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

To better understand the responses of subsoil CO2 to maize (Zea mays L.) phenology and N fertilization, a field experiment was conducted from 2014 to 2015 in the Changwu Agri-Ecological Station, Shaanxi, China. The experiment included four treatments: unplanted and N-unfertilized soil (C0), unplanted soil amended with 225 kg N ha−1 (CN), maize planted and N-unfertilized soil (P0), and maize planted soil fertilized with 225 kg N ha−1 (PN). Soil CO2 concentration at 0-50 cm soil depth, at a resolution of 10 cm, was measured, and the CO2 effluxes were calculated using the gradient method. Soil CO2 concentrations and fluxes in the planted treatments corresponded with maize growth; they rapidly increased from the jointing stage, peaked around the milk stage, and then slowly decreased with plant maturity. CO2 concentrations and fluxes in the planted soil were significantly higher compared to those in the unplanted soil. N inputs significantly decreased (P < 0.05) the CO2 concentrations of the planted soil at depths of 10, 20, and 30 cm in 2015 and increased total CO2 fluxes of the 0-50 cm soil layers during the maize growing season by 6% (P = 0.29) in 2014 and by 18% (P < 0.01) in 2015, with the cumulative plant-derived CO2 fluxes enhanced by 20% (P = 0.05) and 29% (P = 0.07), respectively. In unplanted soil, the CO2 concentrations and fluxes of the 10 cm soil layer were slightly (P > 0.05) decreased with N inputs in both years. The contributions of the plant-derived CO2 effluxes to the total CO2 effluxes of the 0-50 cm soil layers were affected by maize growth, with two year mean values of 0.49 for the P0 treatment and 0.55 for the PN treatment, respectively. The results indicated that subsoil CO2 fluxes were affected by maize phenology and that application of N fertilizer enhanced subsoil CO2 effluxes mainly by increasing plant-derived CO2 effluxes during the late growing season.