Within‐site difference in nitrogen status between mixed forests and larch plantations: Evidence from multiple indicators

Conversion of natural and secondary mixed forests to plantation monocultures can alter soil physical, chemical, and biological properties, which may lead to changes in ecosystem nitrogen (N) status by altering soil N availability. However, assessing and comparing N status between forest types within and across sites remain challenging because different indicators often yield contradicting conclusions. We developed a multiple-indicator approach to evaluate N status across sites and between larch monocultures and mixed secondary forests paired at three sites (Daxinganling, Maoershan, and Qingyuan) in northeastern China. The selected indicators were foliar N content (%N), soil carbon-to-N ratio (C/N), soil inorganic N, foliar N-15 natural abundance (delta N-15), and delta N-15 of bulk soil, ammonium, and nitrate in the soil profiles. Our results, as shown mainly by C/N of soil organic layer and the fraction of nitrate in soil inorganic N, showed that N availability increased from north to south in the order Daxinganling < Maoershan < Qingyuan, along with the increasing climatic (mean annual temperature and mean annual precipitation) and N deposition gradients. We also found that the mixed forests had lower N availability than the larch plantations at the two north sites (Daxinganling and Maoershan), while the opposite was found in the south site (Qingyuan). The site-specific pattern of N availability in the larch versus mixed stands reflects the difference in their succession and management histories. Our study demonstrated that a multiple-indicator approach is needed for a comprehensive comparison of the N status of different forest types both within and across sites.