Amazon's growth limited by lack of phosphorus

Assistant Professor Kelly Andersen who is part of the Amazon Fertilisation Experiment (AFEX)

Growth of the Amazon rainforest in our increasingly carbon-rich atmosphere could be limited by a lack of phosphorus in the soil, new research shows.

Higher concentrations of carbon dioxide (CO2) cause plants to grow more quickly, meaning they store more carbon.

This storage – especially in huge forests like the Amazon – helps to limit rising CO2 levels, slowing climate change.

However, plants also need nutrients to grow, and the new study shows that availability of a particular nutrient, phosphorus, could limit the Amazon's ability to increase productivity (growth rate) as CO2 rises.

This could also make the rainforest less resilient to climate change, the researchers warn.

The study, published in the journal Nature, was carried out by an international team led by Brazil's National Institute of Amazonian Research (INPA), the University of Exeter, and Nanyang Technological University.

“Our results question the potential for current high rates of carbon uptake in Amazonia to be maintained” said lead author Hellen Fernanda Viana Cunha, who is currently finishing her PhD at INPA.

"About 60% of the Amazon basin is on old soils with low phosphorus content, but the role of phosphorus in controlling productivity was unclear because most fertilisation experiments in other parts of the world have been in more phosphorus-rich systems.

Assistant Professor Kelly Andersen with Amazon Fertilisation Experiment (AFEX) students

"Our experiment, the Amazon Fertilisation Experiment (AFEX), examined the effects of adding phosphorus, nitrogen and base cations (other potentially key nutrients) in an old-growth, low-phosphorus area of the rainforest.

"Only phosphorus led to increased productivity in the first two years of the experiment.

"Having such rapid and strong responses to phosphorus, both above and below ground, is an indication that the whole system was functioning under severe phosphorous limitation."

Soil in tropical regions such as the Amazon generally formed millions of years ago, and nutrients like phosphorus and cations, which come from chemical break down of rocks, can be lost over time.

While nutrients such as nitrogen can be absorbed from the air by micro-organisms associated with certain plants and soils, phosphorus is not available as a gas in the atmosphere – so once it is lost there is little opportunity for levels to increase.

In the new AFEX experiment, two years with extra phosphorus caused significant increases in fine root growth (29%) and canopy productivity (19%).

Stem growth did not increase. Ms Cunha said this may be because roots and leaves require more phosphorus than stems, and stem growth is a slower process.

Long-term monitoring of the experiment is required to determine whether a stem wood productivity response becomes apparent.

The findings have major implications not only for carbon storage, but also for the forest’s resilience to global changes.

“Plants grow faster with more CO2 in the atmosphere. However, results from AFEX suggest that the ability of the Amazon Forest to take advantage of increasing atmospheric CO2, which would enhance the forest’s resilience, will be ultimately limited by soil phosphorus availability,” said Assistant Professor Kelly Andersen, from Nanyang Technological University.

“Therefore, tropical forests growing on low nutrient soils may be experiencing increased risks caused by global change, with their resilience diminishing over time.

The AmazonFACE experiment – which is elevating CO2 at site near the AFEX experiment and whose international team includes researchers from the AFEX experiment– is working to address this key priority.

However, Dr Andersen said there is an urgent need to establish large-scale nutrient addition experiments across a range of tropical forest regions to better understand how soil nutrients control forest productivity and carbon cycling across the tropics.

“Whether soil phosphorus, or other nutrients, determines tropical forest productivity in low soil nutrient forests of Southeast Asia, remains uncertain.” Dr. Andersen is working with colleagues to address this research gap.

The new study was funded by the Natural Environment Research Council.

The paper is entitled: "Direct evidence for phosphorus limitation on Amazon forest productivity."