Greenhouse gas emissions from rice straw burning and straw-mushroom cultivation in a triple rice cropping system in the Mekong Delta

The Mekong Delta produces 21Mt of rough rice (Oryza sativa L.) and an estimated 24Mt of straw (dry weight) annually. Approximately one fourth of the straw is burned on the field, which is a common practice in intensive rice cultivation systems in this region because there is limited time to prepare the field for the next crop. The spread of intensive rice production in the Delta may increase the total biomass of burning crop residues, significantly impacting greenhouse gas (GHG) emissions in Vietnam. In this study, GHG emissions from the major uses of straw (burning and mushroom beds) were monitored in a triple rice cropping system located in the central Mekong Delta. Between September 2011 and November 2012, both wind tunnel and closed chamber methods were used to measure the emissions of major GHGs from straw-burning and straw-mushroom cultivation systems, respectively. The global warming potential (GWP) was then determined. Methane (CH4) and non-methane volatile organic carbon emissions (NMVOC) increased with lower modified combustion efficiency [MCE: emissions ratio of Carbon composing carbon dioxide (CO2-C) and carbon monooxide (CO-C) (CO2-C/(CO-C + CO2-C))]. Furthermore, higher moisture straw stacks generated lower nitrous oxide (N2O) emissions. Small straw stacks (5 or 10kg dry straw) with higher moisture content emitted more carbon monoxide (CO), CH4 and NMVOC. These results suggest that factors that increase the straw moisture content, such as rainfall, can cause smoldering combustion in small straw stacks or when straw is scattered on the ground, thereby inhibiting N2O emissions but enhancing CO, CH4 and NMVOC. The measured N2O emissions contributed negligible amounts to the GWP compared with measured CO and CH4, which are relatively intense GHG emissions; this was likely a result of the slow and inefficient burning that was observed from the smaller straw stacks with higher moisture content. In this study, rice straw burning threatened to generate more GHGs than straw-mushroom (Volvariella volvacea (Bul. ex Fr.) Singer) cultivation under the studied agroecosystems.