Proper soil management could make Amazon pasturelands a better methane sink

Grass-covered pasture captures more methane than bare soil, albeit much less than forest-covered soil. Researchers at the University of São Paulo showed that proper pasture management strategies have the potential to reduce the impact of livestock production and mitigate global warming.

By Ricardo Muniz  |  Agência FAPESP – A study published in Science of the Total Environment shows that grass-covered pasture in the Amazon is a far better methane sink than bare soil. 

Methane (CH₄) is one of the most important greenhouse gases, with 21 times more capacity to trap heat in the atmosphere than carbon dioxide (CO₂). Pasture management strategies, therefore, have the potential to mitigate global warming.

“We studied the consequences of deforestation in the Amazon followed by the establishment of pasture, focusing on methane fluxes between soil and atmosphere,” said Leandro Fonseca de Souza, first author of the article and currently a postdoctoral researcher in ecogenomics and environmental microbiology at the University of São Paulo’s Luiz de Queiroz College of Agriculture (ESALQ-USP) in Brazil.

“We found grass-covered pasture to be capable of capturing much less methane than forest-covered soil but much more than degraded areas of pasture with no vegetation,” Souza said.

His PhD research, which is reported in the article, was supported by a scholarship from FAPESP and supervised by Tsai Siu Mui, a professor at the Center for Nuclear Energy in Agriculture (CENA-USP).

One of the reasons for this effect, according to the authors, is that the rhizosphere – the soil layer directly in contact with the roots of the grass – contains about ten times fewer of the methane-producing microorganisms known as methanogenic archaea. Both methane-producing and methane-consuming microorganisms live in the soil. The switch from forest to pasture inhibits the latter and fuels the former. In properly cultivated pasturelands, the roots of the grass reduce the quantity of methane-producing microorganisms.

“Our analysis of the process shows that soil management can mitigate the impact of livestock production on emissions of CH₄,” Souza said.

The researchers also discovered that the typically acid pH of forest-covered soil is important to its ability to act as a methane sink. “When we limed the soil [using a traditional agricultural technique that entails adding mineral calcium and/or magnesium to the soil to lower acidity and increase crop yields], its capacity to capture methane from the atmosphere diminished. In some cases, it even became a methane emitter,” Souza said. 

Research has shown that 60%-80% of the Amazon’s deforested areas are used as pasture and 40%-60% are degraded to some extent.

Greenhouse and field experiments

In the study led by Souza, methane fluxes from forest and pastureland soil were analyzed in moisture-controlled greenhouse experiments. The soil samples were collected from western and eastern Amazonia. Some were used to grow typical grazing signal grass (Urochloa brizantha cv. Marandu), and some were limed. 

The researchers also assessed changes in the microbial communities of both bare and pasture soils, using high-throughput gene sequencing and measurement of methane-cycling microbiota by marker genes associated with methane production (mcrA) or oxidation (pmoA). Concurrent field studies confirmed the greenhouse data.

The presence of grass cover increased methane uptake by up to 35% in pasture soil and reduced the abundance of the methane-producing community. Liming to raise pH compromised the capacity of forest and pasture soil to act as a methane sink, instead converting formerly methane-consuming forest soil into a methane source in 40-80 days.

Deforestation rates have increased significantly in the Amazon since measurements began in 1988. The worst period was between 1995 (29,059 sq. km. per year) and 2004 (27,777). The rate fell 84% in the eight years after 2004, bottoming out in 2012 (4,571) and rising sharply until 2019 (more than 10,000). 

Some 37 million hectares of pastures in the Amazon, or about 70% of the total pastureland in the region, have resulted from deforestation in the past 35 years.

The article “Maintaining grass coverage increases methane uptake in Amazonian pastures, with a reduction of methanogenic archaea in the rhizosphere” is at: www.sciencedirect.com/science/article/abs/pii/S0048969722033228.