Soil C and N Stocks Under Burned and Unburned Sugarcane Harvesting Systems In Brazil.

The demand for biofuels that contributes for the reduction of greenhouse gases emissions associated to agricultural sustainability has contributed to the wide expansion of bioethanol industry in Brazil. The unburned sugarcane harvesting is considered a more environmental friendly practice compared to the burned system not only because it avoids greenhouse gas emissions from the biomass burning but also enhances the amount of organic residues left on the soil after harvesting. In this context, it is possible that residue inputs left on the soil surface lead to an increase in soil C and N stocks and enhance soil fertility. The aim of this study was to quantify and compare total soil C and N stocks and soil microbial biomass, as well as the potential changes in soil chemical attributes due to burned and unburned sugarcane residues management. We selected representative sugarcane areas in Brazil, cultivated for 6 and 12 years, both with burned and unburned management practices, which constitute the evaluated chronosequence. As a reference for the management systems it was selected a native vegetation area, neighboring the sugarcane areas. Soil samples were analyzed for bulk density, soil texture, total soil C and N stocks, soil microbial biomass C and N and soil chemical characterization. The results showed an increase in soil C stocks in the unburned system in all depths when compared to the burned management system. The increase between unburned areas with 6 and 12 years was 0.47 and 0.07 Mg ha-1 y-1 of C and N, respectively. There were no significant differences on chemical properties under burned or unburned trash management, except for P and K in soil surface that presented increased contents under unburned sugarcane harvesting system. For the microbial biomass analysis it was also observed an increase in C and N contents in the surface layer of the unburned management areas, mainly when during the rainy season. Keywords: soil carbon stock; soil nitrogen stock; Microbial biomass; Unburned sugarcane management; Sugarcane; Soil fertility, Brazil