Reduced use of burning in cane fields produces results

A change in the way sugarcane is harvested has led to a reduction in greenhouse gas emissions in São Paulo.

By Elton Alisson

Agência FAPESP – As a result of efforts over the last six years to replace manual sugarcane harvesting with mechanized collection, through the Agroenvironmental Protocol for the Sugarenergy Sector, São Paulo State has seen a greenhouse gas emissions in the agricultural sector decrease at an accelerating rate.

If the sector maintains this pace of conversion to mechanization in the next few years, depending on the type of raw sugarcane handling adopted, it could contribute to more than half of the target reduction in greenhouse gas emissions in the state.

This estimate is a product of studies conducted by researchers in the School of Agrarian and Veterinary Sciences at the Universidade Estadual Paulista (UNESP/Jaboticabal), under the auspices of the Thematic Project, funded through FAPESP’s Program for Research on Global Climate Change (PFPMCG). The results were presented on September 12 during the 1st National Conference on Global Climate Change, organized by FAPESP in partnership with the Brazilian Research Network on Global Climate Change (Rede Clima) and the National Institute of Science and Technology on Climate Change (INCT-MC).

“Greenhouse gas emissions per unit of area planted with sugarcane in São Paulo are decreasing because of the switch from sugarcane burning to raw sugarcane collection,” said Newton la Scala Júnior, UNESP professor and project coordinator.

According to the data presented by the researcher and obtained by the Canasat Project of the National Institute for Space Research (INPE), of the 4,658,316 hectares of sugarcane harvested in the 2012 crop, 1,277,003 hectares (27.4%) were harvested through burning, and 3,381,313 ha (72.60%) where harvested mechanically. In contrast, in 2006, just as the Agroenvironmental Protocol was being implemented, some 65.76% of the area planted with sugarcane in São Paulo was harvested through burning and 34.24% was harvested by mechanized harvest.

In estimating the greenhouse gas emissions in sugarcane-growing areas in São Paulo during the period 2006–2012, using the methodology recommended by the Intergovernmental Panel on Climate Change (IPCC), the researchers found that the emissions per unit of planted area fell despite the increase in the number of hectares of sugarcane harvested and despite the fact that mechanized harvesting emits greenhouse gases due to the use of synthetic fertilizers and diesel fuel.

In 2006, the agricultural sector in São Paulo emitted 2,300 to 2,400 kilograms of CO2-equivalent per hectare (greenhouse gas emissions multiplied by their global heating potential). By 2012, this number had dropped to 2,100 kilograms per hectare.

“This reduction per unit of area has occurred precisely because of the conversion of burned sugarcane areas to raw sugarcane harvesting. The tendency is for greenhouse gas emissions from sugarcane areas to fall even more,” said Scala.

Mitigation targets

The researchers conducted another study, to be published in an upcoming edition of the magazine Biomass & Bioenergy, in which they estimate the potential reductions in greenhouse gas emissions from sugarcane collected in São Paulo, on the basis of three conversion scenarios for mechanized harvesting.

In the first scenario, they calculated the possible reduction in greenhouse gases between 2011 and 2014 if raw harvesting replaced burning for 568,840 hectares of sugarcane, in compliance with the Agroenvironmental Protocol for the Sugarenergy Sector, targeted at ending all burning in the state between 2014 and 2017.

In the second scenario, greenhouse gas mitigation was projected assuming that raw harvesting was implemented for 167,042 hectares by 2021, in compliance with the São Paulo State Climate Change Plan (PEMC), which established goals for reducing greenhouse gas emissions in the state by 20% across all sectors by 2020, compared to 2005 levels. In the third scenario, the researchers estimated the reduction in greenhouse gas emissions through 2029 based on the average rate of conversion to raw harvesting observed today—92,294 hectares per year, on average.

In the three scenarios, they considered the possibilities of conversion through conventional handling, reduced handling, and a combination of reduced preparation of the soil with crop rotation and synthetic fertilizers not being applied over a harvest cycle of 5 to 6 years.

The results of the study revealed that if the current pace of conversion continues in the three scenarios and if the conversion occurs based on reduced soil handling, it will be possible to meet the targets set under both the Agroenvironmental Protocol for the Sugarenergy Sector and the PEMC. If the switch occurs through reduced soil preparation combined with crop rotation, it will be possible to not only meet the two targets but also prevent 70 megatons of CO2 equivalence in emissions from entering the atmosphere.

“This is a significant number and is the equivalent of more than half of the reduction targets for greenhouse gas emissions that São Paulo will have to meet in the coming years,” said Scala.

“With the change in São Paulo’s sugarcane production scenario alone, it will be possible to reach half of the greenhouse reduction target,” said Scala.

The importance of straw

According to the researcher, 60% of the emissions reduction that the state would achieve by reduced soil preparation combined with crop rotation corresponds to the accumulation of carbon in the soil.

According to Scala, sugarcane absorbs a large quantity of CO2 from the atmosphere during photosynthesis and incorporates some of the absorbed CO2 into its phytomass (roots and straw), which returns it to the atmosphere. Furthermore, the soil in which the crop is planted absorbs CO2 and emits some of it to the atmosphere.

By converting to raw sugarcane harvesting, the stock of carbon in the soil increases because large quantities of plant residue (such as straw) remain on the surface and are incorporated into the soil in the form of carbon.

“If the sugarcane residue stays on the soil surface after harvesting, it can offset almost all of the greenhouse gas emissions associated with using synthetic fertilizer or the diesel used in harvesters, for example,” affirmed Scala.

In field studies and laboratory tests with soil samples conducted in the last 15 years, the researchers have shown that the average carbon emission values from the soil in areas where harvesting is mechanized were significantly lower than those where burning and straw have been eliminated.

In a 50-day experiment conducted on a plantation that was split into three areas—one 50% covered by straw, the second covered 100% by residue and the third without straw—the researchers observed that the areas covered with straw emitted 400 kilograms less carbon (corresponding to almost 1,500 kilograms less carbon gas) than areas where the straw was removed.