A group of scientists found that much of the nitrogen fertilizer applied to soybean and corn plantations in the Amazon is stored up to six meters below the ground – but until now it is not known – informs the Environmental Research Institute of Amazonia (IPAM). The study, published earlier this month on the Scientific Reports website of the British journal Nature, provides more information on what it means to intensify agriculture in the region.
The researchers, led by KathiJo Jankowski of the United States Geological Survey, analyzed what happens to fertilizer applied to plantations at Tanguro Farm in Mato Grosso, where IPAM coordinates scientific work in ecology.
When up to 80 kilograms of nitrogen per hectare is applied to corn plantations, the plant absorbs the fertilizer almost completely. When the quantity is greater than that – 120, 160 and up to 200 kg of nitrogen per hectare -, productivity does not rise and the surplus is stored.
Intensification of agriculture is a way to avoid deforestation of new areas for production. However, its environmental impacts are still poorly understood, including those resulting from the use of more fertilizers. The study now published brings a little more knowledge about this topic.
Scientists fear that when the soil is saturated, this nitrogen will reach the water bodies of the Amazon, with consequences still unknown to biodiversity and climate.
The concern is right: in the United States, for example, the overuse of fertilizer in the Mississippi River basin has knocked out water quality in that region and created a dead zone where the river empties into the Gulf of Mexico, which hit last year more than 22,000 square kilometers – the concentration of nitrogen and phosphorus is so high and the oxygen rate so low that marine life is unfeasible there.
Jankowski explains that there are differences between the American and the Brazilian situation, such as the form and amount of nitrogen applied in agriculture, as well as the type of soil. However, there are sufficient similarities between the two cases to raise a flag of attention, as well as a determining factor to maintain the constant analysis of the situation: the novelty of a practice being applied at a high pace and extensively by a very large region.
“The conversion (from native vegetation) to soybean and corn fertilized with nitrogen is much more recent in the Amazon and Cerrado than in the Mississippi basin. The cultivated area of soy and corn has grown more than tenfold in Mato Grosso since 2001, “says Jankowski.
The type of soil where the study was made, and which worked as a “sponge” for nitrogen, is the weathered latosol, formed in hot and humid places. It is the most common in the transition between Cerrado and Amazon, where agriculture has intensified in recent years. By 2015, about 2.3 million hectares of soybean in the region (or 68 percent of total cultivated) and 4.9 million in married soybean and corn (80 percent of the total) are in this type of soil.
Researcher Christopher Neill of the Woods Hole Research Center, who also participated in the study, points out that most of the Amazonian monoculture has expanded on a type of soil that has hitherto prevented excess nitrogen from moving into rivers and streams.
“This cropping system is less than 20 years old in the region and we do not know how much soil protection is available, how long this protection lasts or what happens if the accumulated nitrogen content is greater than the retention capacity,” says the scientist. “These results suggest that it is possible to use some degree of fertilizer in the Amazon, but these are the critical questions that will determine the environmental sustainability of a more intensive model of agriculture in the region.”
In case the amount of nitrogen exceeds the retention capacity of the soil, the maintenance of riparian forests is pointed by the scientists as fundamental to reduce the risk of contamination of rivers and streams. The vegetation, in this case, can serve as a filter.
This text was translated by machine from Brazilian Portuguese.