Tropical forests, critical for climate stability, can significantly increase carbon absorption with a simple boost of nitrogen, according to new research published in Nature Communications. The study demonstrates that nitrogen fertilization can nearly double tree growth in recently reforested areas, and even increase growth by nearly 50% in forests recovering for a decade. This suggests that nutrient depletion – a long-understood but under-experimented issue – is a major constraint on tropical forest regeneration.
The Nutrient Depletion Problem
When tropical rainforests are cleared, often for agriculture, the soil loses key nutrients like nitrogen and phosphorus. Unlike temperate soils, these tropical soils don’t rapidly replenish these nutrients naturally, even after decades of regrowth. This limits the forests’ ability to re-establish as effective carbon sinks. Researchers at the Smithsonian Tropical Research Institute (STRI) and the University of Leeds tested this hypothesis rigorously over a four-year period near the Panama Canal.
They monitored plots ranging from year-old pastures to 600-year-old forests, applying nitrogen, phosphorus, or a combination of both. The results were striking: nitrogen nearly doubled growth in the youngest forests and boosted growth in 10-year-old forests by almost half. Older forests showed no additional benefit, and phosphorus had no effect.
Why This Matters: Carbon Storage and Climate Change
Tropical forests store roughly half of all forest carbon and absorb about 20% of global carbon emissions. Whether they continue to do so is uncertain, and nutrient availability is a key factor. The world’s forests absorb 3.5 pentagrams of carbon annually, with tropical forests accounting for the largest share. Recovering tropical forests are particularly important, absorbing approximately 2.5 pentagrams of carbon each year.
The study confirms decades-old observations that tropical soils are easily depleted and slow to recover. Richard Birdsey, a senior scientist at the Woodwell Climate Research Center, notes that nutrient limitations in these forests have been recognized for 50 years but lacked rigorous experimental proof until now.
Practical Implications: Nitrogen-Fixing Trees
Rather than widespread fertilization, the findings suggest a more sustainable solution: planting more nitrogen-fixing trees. These species can convert atmospheric nitrogen into usable nutrients, naturally enriching the soil. Jefferson Hall, director of the Agua Salud project at STRI, explains that this is a far more practical approach to enhancing carbon sequestration in tropical forests.
“The natural way of enhancing the nitrogen system would be to plant more nitrogen-fixing trees.”
The study provides critical evidence supporting the role of nutrient management in tropical forest recovery, offering a pathway to enhance carbon storage in one of the planet’s most important ecosystems.
