Forest soil respiration showed a saturation threshold in response to increasing nitrogen deposition: Evidence from a mesocosm experiment

• Soil respiration is enhanced by N deposition. • N deposition increases autotrophic but decreases heterotrophic respiration. • Soil respiration has a N saturation threshold in response to growing N deposition. • Tree growth accounts for the non-linear trend of soil respiration along N gradients. Whether there is a N saturation threshold for the response of soil respiration (R s ) to increasing nitrogen (N) deposition remains unclear in forest ecosystems. In this work, a simulated N deposition experiment involving five levels of N addition (0 to 44.8 g N m −2 yr −1 ) was performed in a mesocosm experiment platform with young Chinese fir ( Cunninghamia lanceolata ) trees planted on it. R s and its autotrophic (R a ) and heterotrophic (R h ) components were measured for four years, and the N saturation threshold was estimated using a quadratic-plus-plateau model. With increasing rate of N addition, R s and R a first increased dramatically until the critical N addition rate of 12.2 and 13.6 g N m −2 yr −1 , respectively, and then leveled off. A similar non-linear increase in tree growth was also detected with N addition. In contrast, R h continued to decrease as N addition increases, primarily a consequence of the N-increased recalcitrance of soil organic C, as evidenced by the decreased gram-negative bacteria biomass coupled with increased polyphenol oxidase activity. Our results indicated (1) that there was a N saturation threshold for the soil respiration, before which respiration rate increased with N addition and such an increase vanished thereafter; and (2) that the N saturation threshold of soil respiration was driven by autotrophic rather than heterotrophic respiration. This study advanced our understanding of the response of forest carbon cycle to growing N deposition.