Effects of long-term nitrogen addition on the δ15N and δ13C of Larix gmelinii and soil in a boreal forest

Background Natural abundance of carbon (C) and nitrogen (N) stable isotope ratios (delta C-13 and delta N-15) has been used to indicate the state and cycle of ecosystem C and N. However, it is still unclear how C and N cycle of boreal forests respond to the N deposition. Results We conducted an 8-year continuous N addition field experiment in a Larix gmelinii forest in Greater Khingan Mountains, Northeast China. Four N treatments (0, 25, 50, 75 kg N ha(-1) year(-1)) were built. The effects of N addition on the delta C-13 and delta N-15 of needle, branch, bark, and fine root of Larix gmelinii and soil were studied. The result of the balance between the N input and output flux showed that N addition significantly increased the delta N-15 in each organ of Larix gmelinii, but did not change the delta N-15 of soil. We also found that the N absorption by needles of Larix gmelinii could increase the needle photosynthesis rate and delta C-13 by increasing carboxylation, but N addition had no significant effect on the delta C-13 of soil and other organs. In addition, both the soil delta N-15 and delta C-13 increased with the soil depth. Conclusions Long-term N addition may lead to more open C and N cycles and further affect plant nutrient acquisition strategies in boreal forest ecosystems.