Response of soil respiration to nitrogen fertilization: Evidence from a 6-year field study of croplands

Abstract Agroecosystems receive large amounts of nitrogen (N), which can have a profound impact on soil respiration (Rs). Although Rs is the largest carbon dioxide (CO2) flux from terrestrial ecosystems to the atmosphere, the potential effects of N fertilization on Rs and its components (heterotrophic respiration (Rh) and autotrophic respiration (Ra)) remain controversial, and information on the impacts of N fertilization on C emission from long-term experiments is lacking. In this study, we conducted a field experiment during 6 growing seasons (2012–2018) in which we applied N fertilizer at three rates (0, 180 and 360 kg N ha−1, referred to as N0, N180 and N360, respectively) to two wheat varieties (Changhan No. 58 (CH) and Zhengmai No. 9023 (ZM)) and investigated the effects of N fertilization on Rs and its components by continuous half-hourly observations. We found that over the 6 growing seasons, N180 and N360 increased the mean growing season soil C emission by 29.7% and 11.6%, respectively, due to the increased C emission through Ra under fertilization, which was related to an increased leaf area index (LAI) and canopy photosynthesis (Pn). Significant seasonal variations in the Rs rate were observed; both Rh and Ra contributed to these variations, and the variations were significantly correlated with both soil and air temperature and plant-related traits (LAI and canopy Pn). In addition, the C emission through Rs during the growing season increased throughout the experiment, mainly because of an increase in C emissions through Rh that was caused by increasing temperature during the observation period due to climate change at the experimental site. However, C emission through Rs increased relatively rapidly under the N180 treatment due to the increased Rh throughout the experiment. Furthermore, interaction effects of soil moisture and temperature on the Rs rate and its components were also observed. Overall, our findings highlight the significance of N fertilization in term of soil C emission and its feedback under global warming in agroecosystems.