Indiscriminately removing sugarcane waste—the dried leaves that are left over in the field after sugarcane is harvested—can reduce soil carbon stocks and increase greenhouse gas (GHG) emissions. Previous research has already shown that sugarcane waste—which many sugar and ethanol mills are now collecting to produce cellulosic (G2) ethanol and electricity—provides a wide range of ecosystem services, including soil water retention and erosion control. Now, a study by the National Biorenewables Laboratory at the Brazilian Center for Research in Energy and Materials (LNBR-CNPEM) in Campinas has revealed that leaving sugarcane waste in the field plays a key role in maintaining soil carbon stocks.
As sugarcane grows, it captures carbon dioxide (CO2) from the atmosphere, storing it in the leaves, stalk, and roots. After harvest, the leftover leaves gradually transform the stored CO2 into stabilized carbon in the soil. This carbon transfer from the atmosphere to the soil improves the overall emissions balance in sugarcane growing.
“This is the first study to factor in soil carbon stocks when calculating GHG emissions in the life cycle of waste-derived bioenergy,” explains Ricardo Bordonal, a crop scientist and the lead author of a study published in Science of the Total Environment in July. “Using modeling and life-cycle assessments, we found that the environmental benefits of GHG reduction vary depending on the amount of sugarcane waste that is removed,” he adds.
The researchers modeled three scenarios to evaluate carbon balance: 100% waste removal, 50% removal, and 0% removal. “There is no net emissions benefit from removing sugarcane waste to generate bioelectricity” Bordonal explains. “Brazil already has a clean, low-carbon grid, so it’s more beneficial to leave the waste on the field, allowing its carbon to be fixed in the soil,” he says.
However, according to the study—funded by FAPESP and the Sugarcane Renewable Electricity (SUCRE) project run by the United Nations Development Program—removing waste for G2 ethanol production can be beneficial. “By selectively removing 50% of the sugarcane waste for cellulosic ethanol, GHG emissions are reduced, because replacing gasoline with ethanol lowers vehicles’ CO2 emissions, and this offsets the carbon that would have been sequestered into the soil,” explains Bordonal. However, he warns that removing all of the residue causes a larger loss of soil carbon sequestration, which is not worth the trade-off.
“This research delivers a strong message to the sector: there’s no zero-cost option when removing sugarcane waste for G2 ethanol or bioelectricity production,” says Maurício Cherubin, a crop scientist at the Luiz de Queiroz School of Agriculture at the University of São Paulo (ESALQ-USP) who is also the deputy coordinator of the Tropical Agriculture Carbon Study Center (CCarbon-USP), a FAPESP-funded Research, Innovation, and Dissemination Center (RIDC). “Leaving the sugarcane waste in the field can sequester between 400 and 500 kilograms of carbon per hectare per year,” he adds.
Integrated crop-livestock-forestry systems are another strategy for cutting greenhouse gas emissionsGisele Rosso / EMBRAPA
Reducing emissions
The study is part of an industry effort to improve soil management and cut GHG emissions in agriculture, which accounted for 27% of the country’s 2.3 billion metric tons of carbon dioxide equivalent (CO2e) emissions in 2022, or 9.6 metric tons per hectare—CO2e is a global standard that measures the combined impact of all GHGs (like methane and nitrous oxide) in CO2 terms.
“Sustainable farming practices could enable Brazilian agriculture to shift from a net GHG emitter to a key player in the country’s climate change efforts,” says Carlos Eduardo Cerri, a crop scientist at ESALQ and head of CCarbon-USP. “These techniques replace single-crop systems with models that enhance biodiversity. They improve soil health, reduce GHG emissions, and promote carbon sequestration in soil,” he explains. Officially launched in September 2023, the center, based at ESALQ in Piracicaba, involves around 40 researchers and 90 grant beneficiaries.
