Effect of converting native vegetation into sugar cane crop on soil microbial biomass carbon in Alagoas, Brazil

The cultivation of sugarcane is one of the main agricultural activities in Brazil. One of the factors that made the sugarcane sector important for the national economy was the conversion of native vegetation areas into sugarcane planting areas. The adoption of mechanized harvesting has replaced the burnt harvesting system, which can influence the quality of soil organic matter (SOM), especially regarding the carbon of microbial biomass. This study aimed to evaluate the effects of converting native vegetation to a burnt and unburnt sugarcane harvesting system on the carbon of microbial biomass in the Alagoas state. The study was conducted on three farms (F1, F2, and F3), each having one area of burnt sugarcane (BS), two areas with a mechanized harvesting system (unburnt sugarcane) (US), and a native vegetation (NV). Soil samples were stored at 4°C for the analysis of microbial biomass carbon. There were no significant differences in microbial biomass carbon stocks between sugarcane treatments (p>0.05). However, there was a significant decrease (p<0.05) when comparing the sugarcane areas to NV. There were losses of 58%, 59%, and 52% in the unburnt sugarcane systems with 12 and 17 years of sugarcane cultivation, as well as 52% in the burnt sugarcane when compared to NV. In F2, the reductions were more pronounced, with a loss of 83% for unburnt sugarcane with 3 years and 76% and 74% for mechanized sugarcane with 9 years and burnt sugarcane, respectively. On this farm, we observed a significant difference of 36% between the mechanized sugarcane with 3 years and the burnt sugarcane. F3 showed differences of 0.43, 0.42, and 0.50 Mg ha⁻¹ for mechanized harvesting areas with 4 and 8 years and burnt sugarcane, respectively, compared to native vegetation (p<0.05). Although there were no significant differences between the treatments under sugarcane, the unburnt sugarcane systems showed an average increase of 0.07 Mg ha⁻¹ compared to the burnt sugarcane. Generally, the conversion of native vegetation into sugarcane cultivation reduced the carbon stocks of microbial biomass, indicating that land-use change negatively affects soil quality. Therefore, it is crucial to seek alternatives to mitigate carbon losses in these areas.