Researchers at the University of Texas at Arlington have found that widespread conversion from rainforest to pastureland in the Amazon may lead to the reduction of this area as a reservoir for greenhouse gas. The amazon rainforest is the largest sink for carbon dioxide, which has been contentiously associated with climate change.
By way of photosynthesis, the Amazon takes in 1.5 billion tons of CO2 from the air every year in a process that requires an input of nitrogen. The nitrogen comes from a process known as nitrogen fixation, whereby microbes pull nitrogen from the air into the soil. Nitrogen fixation is important for plant growth. The researchers took a first-time look at the reaction of free-living nitrogen-fixing organisms known as diazotrophs to the deforestation.
Assistant professor of biology at UT Arlington, Jorge Rodrigues, organized the work for the study, and former postdoctoral fellow Babur S. Mirza is the paper’s lead author.
According to Rodrigues, “This study shows that although the diversity of diazotrophic microorganisms remains the same with the conversion from forest to pasture, the types of species found are different. Our next step is to measure how the rates of biological nitrogen fixation are influenced by community changes. Because the carbon and nitrogen cycles are so strongly linked, our previous results indicated that changes in carbon dioxide sequestration will occur.”
Soil samples were collected from the Fazenda Nova Vida site in Rondonia, Brazil. Rondonia is one of three states that accounts for more than 85 percent of deforestation from the late 90s to 2005. Researchers analyzed soil samples from a primary forest that had become a pasture in 2004 and a secondary forest that came from an abandoned pasture in 1999. They used DNA analysis of the nifH gene which is characteristic of diazotrophs to measure communities in their samples.
The analysis revealed a ten-fold increase in the number of diazoptrophs in the pasture established in 2004 when compared to primary forests. The researchers suggest that pasture ecosystems rely on diazotrophs more so for nitrogen because of cattle grazing which requires constant regrowth of grasses.
Mirza notes, “We observed a complete shift in the diazotrophic microbial community composition in response to the Amazon rain forest conversion to a pasture. These differences can be attributed to the shift in the above ground plant community because we did see partial recovery of diazotroph community composition in the secondary forest, which have more plant species as compared to pasture.” Mirza points out that they are continuing their work with more sophisticated sequencing technologies and in-depth sampling.
The current results show promise for maintaining the carbon cycle in these regions in that pastures that had been abandoned where secondary forest grew, partial restoration of the original diazotrophic organisms was achieved. Rodrigues points out, growth of secondary forest continues for about 50 percent of the abandoned pastures in the Amazon. However, more needs to be done to encourage secondary forests and limit deforestation. He goes on further to say, “There is still time to recover if we act now.”
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