Sugarcane harvesting systems in Brazil: Effects on soil carbon stocks and ethanol payback time

Abstract

Replacing the burnt sugarcane harvesting system with unburnt sugarcane is important for the sustainability of the sugarcane sector in Brazil. Thus, quantifying the impact of the change in the sugarcane harvesting system on soil organic carbon (SOC) stock in Brazil is necessary, as it will allow the refinement of data on SOC, which is essential for the preparation of the national inventory of emissions and removal of greenhouse gases (GHGs), in addition to contributing to national public policies. We used data from both soil sampling and literature review in this study, resulting in 210 pairs of comparisons: 84 for the conversion from burnt sugarcane to unburnt sugarcane; 95 for the conversion from native vegetation to unburnt sugarcane; and 31 for the conversion from native vegetation to burnt sugarcane (NV–burnt), which we analysed using a mixed linear model. In Brazil and the South-Centre region, burnt–unburnt conversion results in a progressive increase in SOC stocks over time, in surface and subsurface layers. Over 20 years, the NV–burnt conversion showed SOC losses between 15% and 32%, and the NV–unburnt conversion showed losses between 27% and 35%. SOC change rates showed gains of 0.32 and 0.59 Mg C ha⁻¹ year⁻¹ for burnt–unburnt, and losses ranging from 0.82 to 1.06 Mg C ha⁻¹ year⁻¹ for conversions from native vegetation. The time required to offset the negative carbon balance of the NV–unburnt conversion is 6.4 and 8.2 years, being shorter than the payback time of the NV–burnt conversion, which is 9.9 and 9.2 years, in the 0–30 and 0–50 cm layers, respectively.