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 (797–1214 g C m−2 season−1) were significantly higher than from inter-row (289–403 g C m−2 season−1) and bare soil (148–241 g C m−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 −89 g C m−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.