Accumulation rates and sources of external nitrogen in decaying wood in a Norway spruce dominated forest

Microbial respiration in dead wood contributes substantially to the long-lived forest carbon (C) pool and has a significant role in the forest nitrogen (N) cycle. Wood N content has been found to increase during the decay process; however, temporal dynamics and the sources of this external N remain unclear.
To examine N dynamics at various stages of decomposition, we combined high variety of analytical methods on Norway spruce logs, including wood δ15N, N%, 14C-dating, fungal composition and N2 fixation rate. For N2 fixation rate, we also determined its dependency on ambient temperature and decay class, when estimating annual N2 fixation rates for our study site.
N2 fixation was observed to have a major role in increasing wood N content during decay. For the most decayed wood, it accounted for 60% of the total N accumulation. Compared to other reports, where the annual temperature was similar to our site, the calculated annual fixation rate of 85 g N ha−1 year−1 is a low estimate. How...