Implications of Sugarcane Straw Removal for Soil Greenhouse Gas Emissions in São Paulo State, Brazil

The use of sugarcane straw is a promising option to enhance bioenergy production. But the implications of straw removal for soil greenhouse gas (GHG) emission are not yet fully understood due to scarcity of studies under Brazilian conditions. Four field experiments were designed to assess soil N2O and CH4 emissions derived from nitrogen (N) fertilization under scenarios of sugarcane straw removal in São Paulo state, Brazil. Our focus was also to derive the direct N2O emission factor (EF) that represents the regional conditions of sugarcane production, taking into account the data obtained from this study and those from the literature. In each field study, four straw removal rates (no removal (NR); low removal (LR)—removal of 5 Mg ha−1; high removal (HR)—removal of 10 Mg ha−1; and total removal (TR)—removal of 15 Mg ha−1) were arranged in a randomized block design. This study shows a clear evidence that CH4 fluxes are very low for all assessed sites regardless of straw removal rates, indicating a predominance of CH4 consumption by the soil. Cumulative N2O emissions ranged from 0.20 to 4.09 kg ha−1 year−1 and were significantly affected by straw removal in two sites, indicating that straw removal reduces N2O emissions. The average direct N2O EFs obtained from this study and from the literature were 0.28, 0.44, 0.70, and 0.56% for TR, HR, LR, and NR treatments, respectively, which are consistently lower than the EF of 1% suggested by the IPCC. Based on our regional-specific EF (Tier 2), the direct N2O emissions derived from N fertilization under scenarios of straw removal showed a reduction of at least 50% in relation to IPCC approach. Our findings are a step forward in providing regional-specific data to reduce the high level of uncertainty concerning N2O emission assessments of sugarcane ethanol in Brazil, but further studies are needed to evaluate how straw removal for bioenergy production and the associated changes in soil organic carbon stocks affect the GHG balance of sugarcane.