Crop scientist Guilhermo Congio sees the new carbon research center for tropical agriculture as a potential boon for the country: “Beyond cutting GHG emissions, CCarbon-USP can tackle critical issues like food security, low-carbon economies, and social development,” he notes. Congio works at the Noble Research Institute in the US, where he develops methods to minimize the environmental impacts of beef cattle production. “One of our projects aims to measure soil health in pastures and connect those metrics to remote sensing technology. We’re also studying how ranching practices impact soil health and carbon sequestration in native and managed pastures,” he explains.
Nature-based solutions (NBS)—productive systems that mimic natural processes—enhance sustainability, productivity, and provide environmental services like carbon sequestration, according to CCarbon-USP researchers in a March 2023 study published in Green and Low-Carbon Economy. Examples of NBS include integrated systems that combine farming, livestock, and forestry on the same land (see Pesquisa FAPESP issue n° 314), the use of biofertilizers, and biological pest control.
“We now have the chance to swap an unsustainable production model with one that harnesses plants’ natural capacity to capture atmospheric carbon and soil’s ability to store it,” says Cherubin. He adds that healthy agricultural soils can hold carbon for long periods: “Carbon adds nutrients to the soil, enhancing yield.” Greater plant growth, in turn, captures more CO2, creating richer, more productive land.
Conversely, degraded soil results in lower yield and a diminished capacity to store carbon, much of which escapes back into the atmosphere as CO2. The more degraded the soil, the more it depends on nitrogen fertilizers to stimulate plant growth. These fertilizers are derived from petrochemicals, and their production emits significant amounts of greenhouse gases. Moreover, applying nitrogen fertilizers to crops releases nitrous oxide (N2O), a GHG that is 300 times more potent than CO2.
A field of rattlepods, a fast-growing legume species used to fix nitrogen in the soil, rotated with cottonValdinei Soffiati / EMBRAPA
Microbial biodiversity
Soil health relies on both the soil’s mineral composition and the biodiversity of its plant and microbial communities. Intensive farming systems based on single crops—like grains, sugarcane, or cattle pasture—deplete the soil. A research initiative at CCarbon-USP is examining how shifts in soil microbiome composition and activity could influence carbon sequestration in agricultural systems.
“We will apply well-established microbiological methods, such as gene sequencing, high-throughput gene quantification, metagenomics [the study of microbial communities in a given ecosystem], and bioinformatics,” explains Fernando Dini Andreote, a crop scientist at ESALQ and CCarbon-USP. One of the goals of the initiative is to develop methods for reducing the use of nitrogen fertilizers and pesticides, which would lower GHG emissions.
Brazilian farmers already use a wide range of techniques to preserve biodiversity and promote soil health, such as crop rotation—alternating crops on the same field—and no-till farming, in which crop residue is left on the soil and seeds are sown without disturbing the soil mechanically. According to Cerri, no-till farming can absorb up to half a ton of CO2 per hectare each year.
Converting degraded pastures and conventional farmland into integrated crop-livestock-forestry (ICLF) systems—or crop-livestock systems (ICL) without trees—can also reduce GHG emissions. “Soil in integrated systems can act as a methane [CH4] sink, absorbing between 0.8 and 1 kilogram of methane per hectare each year. Transitioning from monoculture pastures to integrated systems has also cut nitrous oxide emissions by up to 1.63 kilograms per hectare annually,” explains Wanderlei Bieluczyk, a crop scientist at the Center for Nuclear Energy in Agriculture (CENA) at USP and the lead author of a June study reporting on these findings in the Journal of Cleaner Production. Methane, a gas 30 times more damaging than CO2, is produced during the digestion of cattle and is primarily released through belching.
Funded by the Research Center for Greenhouse Gas Innovation (RCGI) and FAPESP, the study found that converting degraded pasture to integrated systems can significantly lower enteric methane emissions from cattle, with reductions of up to 122 grams of methane per kilogram of average daily weight gain. “Essentially, you produce the same amount of meat, but with about a 25% reduction in enteric methane emissions,” estimates Bieluczyk. Brazil boasts the world’s largest commercial cattle herd, at roughly 220 million head.
