CO₂ emission in a subtropical red paddy soil (Ultisol) as affected by straw and N-fertilizer applications: A case study in Southern China

Soil sequestration of atmospheric CO₂ through land application of inorganic N fertilizer along with organic residues may have beneficial effects as a strategy to offset the increase in the concentration of CO₂ in the atmosphere. A field study was conducted to assess the effect of application of N fertilizer and rapeseed (Brassica napus L.) straw in a paddy field. To understand rice-, rhizosphere- and N-induced CO₂ flux, CO₂ flux was measured during the growth stages of rice (Oryza sativa L.) from row, inter-row and bare soil at the experimental station of Heshengqiao at Xianning, Hubei, China. The study included seven treatments: (CK) control, (N0) fertilizer PK, (N1) fertilizer NPK (50% N), (N2) fertilizer NPK (100% N), (N3) fertilizer NPK (200% N), (N0+S) fertilizer NP+traw, (N2+S) fertilizer NPK (100% N)+straw. There was a distinct variation in soil CO₂ fluxes, with the higher values being observed during the reproductive stage of crop growth while the lower fluxes were observed during the maturity stage. Soil CO₂ fluxes from row (797-1214gCm⁻² season⁻¹) were significantly higher than from inter-row (289-403gCm⁻² season⁻¹) and bare soil (148-241gCm⁻² season⁻¹), due to the contribution of rhizosphere respiration. Among different treatments, N fertilization significantly increased the CO₂ flux from row with the highest being observed from N2+S and lowest from N0+S treatment. No significant differences among different treatments were observed from inter-row and bare soil. From bare soil, soil CO₂ flux was decreased in response to N fertilizer application; this suggested suppression in microbial activity in response to increased N fertilizer application. Soil temperature accounted for 68 and 38% of CO₂ flux variability from row and inter-row, respectively, while no significant correlation was found from bare soil. Soil temperature explained 69% of N-induced CO₂ flux variability from row, while no effect was observed from inter-row and bare soil. Soil temperature was also significantly correlated with rice- and rhizosphere-induced CO₂ flux accounting for 42 and 31% of CO₂ flux variability, respectively. The amount of soil carbon sequestration was estimated by taking the difference between net primary production (NPP) and the amount of carbon in harvested rice. The values ranged from -176 to -89gCm⁻² season⁻¹ with the highest value observed from N2+S treatment; this suggested that N fertilizer application with straw has the potential to mitigate the global carbon budget. The current findings indicate that N addition increases the CO₂ flux. However, integrated use of N fertilizer along with rapeseed straw may be a preferred strategy in sequestering C in red soil.