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.

枯木中的微生物呼吸作用对长期留存的森林碳（C）库具有显著贡献，并在森林氮（N）循环中发挥关键作用。研究发现，枯木在腐解过程中其氮含量会逐步升高，但这一过程的时序动态特征以及外源氮的来源仍尚不明确。
为探究腐解不同阶段的氮素动态，我们针对挪威云杉（Norway spruce）原木采用了多种分析方法，包括木材δ15N、全氮占比（N%）、14C定年（14C-dating）、真菌群落组成以及固氮速率（N2 fixation rate）。在估算本研究样地的年固氮速率时，我们同时明确了固氮速率对环境温度与腐解等级的依赖性。
研究观察到，腐解过程中固氮作用是提升枯木氮含量的核心驱动因素之一。在高度腐解的木材中，固氮贡献了总氮积累量的60%。与其他年平均温度与本样地相近的已有研究相比，本研究估算的年固氮速率85 g N·ha⁻¹·yr⁻¹属于偏低的估计值。但既往研究均未恰当考虑固氮速率对环境温度与腐解等级的依赖性。
本研究用于解析外源氮来源的δ15N模型、统计分析结果以及腐解木材的真菌DNA组成均显示，枯木中积累的其余外源氮来自觅土型木腐真菌与菌根真菌（mycorrhizal fungi）。
本研究深化了对枯木腐解进程中氮素积累的时序动态特征、潜在外源氮来源及其相对重要性的认知。上述所有要素对于研究生态系统对气候变化响应的氮循环与碳循环模型均具有重要参考价值。