Response of tree growth to nitrogen deposition in spruce-fir-Korean pine virgin forest in Lesser Khingan Mountains in northeastern China

In this article, through field control experiment, four gradient nitrogen treatments were set as control (CK, no nitrogen (N), low-N (T_L, 5 g/(m²·year)), medium-N (T_M, 10 g/(m²·year)) and high-N (T_H, 15 g/(m²·year)), in order to study the responses of DBH growth of main species to the increasing nitrogen deposition in spruce-fir-Korean pine virgin forest. The results showed that, with the increase of nitrogen adding gradient, the DBH growth of Betula costata appeared a trend of promoting first and then suppression. The DBH growth of Abies nephrolepis and Korean pine would be suppressed due to the increase of nitrogen adding gradient, and long-term nitrogen adding can lead to the degradation or death of A. nephrolepis. While Acer ukurunduense appeared more complex response, which was possibly related to the different response mechanism of this species to the varied nitrogen adding gradients. Meanwhile, it was also found that there existed a certain relationship between the amount of DBH and DBH growth, which was possibly related to tree age and its situated niche. Our study showed that nitrogen deposition could change the growth rate and mortality of trees, and then influence the aboveground ecosystem carbon pool and the carbon cycle of whole ecosystem.