Low phosphorus soil limits the Amazon’s ability to absorb more atmospheric carbon dioxide (COtwo), one of those responsible for global warming. The conclusion is contained in a study published in the journal Nature.
Conducted by scientist Hellen Fernanda Viana Cunha, a doctoral student at Inpa (National Institute for Research in the Amazon), the research, which was published on the 10th of last month, showed that the low availability of phosphorus, a natural characteristic of 60% of Amazonian soils, can limit the capture of greenhouse gases.
In recent years, scientists have observed that the Amazon has become a source of carbon for the atmosphere, considering the balance between emission and absorption of COtwo —that is, the forest emits more than it absorbs. The causes are the increase in fires, which release carbon directly into the atmosphere, and deforestation, according to climatologist Carlos Nobre, from the Institute for Advanced Studies at USP (University of São Paulo).
More than 40 billion tons of COtwo are released annually into the global atmosphere. Most of it comes from burning fossil fuels. Of this total, forests remove about 13 billion tons (32%), and the oceans, 9.2 billion (23%), according to Nobre.
The scientist says that the mathematical models for the analysis of climate change until then, they did not take into account the growth limitation of the forest’s absorption capacity imposed by phosphorus. “This research is showing that it is necessary to place this [fator] limiter”.
“The Amazon took out more than two billion tons of atmospheric carbon dioxide in a year, but it is losing this capacity due to the increase in deforestation and degradation”, Nobre reiterated.
About 18% of the forest is deforested and 17% degraded (impoverishment of the forest with removal of trees and fires). Adding the values, more than a third (35%) of the forest is already compromised, regrets the scientist.
The deforested area is being replaced by grasses for livestock and starts to remove only 4% of COtwo that the original forest performed.
Phosphorus in the Amazon originates from rocks that have been weathered by the action of microorganisms, chemical reactions and physical processes over millions of years. The mineral is gradually depleted in the soils, being carried away by rain across the Amazonian plains, or it becomes unavailable to plants by chemically reacting with iron and aluminum.
As it is located in a humid tropical region, the Amazon has intense biological activity that accelerates soil and rock wear and reduces the availability of nutrients, especially phosphorus. Soil acidity (pH below 5.5) contributes to the picture.
The Sahara desert also provides phosphorus to the Amazon, as detected by NASA satellites. The mineral travels in the sand raised by the winds that blow from North Africa, cross the Atlantic and land in the Amazon, fertilizing it.
The process has been going on for thousands of years and scientists estimate that the 22,000 tonnes of phosphorus that arrive annually are comparable to the hydrological losses of the mineral in the basin, suggesting an important role for African dust in preventing further depletion of phosphorus. The Sahara was a green region in the past, turned into a desert, with parts rich in nutrients.
Phosphorus acts in photosynthesis, cell division, transport of substances and in the genetic structure of plants. Essential for the roots and maturation of plants, its lack reduces height and delays the emergence of leaves, according to agronomist Edilson Carvalho Brasil, a soil specialist at Embrapa (Brazilian Agricultural Research Corporation).
To conclude that phosphorus is the main nutrient limiting forest growth, Inpa scientists fertilized plots of land with different nutrients, in an area of ​​80 thousand square meters of untouched forest, 80 km north of Manaus. The region has a low concentration of phosphorus (87.5 mg kg−1), characteristic of more than half of the Amazonian soils.
After measuring almost 5,000 trees, during two years of experience, a 29% increase in the growth of fine roots and a 19% increase in leaves was observed in the treetops of the land that received phosphorus in relation to that observed in the trees of the land that did not receive the product.
Having more roots makes plants able to get more water and nutrients. In a drought situation, for example, they are less vulnerable. The growth of leaves, in turn, enables them to obtain more sunlight for photosynthesis and thus incorporate more COtwo atmospheric.
Hellen said that experience has shown that the “Amazon rainforest is currently growing under intense stress [devido à carência de fósforo]”.
The experiment is part of the Amazon Fertilization Experiment (AFEX), made up of researchers from Brazil, Panama, Australia and the United Kingdom, funded by the Natural Environment Research Council (NERC, United Kingdom).
The researcher says that the project’s intention is not to prepare a possible fertilization of extensive Amazonian areas, but to understand the behavior of the forest in the face of natural phosphorus deficiency and which strategies it uses to adapt, such as associations with bacteria, in the roots, capable of efficiently extract phosphorus from dead leaves on the ground.
I have over 8 years of experience working in the news industry. I have worked as a reporter, editor, and now managing editor at 247 News Agency. I am responsible for the day-to-day operations of the news website and overseeing all of the content that is published. I also write a column for the website, covering mostly market news.
