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

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.