Objective To study whether there is a response of biomass growth rate of Larix gmelinii forest to nitrogen deposition, whether there is difference among years and whether different diameters can affect the response.

Method This study simulated the change of atmospheric nitrogen deposition by artificial nitrogen addition method to investigate the response of biomass growth in three years to nitrogen deposition in L. gmelinii forests during the period of 2014-2016. The nitrogen was added from 2012, 4 nitrogen addition levels were set as control (CK, 0 g·m^-2·a^-1), low nitrogen (TL, 2.5 g·m^-2·a^-1), middle nitrogen (TM, 5 g·m^-2·a^-1) and high nitrogen (TH, 7.5 g·m^-2·a^-1). The growth season (May-October) will dissolve NH₄NO₃ in 32 L distilled water monthly, spray evenly on the sample plot with back atomize, and spray the same amount of pure water in the same way on the CK.

Result (1) The biomass growth rate of every treatment was significantly higher in three years (P < 0.05). The annual biomass growth rate was significantly different (P < 0.05), and the growth rate of 2016 was significantly higher than that of the previous two years (P < 0.05). (2) There was no significant difference between the three treatments in 2014 (P < 0.05). The growth rate of TL in 2015 was significantly higher than that of TM and TH(P < 0.05). The growth rate of TL in 2016 was significantly higher than that of TH (P < 0.05). (3) Different treatments had different effects on different diameter class. Between 2014 and 2015, TL significant increased the biomass growth rate of L. gmelinii (P < 0.05) in 10 15 cm diameter. TM had a significant effect on the biomass growth rate of L. gmelinii of 10 15 cm and 15 20 cm diameter in all the 3 years (P < 0.05). In 2016, the biomass growth rate of L. gmelinii significantly increased (P < 0.05) in the 15 20 cm diameter. TH had more significant effect on the biomass growth rate of three diameters class (10 15, 15 20, and > 20 cm) than that < 10 cm (P < 0.05) in all the 3 years.

Conclusion (1) Long-term nitrogen addition will lead to a growth rate gradually reduce or even suppress, which is likely to change the boreal forest carbon source & sink dynamic; (2) Since different diameter trees have different responses to the nitrogen deposition, it should be as the basal factor in simulation in order to increase the prediction accuracy about nitrogen deposition on the boreal forest carbon pool.