Carbon dioxide fluxes in soil profiles as affected by maize phenology and nitrogen fertilization in the semiarid Loess Plateau

•Subsoil CO2 flux and its components at the depths of 0–50cm were evaluated using the gradient method.•The CO2 concentration and fluxes within the soil profile were significantly increased by the presence of maize.•The dynamics of the contributions of the plant-derived CO2 flux to the total soil CO2 flux corresponded with maize growth.•N inputs increased total CO2 effluxes for the 0–50-cm soil layer, mainly by stimulating the plant-derived efflux during the late growing season.