Benefits of organic amendments on soil C stock may be offset by increased methane flux in rice paddy field

Periodic application of organic manure (OM) has been suggested to increase soil carbon stock and mitigate global warming. However, in irrigated rice fields, OM amendment greatly increases methane (CH₄) emissions, which in turn may offset the effect of increased soil carbon storage on mitigating global warming. Research on the effects of OM application on net global warming potential that integrates greenhouse gas (GHG) fluxes and soil carbon stocks change in rice cultivating systems has been elusive. To determine the impact of organic amendments on global warming in a rice paddy, chemical (NPK) and organic fertilization were installed, and straw removal and recycling plots were added in two fertilization treatments. Annual CH₄ and nitrous oxide (N₂O) fluxes were evaluated using the static closed chamber method. Soil C stock changes were estimated using the net ecosystem C budget (NECB) which indicates the difference between C input and output. In chemical fertilization, straw removal decreased soil C stock by an average of 751 kg C ha⁻¹ year⁻¹, but straw recycling significantly increased soil C stock by an average of 464 kg C ha⁻¹ year⁻¹. In organic fertilization, soil C stock was strongly affected by cover crop productivity. Under favorable climatic conditions for cover cropping, soil C stocks were greatly increased by green manuring and even more so by straw recycling. However, under an unfavorable climate, soil C stock was not statistically increased, regardless of straw addition. Straw recycling increased annual CH₄ flux by approximately 220–310 and 130–190% over straw removal in chemical (7.9 Mg CO₂-eq. ha⁻¹ year⁻¹) and organic fertilization (40 Mg CO₂-eq. ha⁻¹ year⁻¹), respectively, but negligibly affected N₂O flux. Irrespective of fertilization background, net GWP was mainly decided by CH₄ flux with 54–98% coverage, and then followed by soil C stock change and N₂O flux. As a result, straw addition increased net GWP by around 110–140 and 120–150% over straw removal in chemical and organic fertilization, respectively. Rice yield was stable between years in the chemical fertilization but showed a big difference in organic fertilization, due to the difference in cover crop biomass productivity. However, rice productivity was not significantly affected by straw recycling. Organic amendment and straw applications increased greenhouse gas intensity (GHGI) by an average of 131% and 269% in a rice paddy, respectively, due to a high increase of CH₄ flux but a low increase of soil C stock. In conclusion, more effective OM management which can decrease CH₄ emissions and increase SOC stock is required in rice paddies.