The growth of the Amazon rainforest in our increasingly carbon-rich atmosphere may be limited by a lack of phosphorus in the soil, according to new research.
Higher concentrations of carbon dioxide (CO2) cause plants to grow faster, which means they store more carbon.
This storage – especially in huge forests like the Amazon – helps limit rising CO2 levels, slowing climate change.
However, plants also need nutrients to grow, and the new study shows that the availability of a particular nutrient, phosphorus, could limit the Amazon’s ability to increase productivity (growth rate ) as CO2 rises.
It could also make the rainforest less resilient to climate change, researchers warn s.
The study, published in the journal Character, was carried out by an international team led by the Brazilian National Institute for Amazonian Research (INPA) and the University of ‘Exeter.
“Our findings call into question the potential for sustaining current high rates of carbon uptake in the Amazon,” said lead author Hellen Fernanda Viana Cunha, from INPA.
“Approximately 60% of the Amazon basin is on ancient soils with low phosphorus content, but the role of phosphorus in controlling productivity was unclear as most fertilization experiments in other parts of the world have been performed in higher phosphorus-rich systems.
“Our experiment, the Amazon Fertilization Experiment (AFEX), examined the effects of adding phosphorus, nitrogen and base cations (other potentially essential nutrients) in an area ancient low-phosphorus rainforest.
“Only phosphorus led to increased productivity in the first two years of the experiment.
“Having such rapid and strong phosphorus responses, both above and below ground, is an indication that the entire system was operating under severe phosphorus limitation.”
Soil in tropical regions such as the Amazon was typically formed millions of years ago, and some nutrients may 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 runs out levels are unlikely to rise.
In the new experiment, two years with a phosphorus supplement resulted in s significant increases in fine root growth (29%) and canopy productivity (19% ).
Stem growth has not increased. Cunha said this could be because roots and leaves require more phosphorus than stems, and growing stems is a slower process.
A Very long-term monitoring of the experiment is needed to determine if a stemwood productivity response becomes apparent.
The results have major implications not only for storage carbon, but also for the resilience of the forest to climate change.
“To deal with and recover from growing threats such as droughts, we need the forest is growing better than before,” said Professor Iain Hartley, from the Department of Geography at the University of Exeter.
“CO2 fertilization could increase resilience of the forest, but our results suggest that the availability of phosphorus will limit this effect – and therefore the risks caused by climate change atic are becoming increasingly important.
“In short, some parts of the rainforest that grow in low-fertility soils may be more vulnerable than currently recognized. .”
Testing this recommendation is an urgent research priority. The AmazonFACE experiment – whose international team includes researchers from INPA and Exeter – is working to address this key priority.
The new study was funded by the Organic Ecosystem Investigate Council.
The paper is titled: “Direct Evidence for Phosphorus Limitation on Amazon Forest Productivity.