Changes in carbon and nitrogen cycles concern researchers

The process of change has intensified with the use of nitrogen fertilizers and the burning of fossil fuels.

By Karina Toledo

Agência FAPESP – To meet the growing demand for food and energy, humans have altered the cycles of two important nutrients for life on Earth: nitrogen and carbon. Among the undesirable effects of this change are acid rain, increased concentrations of greenhouse gases in the atmosphere and the resulting increase in temperatures around the world.

This topic was a highlight of the recent meeting of the 2014 Biota-FAPESP Education Conference Cycle held on June 25 in São Paulo.

As explained in a lecture from University of Brasília (UnB) professor Gabriela Bielefeld Nardoto, terrestrial nutrients are stored in four large “systems” on the planet: the atmosphere, lithosphere (the outermost crust of the Earth), biosphere and hydrosphere. In the case of nitrogen, a molecule that contributes to the composition of proteins and nucleic acids – both essential to humans – most has been stored in the atmosphere for millions of years as inert N2 molecules.

“Nearly 78% of the air we breathe is composed of N2. For nitrogen to enter the ecosystem and the food chain, it needs to be converted into ammonium (NH4) or nitrate (NO3), and a very small group of nitrogen-fixing bacteria is responsible for this conversion. In these forms, nitrogen is then able to be used by plants, the primary producers of food, and other terrestrial system microorganisms, or it enters the aquatic system,” Nardoto explained.

The return of nitrogen to the atmosphere as N2 occurs due to the action of other denitrifying bacteria. However, this natural cycle began to be altered by humans 10,000 years ago with the advent of agriculture; leguminous plants together with fixing bacteria are able to fix large quantities of nitrogen in the terrestrial system.

The farming of soybeans, for example, fixes 70 to 250 kilograms of nitrogen per hectare per year (kg/ha) in the soil, explained Nardoto. By contrast, one hectare of Amazon rainforest fixes only 3 to 7 kilograms of nitrogen per year.

This process has intensified in the last 150 years with the increase in agricultural productivity, the use of nitrogen fertilizers and the burning of fossil fuels to generate energy.

A small portion of the anthropogenically fixed nitrogen is converted into protein along the food chain. However, large quantities are lost and returned to the atmosphere, not as N2 but as nitric oxide (NO), which reacts with water vapor to give rise to acid rain, or as nitrous oxide (N2O), one of the greenhouse gases.

“Nitrogen could be carried into the aquatic medium as nitrate, causing the eutrophication of this environment; in other words, the growth of algae due to excess nutrients, thus reducing the oxygen available for other organisms,” Nardoto explained.

During her presentation, the UnB professor commented on an article published in the journal Nature in 2009 written by Johan Rockström (University of Stockholm, Sweden) and colleagues that proposed the existence of nine “planet limits” that humans have to respect so as not to destabilize essential terrestrial systems and avoid sudden and catastrophic climate changes.

Scientists say that three limits have already been surpassed: global warming, species extinction and alterations in the nitrogen cycle.

Nardoto went on to say that the nitrogen cycle is closely related to the carbon cycle. “Carbon and nitrogen are both needed for the primary production of food by plants. Carbon is taken in as carbon dioxide (CO2) during photosynthesis, but this process requires an enzyme that is composed of nitrogen. That is why nitrogen fertilizers are used to increase agricultural yield,” Nardoto explained.

Carbon sequestration

The impacts of anthropogenic changes to carbon stocks were the topic of the lecture by researcher Simone Aparecida Vieira at the Center for Environmental Studies and Research (Nepam) of the University of Campinas (Unicamp), also held during the Biota-FAPESP educational meeting.

“Today, there are twice as many carbon dioxide (CO2) molecules due to our activities as there were when Charles Darwin first came to Brazil (19th century),” Vieira said.

According to the researcher, this is because human activities since the Industrial Revolution have released large quantities of carbon that had been stored in the lithosphere as coal, petroleum and natural gas into the atmosphere and the terrestrial system, mainly in forests.

“When we refer to the carbon cycle on Earth, forests provide two important ecosystem services: carbon sequestration from the atmosphere, which occurs during photosynthesis, and the capture and storage of this nutrient,” said Vieira.

“The system’s capacity to store carbon varies according to the area of forest and the various forest ecosystems in decline due to climate conditions, type of soil, species found in the location and disturbances. Species experiencing rapid growth can sequester carbon more rapidly than those that are growing slowly. However, the carbon that enters the system rapidly is also able to leave rapidly through respiration or the process of leaf and branch decomposition,” Vieira explained.

According to the Unicamp professor, the Central Amazon regions store nearly 260 tons per hectare of dry biomass in the vegetation – allowing for nearly 180 tons of carbon stock per hectare.

In the areas of the arc of deforestation, such as Santarém (PA) or the state of Acre, where there is a clear dry season, the stock is similar to that found in the Atlantic Forest: approximately 250 tons per hectare of dry biomass, with between 125 and 140 tons of carbon stock per hectare.

The quantification of Atlantic Forest carbon stocks was carried out during the thematic project entitled, “Floristic composition, structure and functioning of the Dense Rainforest nuclei of Picinguaba and Santa Virgínia Park,” led by Unicamp professor Carlos Alfredo Joly, who also coordinates the Biota-FAPESP program.

“We used 14 permanent one-hectare parcels established at an altitudinal gradient of Atlantic Forest between the cities of Ubatuba and São Luiz do Paraitinga, in which we identified and measured the diameter and height of all individual trees with a diameter exceeding 4.8 cm. Based on this information and the density of the timber, obtained from information about the species, we were able to estimate the forest biomass of the area. We were also able to monitor factors such as the rate of growth and mortality of the vegetation,” Vieira explained. “Thus, it was possible to compare the variation over the years and assess carbon flow.”

The study also showed that although the quantity of carbon stocks of trees is larger in the Central Amazon region, the Atlantic Forest has a greater quantity of carbon stock in the soil. “It is possible that the processes of decomposition in the Atlantic Forest are slower because of the lower temperature and shallow soils. In addition, there is less nutrient loss through percolation,” she explained.

“Tropical forests store large quantities of carbon and other nutrients, and when they are deforested, all this material is lost. We also lose the service of carbon sequestration from the atmosphere because there is no photosynthesis,” Vieira said.

The third and final lecture of the meeting was presented by researcher Plinio Barbosa de Camargo from the Center for Nuclear Energy in Agriculture (Cena) at the University of São Paulo (USP).

Camargo discussed the cycling of carbon, nitrogen and other nutrients in aquatic systems and discussed projects that are attempting to identify parameters to assess water quality and measure the impact of the reforestation of water sources.

For the purpose of contributing to educational improvement in the teaching of science in elementary and high school, the conference cycle was organized by the Biota Program in 2014 with a focus on ecosystem services. Topics addressed included pollination, the protection of hydric resources and climate change. The lectures are available on the FAPESP website.

For more information, visit: www.fapesp.br/8441 (in Portuguese).