According to Congio, it is important that carbon balance estimates for Brazilian agriculture use standardized units for GHG fluxes, as recommended in Bieluczyk’s paper. “Many studies rely on GHG conversion factors recommended by the IPCC [Intergovernmental Panel on Climate Change], which are typically based on research from temperate climates and different farming systems than those used in the tropics,” Congio explains.
One of CCarbon-USP’s aims is to identify plant combinations and land-use methods that promote greater carbon retention, enhance soil health, and improve yields. Researchers are particularly focused on cover crops like brachiaria, crotalaria, millet, and sorghum, which are planted between main crop cycles.
“As their name suggests, farmers plant one of these crops to literally cover the soil,” explains Cherubin. “These crops are critical because they help cycle nutrients, fix atmospheric nitrogen, sequester carbon, control nematodes, and shield the soil from rain impact and erosion.” During the previous growing season, several farms in Mato Grosso had to replant three times because of high temperatures and the absence of crop residue covering the soil.
In July, the soil management and health research team at ESALQ, in collaboration with Carbon-USP, released an e-book titled Guia prático de plantas de cobertura: Espécies, manejo e impacto na saúde do solo (Practical guide to cover crops: Species, management, and impact on soil health), designed to help farmers optimize their cropping schedules. “We printed 3,000 copies and handed them out to farmers during an event in Bahia,” says Cherubin. He notes that agriculture is especially vulnerable to climate change. “Currently, agriculture is contributing to the problem by emitting GHGs. Our goal is to demonstrate to farmers that by adopting sustainable crop management practices, they can become part of the solution—by sequestering carbon and translating that into improved yields. Ultimately, the biggest winner will be farmers themselves.”
The story above was published with the title “Carbon as an ally” in issue 343 of September/2024.
Projects
1. Research Center for Carbon in Tropical Agriculture (CCarbon) (n° 21/10573-4); Grant Mechanism Research, Innovation, and Dissemination Centers (RIDCs); Principal Investigator Carlos Eduardo Pellegrino Cerri (USP); Investment R$26,319,364.85.
2. Effect of changing land use and sugarcane management practices on soil C, soil health, and associated ecosystem services: A summary of the evidence (nº 23/11337-8); Postdoctoral Fellowship; Supervisor Mauricio Roberto Cherubin (USP); Beneficiary Carlos Roberto Pinheiro Junior; Investment R$244,824.36.
3. Implications of the expansion and intensification of sugarcane cultivation on soil ecosystem services (n° 18/09845-7); Grant Mechanism Regular Research Grant; Principal Investigator Mauricio Cherubin (USP); Investment R$158,472.12.
4. Soil carbon dynamics and greenhouse gas balance: Implications of the removal of sugarcane leaves for bioenergy production (n° 17/23978-7); Grant Mechanism Regular Research Grant; Principal Investigator Ricardo Bordonal (CNPEM); Investment R$83,507.09.
Scientific articles
BORDONAL, R. O. et al. Carbon savings from sugarcane straw-derived bioenergy: Insights from a life cycle perspective including soil carbon changes. Science of the Total Environment. July 11, 2024.
DENNY, D. M. T. et al. Carbon farming: Nature-based solutions in Brazil. Green and Low-Carbon Economy. Vol. 1, no. 3, pp. 130–7. May 4, 2023.
BIELUCZYK, W. et al. Greenhouse gas fluxes in Brazilian climate-smart agricultural and livestock systems: A systematic and critical overview. Journal of Cleaner Production. Vol. 464, 142782. July 20, 2024.
Book
CHERUBIN, M. Guia prático de plantas de cobertura: espécies, manejo e impacto na saúde do solo. USP. June 2024.
