Amazon Deforestation Stimulates Soil Microbes to Produce More Greenhouse Gases

UMass Amherst microbiologist and international team track down major methane source
Microbiologist Maria Kroeger makes field repairs to a tablet computer used in methane measurements.
Microbiologist Maria Kroeger makes field repairs to a tablet computer used in methane measurements.
A single Brazilian nut tree, Bertholletia excels, is left after the Amazon rainforest has been removed and converted into a pasture.
A single Brazilian nut tree, Bertholletia excels, is left after the Amazon rainforest has been removed and converted into a pasture.

AMHERST, Mass. – A team of researchers led by microbiologists Klaus Nüsslein and Marie Kroeger at the University of Massachusetts Amherst in a new report say they have pinpointed the source – methane-producing microorganisms – for large amounts of methane emitted from rainforest-turned-cattle-pasture in Brazil’s Amazon region.

Deforested areas there are most often converted to cattle grazing, the authors point out, which for decades were known to be large methane sources, but until now the reason was not clear.

Writing in the International Society of Microbial Ecology Journal, Kroeger, Nüsslein and colleagues state that “a significant increase in the abundance and activity of methanogens in pasture soils could drive increased soil methane emissions. Furthermore, we found that secondary rainforests had decreased methanogenic activity similar to primary rainforests, and thus a potential to recover as methane sinks, making it conceivable for forest restoration to offset greenhouse gas emissions in the tropics. These findings are critical for informing land management practices and global tropical rainforest conservation.”

Kroeger, now at Los Alamos National Laboratory, points out that this study is unique for its large-scale sampling across different locations, including at some of the most active deforestation frontiers in northeast and southwest Amazonia in the states of Pará in and around Tapajós National Forest and in Rondônia.

Nüsslein adds, “This is currently the most comprehensive study of methane-cycling microbial systems in systems of tropical land use change and yields new insights to the microorganisms responsible which drive methane emissions.” Methane gas packs an estimated 20 times the climate-warming potency of carbon dioxide, experts say.

For this work, American and Brazilian researchers collaborated to collect intact soil cores from each land use type – rainforest, cattle pasture and secondary rainforest, that is abandoned pasture regrown to rainforest. Nüsslein and Kroeger then used a lab technique called stable isotope probing, as well as metagenomic DNA sequencing, to identify microorganisms actively involved in producing and consuming methane.

They report, “Regardless of location, the abundance of active methanogenesis genes dominated in pasture soils compared to other land-use types” and further, “we were able to ascertain that methanogen abundance and activity is significantly higher in pastures compared to both primary and secondary forests.”

The authors urge that future studies should focus on identifying specific environmental factors such as pH, vegetation, compaction, nutrient inputs from livestock and trace element availability, for example, that may contribute to increased methanogenesis in pasture soils “so that land management can better mitigate methane emissions.”

“With the currently accelerating expansion of land-use change in Amazonia, understanding which players might assist mitigation of concomitant greenhouse gas production is increasingly important for all agricultural management,” they add.

The research was supported by the National Science Foundation and Fundação de Amparo à Pesquisa do Estado de São Paulo, Brazil. In addition to Nüsslein and Kroeger, investigators on the team came from the University of Arizona and its Biosphere 2; the University of Oregon, Eugene; the University of California, Berkeley and Davis; the Planetary Science Institute, Tucson; the University of São Paulo, Brazil; Lancaster University, U.K. and the University of Oxford, U.K.