Effect of conversion from sugarcane preharvest burning to residues green-trashing on SOC stocks and soil fertility status: Results from different soil conditions in Brazil

Land use and land management have a recognized impact on the cycle of elements such as carbon and nitrogen and are thus expected to play an important role in the mitigation against human-induced environmental changes such as global warming. Brazil is the world's largest producer of sugarcane and has in recent years, experienced a shift from traditional cane preharvest burning (B) to mechanized green-trashing (GT), whose consequences on soil organic carbon and nutrients are still uncertain. This study performed in the São-Paulo state, Southeast Brazil, investigated the impact of shifting from B to GT on soil organic carbon and nitrogen stocks, and chemical elements at three 4-year and one 12-year old trial sites. Soil samples were collected from four sites representing different soil types (clayey to sandy-clay Hapludox; Hapudult and sandy Quartzpsamment). Sampling was up to 0.4m (0–0.05; 0.05–0.1; 0.1–0.2; 0.2–0.4m) depths at six randomly selected pits from six plots (of 0.2ha size each) per treatment arranged in randomized block design. The samples were analysed for carbon and nitrogen content and other soil properties such as texture, bulk density, aggregate stability, pH and chemical concentrations. Overall, conversion from B to GT increased fresh organic matter retained on soil surface by 12.8–13.0Mgha−1y−1, and soil organic carbon and nitrogen stocks in the top 0.4m soil layer by 1.06MgCha−1y−1 and 0.11MgNha−1y−1, respectively. The highest carbon sequestration rate (108‰ y−1) occurred in the most carbon depleted soils, which were sandy soils with lowest proportion of macroaggregates and chemical elements such as calcium, magnesium and potassium. All the chemical elements also showed a general increase in concentration, especially within the topmost soil layer, because increased soil organic matter had greater ability to tighten them together. Considering the average sequestration rate of 1.06 ton ha−1y−1, the resulting soil carbon sequestration rate for Brazil where 8.8 million hectares are under green-trashing of sugarcane residues would be 9.3milliontonCy−1. However, intermittent tillage every 4–5years for replanting poses limitations to carbon sequestration because a 12year experiment which was sampled 1year after replanting did not show any carbon sequestration, suggesting that all carbon stored between planting and replanting was lost at tillage. There is need for further research to quantify the dynamics of carbon stocks during and immediately a