As the largest reservoir of terrestrial carbon (C), soil is a source or sink for atmospheric carbon dioxide (CO2). Understanding the processes whereby soil CO2 is released into the atmosphere as a result of using inorganic nitrogen (N) fertilizers may provide us with knowledge of processes to offset the increasing concentration of CO2. The main objective of this study was to investigate the effects of different N levels on soil CO2 efflux with one controlled experiment. A field experiment was carried out in a non-irrigated winter wheat (Triticum aestivum L.) - cropland in Northwest China to investigate the effects of N fertilization on soil CO2 efflux in two consecutive growing seasons (2007-2009). The soil CO2 efflux to which N was applied at four different levels (0, 90, 180, and 360kg Nha-1) was measured during the growing seasons in 2007-2009. At most growth stages during the growing season, the soil CO2 efflux increased significantly with increased N application. The effect of N fertilization on the cumulative soil CO2 efflux was obvious. In the 10-20cm soil layer, the seasonal variations in soil CO2 effluxes were influenced by soil temperature (ST) rather than by soil water content (SWC). When ST >20°C, however, the low soil CO2 efflux was mainly due to low SWC, which was close to the permanent wilting point (8.5g H2O 100g dry soil-1). In addition, soil CO2 effluxes after anthesis were higher than those at seedling stage and were highest nearby anthesis stage. The results indicated that N fertilization probably had a positive effect on both the seasonal and cumulative soil CO2 effluxes during the growing season.
