Impacts of changes in land use on bodies of water

The expansion of sugarcane and soybean cultivation and the replacement of forests by pastureland affect the chemical composition and biodiversity of rivers and lakes, notes USP professor

By Elton Alisson, in Tokyo

Agência FAPESP/– The changes in land use that are currently taking place in most of the world, with the greatest intensity in the tropical regions – caused by population increases and the demand for food and energy –, have had numerous impacts on the chemical composition and biodiversity of bodies of water.

In Brazil, groups of researchers in partnership with colleagues from other countries have studied some of the alterations in rivers and lakes due to the expansion of sugarcane and soybean cultivation and the replacement of forests by pastureland.

“We’ve increasingly attempted to conduct interdisciplinary research studies in an effort to unify the understanding of terrestrial and aquatic systems into a single approach, and include the effects of man, which is very important in land use changes,” said Maria Victoria Ramos Ballester, professor from the Center for Nuclear Energy in Agriculture (Cena) at the University of São Paulo (USP), to Agência FAPESP.

The researcher, who is a member of the FAPESP Research Program on Global Climate Change, gave a lecture at the Japan-Brazil Symposium on Research Collaboration. Organized by FAPESP and the Japan Society for the Promotion of Science (JSPS), the event was held March 15-16 in Tokyo.

According to Ballester, the cultivation of sugarcane may have several environmental impacts. One of them is caused by the use of nitrogen from the vinasse (a by-product of alcohol refining) to irrigate crops, a substance which in excess in river and lake water can lead to the growth of algae.

Another serious problem in relation to the cultivation of agricultural crops, according to the researchers, concerns the issue of water. “In order to produce one liter of fuel, it takes 1.4 thousand liters of water. Hence, ethanol production is very expensive in terms of water,” she explained.

Ballester went on to say that the soot produced by the burning of sugarcane during the harvest has a different type of carbon that may be used to a greater or lesser extent by organisms present in a river, for example.

When deposited in the soil or in an aquatic ecosystem, the material modifies the carbon cycle of the medium. “The sugarcane soot acidifies the soil and water and this has consequences for the ecosystem,” explained Ballester.

In cases where forests are being transformed into pastureland or used for soybean cultivation, like that which is occurring in the western part of Brazil, the two phenomena are having an impact. The difference between them, however, lies in the level of impact.

Through a studyconducted within the scope of a Thematic Project supported by FAPESP, Ballester’s team analyzed nitrogen transfers and its effect on the biodiversity of fish in two interconnected river basins in the state of Rondônia by measuring 800 meters of water that had the same physical conditions. One of the stretches of river was bordered by pastureland while the other was bordered by a riparian area.

The researchers observed that the river whose vegetation coverage had been modified presented only one species of fish while the water that flowed through riparian areas had 35 species.

“When vegetation is removed from the edge of the river, more light and materials enter the body of water causing it to have less oxygen and modifying the local conditions. This affects the biological diversity of the ecosystem,” Ballester explained.

Another issue observed in studies conducted in the state of Mato Grosso is that the riparian areas preserved by producers in the areas where oleaginous crops were grown in order to obtain socioenvironmental certification for product export is very different from other riparian areas found in the same region where the crops were not grown. “The vegetation is not the same,” said Ballester.

Besides this, according to the researcher, soybeans requires 32% more phosphorous than other crops and, like sugarcane, remove a lot of water from the soil.

“The major international concern with the cultivation of soybeans, sugarcane and corn in order to produce agroenergy is that the areas where food is usually grown will be overtaken by areas used for the production of energy, but little attention has been given to the issue of water,” said Ballester.

“Most of the areas used in the production of agroenergy do not have enough water to maintain high crop productivity without resorting to irrigation. This represents another serious problem that will change the water cycle,” she pointed out.