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对长寿森林碳（C）库贡献显著，且在森林氮（N）循环中发挥关键作用。已有研究表明，枯木在分解过程中氮含量会逐步升高，但其时间动态特征与外源氮的来源仍未明确。

为探究不同分解阶段枯木的氮动态变化，我们针对挪威云杉原木开展研究，结合了多类分析手段，包括木材δ15N、氮百分含量（N%）、14C定年、真菌群落组成以及固氮作用（N2 fixation）速率测定。在估算本研究样地的年固氮速率时，我们还明确了固氮速率对环境温度与分解等级的依赖关系。

研究结果显示，固氮作用是枯木分解过程中氮含量升高的核心驱动因素。在高度分解的枯木中，固氮贡献了总氮累积量的60%。与其他年平均温度与本研究站点相近的报道相比，本研究估算的年固氮速率为85 g N ha⁻¹ year⁻¹，属于较低估值。然而，此前的相关研究并未恰当考虑固氮速率对环境温度与分解等级的依赖性。通过构建δ15N溯源模型、开展统计分析以及对腐朽木材的真菌DNA群落组成进行解析，我们推断枯木中累积的其他外源氮来源为觅土型木腐真菌与菌根真菌。

本研究深化了对枯木随分解进程推进的氮累积时间动态、外源氮潜在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响应气候变化的氮循环与碳循环模型均具有重要参考价值。