Straw recycling in rice paddy: Trade-off between greenhouse gas emission and soil carbon stock increase

Abstract

A two-year field study was conducted in rice paddy to determine how straw incorporation influences net global warming potential (GWP) using the sum of methane (CH4) and nitrous oxide (N2O) fluxes and net ecosystem carbon budget (NECB), and greenhouse gas intensity (GHGI) using net GWP per unit of grain yield. Straw was applied by spreading over soil or mixing with soil during the dry fallow season and plowed right before flooding. In the control treatment, straw was removed for comparison. Soil organic carbon (SOC) stock changes were estimated by NECB, which means the difference between C input and output. Net primary production and applied straw were considered as C input source, and harvest C removal and heterotrophic respiration C loss were counted as C output. Straw removal depleted SOC stock at a range of 0.24-1.12 Mg C ha(-1) yr(-1), but straw application significantly increased SOC stock with the rate of 1.48-2.82 Mg C ha(-1) yr(-1). Straw spreading over soil was more effective than straw mixing with soil to increase SOC stock. However, straw application markedly increased GHG emission, in particular, CH4 emission by 27-263 % over straw removal. Between two straw treatments, annual CH4 fluxes were 25-50 % higher in straw spreading over soil than straw mixing with soil. However, the net GWP did not largely differ between straw spreading and straw removal, due to increased SOC stock. In comparison, straw mixing with soil could reduce the net GWP by 50-55 % over straw removal. Straw application increased grain yields by 7-26 % over straw removal. As a result, GHGI did not differ between straw spreading and straw removal, but was significantly lower in straw mixing with soil. Conclusively, straw application could be essential to increase SOC stock without GWP increase in rice paddy. In particular, the aerobic decomposition of straw during the dry fallow season by mixing with soil could reduce the net GWP by approximately 50-52 % over straw removal.