Differential Responses of Soil Respiration and Its Components to Nitrogen Application in Urban Forests

Understanding the impacts of nitrogen (N) deposition on soil respiration (Rs) and its components (autotrophic respiration (Ra) and heterotrophic respiration (Rh)) in urban forests is crucial for predicting the soil carbon dioxide (CO2) emission and evaluating carbon (C) budget in changing environments. In this study, a three-year-long field manipulation experiment was conducted in two urban forests to assess the effect of N application at three rates (0, 50, and 100 kg N·ha−1·year−1) on Rs components. N application did not alter the seasonal dynamics of Rs and its components. Rs and its components showed nonlinear responses to N application; both Rs and Rh increased in year 1 of N application but decreased in year 3. The Ra/Rs ratio increased by 21% in the low N (50 kg N·ha−1·year−1) plots. The mechanism varied across the years of N application; soil temperature and moisture substantially influenced Ra and Rh under N application. Our results indicated that increasing atmospheric N deposition may inhibit soil CO2 emission, and a higher proportion of soil C is released due to root activities. Interannual variations in temperature and rainfall may help predict the efflux of soil CO2 in urban forests in response to atmospheric N deposition.