Data from: 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. However,
previous studies have not taken appropriately into account the dependency
of N2 fixation rate on ambient temperature and decay class. Our δ15N model
describing the sources of external N, statistical analysis and the fungal
DNA composition of decayed wood suggest that other sources of external N
accumulating in wood were soil-foraging wood-decay fungi and mycorrhizal
fungi. Our study improves knowledge of the temporal dynamics of N
accumulation in wood with advancing wood decay, the potential sources of
external N and their relative significance. All of these factors are
important for nitrogen as well as carbon models dealing with ecosystem
responses to climate change.