Sugarcane straw management for bioenergy: effects of global warming on greenhouse gas emissions and soil carbon storage

Global warming can intensify the soil organic matter (SOM) turnover, damaging soil health. Crop residues left on the soil are important to maintain a positive SOM budget and nutrient cycling. But, sugarcane (Saccharum officinarum) straw has been removed from the field for bioenergy purposes. We hypothesize that global warming, together with straw removal, will negatively impact Brazil's ethanol carbon footprint. Thus, we conducted an experiment under controlled conditions to evaluate the impacts of warming and straw removal on greenhouse gas (GHG) emissions, soil carbon and nitrogen storage, and nutrient cycling. Two soils (Rhodic Acrisol and Eutric Nitisol) were tested with three rates of sugarcane straw removal (no removal (NR):equivalent to 12 Mg ha$^{-1}$; medium removal (MR):6 Mg ha$^{-1}$; and total removal (TR):bare soil) and submitted to two temperatures (24$^{\circ}$C and 30$^{\circ}$C) and soil moistures (30\% and 50\%). Straw decomposition was stimulated by lower rates of straw removal, resulting in increases on carbon dioxide (CO$_{2}$) emissions between 5 to 14 times, and N$_{2}$O between 25 and 40\%. There were no significant methane (CH$_{4}$) fluxes. Soil carbon and nitrogen did not change due to straw removal, yet labile carbon fractions (living and non-living) were highly impacted, causing reductions of 15 to 40\% on the carbon management index (CMI). Furthermore, straw removal reduced nutrient cycling between 10 and 30\%. Overall, in a scenario of warming, our findings point to an intensification of SOM dynamic, resulting in increases of 35\% on the GHG emissions and a CMI reduction by 20\%. In practical terms, at least 6 Mg ha$^{-1}$ of straw should be left in the field, guaranteeing raw-material for bioenergy, without causing major impacts on the GHG emission and soil attributes.