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

Abstract

Summary 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.

Carbon flux from decomposing wood and its dependency on temperature, wood N2 fixation rate, moisture and fungal composition in a Norway spruce forest

“…A conversion factor from acetylene reduction to N fixation of 4 was used (Brunner & Kimmins, ). In the present study the N 2 fixation data (Rinne et al, ) is for the first time compared with respiration rate, nifH copy number and fungal community of Lapinjärvi deadwood, and used to calculate the AE of the N 2 fixation rate for the different decay classes. The density, N content and moisture of deadwood used in the present study (Table ) are from Rinne et al ().…”

“…In addition, bacteria are considered to play a minor role in wood decomposition (Clausen, ), but recent studies have proven early hypothesis (Cowling & Merrill, ) that e.g. N 2 fixing bacteria may have a remarkable contribution on N availability in the decomposing wood (Rinne et al, ).…”

“…Apart from being a substantial stock of terrestrial C, deadwood is an important component of nutrient cycling (Laiho & Prescott, ). This includes N, which accumulates in deadwood during the decomposition process mainly via asymbiotic N fixation (Rinne et al, ). N may also be used during the decomposition process via sporocarp and spore production or lost via wood fragmentation and leaching.…”

Increasing N deposition impacts neither diversity nor functions of deadwood‐inhabiting fungal communities, but adaptation and functional redundancy ensure ecosystem function

“…Both angiosperm and conifer deadwoods have very high C:N ratios (377 in Fagus sylvatica , 630 in Picea abies ), thus microbial extraction or utilization of N from deadwood ought to be very difficult (Hoppe et al ., ). Although the C:N ratio decreases over time due to the relative accumulation of N (Rinne et al ., ), at the late decomposition stage it still remains very high (i.e., 194 in F. sylvatica and 423 in P. abies ) compared with other substrates such as soils (ranging from 9 in Greyzems to 26 in Histosols) or leaf‐litter [ranging from 54 ± 38 (mean ± SD) in tropical regions to 79 ± 30 in the Mediterranean] (Batjes, ; Aerts, ; Hoppe et al ., ). In addition, considering that C:N ratios of fungal mycelia are low (i.e., 9–20) (Wallander et al ., ; Brabcová et al ., ) and that production of wood degradation enzyme requires high amount of N, this would mean high demand of N for wood‐inhabiting fungi (WIF) is expected (Hoppe et al ., ).…”

Chemical and microbiological changes in Norway spruce deadwood during the early stage of decomposition as a function of exposure in an alpine setting

“…Indeed, Hoppe et al (2014) found a stronger positive correlation between fungal fructification and nifH diversity during the intermediate stage of decay of P. abies and Fagus sylvatica logs. Another plausible explanation could be related to the moisture content, because it is known to be one of the limiting factors of the biological nitrogen fixation (Rinne et al 2017). In fact, a decrease in wood moisture level was found at the end of the monitoring period in our study.…”