A study published in the journal Science of The Total Environment demonstrated that pastures with grasses in the Amazon increase the methane sequestration capacity when compared to pasture areas with bare soil. CH4 (methane) is one of the most important greenhouse gases—up to 86 times more capable of trapping heat in the atmosphere than CO2 (Carbon Dioxide). Thus, pasture management strategies have the potential to mitigate climate warming.
“We studied the consequences of deforestation in the Amazon, followed by the establishment of pastures, with a focus on methane gas flows between soil and atmosphere”, explains Leandro Fonseca de Souza, who is currently doing his postdoctoral work in the areas of ecogenomics and environmental microbiology at the Department of Genetics at Esalq-USP (Luiz de Queiroz College of Agriculture) at the University of São Paulo.
“We realized that grassy pastures are capable of sequestering more methane—albeit much less than soil under forest—than uncovered pasture soils, without vegetation and in degradation”, says Souza, whose doctoral research, detailed in the article, was supported by a grant from FAPESP and supervised by professor Tsai Siu Mui, at USP’s Cena (Center for Nuclear Energy in Agriculture).
One of the reasons that the authors identified for this effect is that in the soil layer directly in contact with the roots (rhizosphere) of the grasses there are fewer microorganisms that produce methane, the methanogenic archaea. This reduction is of the order of ten times.
Both methane producing and consuming microorganisms inhabit soils. The shift from forest to pasture affects consumers and boosts producers. In a well-tended pasture, grass roots reduce the amount of microorganisms that produce methane. “Understanding this process indicates that soil management has the potential to reduce the impacts of livestock on CH4 emissions”, says Souza.
The team also found that the typically acidic pH of the soil under the forest is important for it to act as a methane sink.
“So much so that when we limed [técnica empregada no preparo do solo agrícola em que materiais de calcário são adicionados para neutralizar acidez, aumentando a produtividade]there was a reduction in their ability to sequester atmospheric methane —in some cases they even started to emit methane”, says Souza. According to the biologist, research indicates that 60% to 80% of the deforested areas in the Amazon are used as pasture. 40% and 60% of them are degraded to some degree.
Greenhouse and field studies
In the study led by Souza, methane fluxes from forest and pasture soils were evaluated in experiments in a greenhouse with controlled humidity with or without grass cover (Urochloa brizantha cv., popularly called marandu grass) and with or without liming.
Changes in the structure of the microbial community in these soils were also evaluated, with the quantification of the methane cycling microbiota by their respective marker genes related to methane generation (mcrA) or oxidation (pmoA).
The experiments used soils from eastern and western Amazonia and a simultaneous field study demonstrated the same trend. The presence of a grass cover not only increased methane uptake by up to 35% in pasture soils, but also reduced the abundance of the methanogenic community.
Soil liming to increase pH compromised the ability of forest and grassland soils to be methane sinks and instead converted formerly CH4-consuming forest soils into sources of the greenhouse gas in just 40 to 80 days.
Deforestation rates in the Amazon have varied greatly since measurement began in 1988. The highest rates occurred between 1995 (29,059 km²/year) and 2004 (27,777 km²/year). In the period between 2004 and 2012, they fell by 84%. After reaching the lowest ever recorded in 2012 (4,571 km²), deforestation levels increased again and exceeded 10,000 km² in 2019.
Currently, 70% (37 million hectares) of the total pastures in the Amazon can be attributed to the deforestation process that took place in the last 35 years.
The article Maintaining grass coverage increases methane uptake in Amazonian pastures, with a reduction of methanogenic archaea in the rhizosphere can be read here.
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