass by about 95% compared with unfertilized plots, effectively doubling growth.

Ten-year-old recovering forests also showed a substantial response: nitrogen fertilisation increased tree growth by around 48%. This means significantly more carbon was stored in trunks and branches when regenerating forests have particularly high nutrient demand.

Older forests reach a nutrient limit

The results differed after the first few decades of recovery. Forests aged 30 years and 600 years received almost no growth benefit from additional nitrogen, suggesting that other constraints were limiting their growth.

Phosphorus, another essential nutrient, produced unexpectedly limited effects at every forest age. No plots recorded a significant growth response when phosphorus was added either alone or alongside nitrogen.

The strongest effects were tightly focused: newly abandoned lands and young regrowth forests were where nitrogen made a real difference.

The pattern indicates that nutrient deficits following deforestation do not last indefinitely. With age, forests appear to regain a growth-supporting balance through the internal recycling of leaves and wood, together with naturally occurring nitrogen inputs, without needing external fertiliser.

Why soils on deforested land recover slowly

When tropical rainforest is cleared for grazing or crops, it loses not just trees but also nutrient reserves accumulated over centuries. Burning and harvesting remove nitrogen and phosphorus from the system, while heavy rainfall can leach the remaining nutrients from uncovered soils.

Scientists can identify the consequences of this loss even decades after land has been left to regenerate. Young trees encounter an effective nutrient bottleneck: although they are capable of rapid growth, the soil contains very few available nutrients.

This issue extends well beyond individual local landscapes. Tropical forests form a major component of the global carbon sink, taking in more carbon dioxide than they release and counteracting part of the greenhouse-gas pollution caused by people.

Regenerating tropical forests alone are estimated to take up a large share of the carbon absorbed by forests worldwide each year.

Improving nutrient management to speed their recovery could therefore deliver climate gains far beyond the Panama study plots.

Replacing fertiliser bags with nitrogen-fixing trees

The scientists are not advocating the application of industrial fertiliser across tropical regions. Such a measure would be prohibitively costly, impractical to deliver and environmentally risky.

Instead, they say that recognising nitrogen's role in limiting early forest recovery can inform more effective reforestation. Their main tool is trees that naturally “fix” nitrogen.

Nitrogen-fixing trees, commonly legumes, form relationships with symbiotic bacteria housed in small root nodules. The bacteria draw nitrogen from the atmosphere - which consists of about 78% nitrogen gas - and transform it into forms plants can use.

By planting more nitrogen-fixing species in young restoration projects, managers can enrich the soil from within and accelerate carbon storage without chemical inputs.

Many native legume trees in tropical forests already fulfil this function. Including them in reforestation mixtures can progressively restore soil fertility and help sustain a diverse canopy of other tree species.

How nitrogen-fixing trees function

Over time, this mechanism can help whole young forest stands escape nitrogen poverty, increasing their productivity and their capacity to capture carbon dioxide.

Implications for climate strategies

The Panama experiment offers rare direct evidence for an idea forest researchers had considered for decades: depleted nutrients can slow tropical forest recovery on former farmland, yet focused nitrogen additions can counteract this constraint.

For climate policy, the findings reinforce the need to safeguard remaining old-growth forests while supporting well-planned reforestation on degraded land. Regrowing forests are not peripheral to the issue; they represent a significant element of the global carbon budget.

Restoration schemes that incorporate nitrogen-fixing species could:

• boost carbon sequestration over the first 10 to 20 years
• lower dependence on industrial fertilisers
• strengthen soil health and drought resilience
• support a more diverse range of tree species over time

These potential benefits are particularly important in areas such as the Amazon and Central America, where extensive pasture and cropland could return to forest if suitable policies and incentives are in place.

Risks, constraints and unanswered questions

Although nitrogen-fixing trees may seem an obvious solution, they must still be used carefully. Establishing too many trees from one fast-growing legume species could lower overall biodiversity or change fire risk. Selected species need to suit local ecosystems, community requirements and land rights.

Timing is another consideration. The clearest gains from extra nitrogen occur during the early decades of regrowth. Restoration planners may therefore need a phased strategy, relying more heavily on nitrogen-fixing trees initially before allowing slower-growing, late-successional species to become dominant as soils recover.

It also remains uncertain how climate change will affect nutrient availability. Higher temperatures, changing rainfall patterns and increasingly frequent droughts could alter the speed at which nitrogen cycles through these ecosystems and the efficiency with which young forests use it.

Key terms climate readers frequently ask about

Carbon sink: An ecosystem or process that removes more carbon dioxide from the atmosphere than it releases. Tropical forests, peatlands and oceans are important natural sinks.

Biomass: The combined mass of living biological material within a particular area, normally expressed as dry weight. In forests, aboveground biomass principally includes trunks, branches and leaves.

Secondary forest: Forest that returns on land previously cleared or severely disturbed by human activity, unlike old-growth forest, which has remained comparatively undisturbed for centuries.

For landowners, NGOs and governments developing tree-planting or natural-regeneration programmes, the findings point to a straightforward practical change: nitrogen-fixing trees should have a central place, particularly in the first planting generation on exhausted pasture or cropland. This decision could determine whether forest returns gradually or rapidly, drawing much more carbon from the atmosphere during the crucial decades ahead.