Mosses reduce soil nitrogen availability in a subarctic birch forest via effects on soil thermal regime and sequestration of deposited nitrogen

In high-latitude ecosystems, bryophytes are important drivers of ecosystem functions. Alterations in abundance of mosses due to global change may thus strongly influence carbon (C) and nitrogen (N) cycling and hence cause feedback on climate. The effects of mosses on soil microbial activity are, however, still poorly understood. Our study aims at elucidating how and by which mechanisms bryophytes influence microbial decomposition processes of soil organic matter and thus soil nutrient availability. We present results from a field experiment in a subarctic birch forest in northern Sweden, where we partly removed the moss cover and replaced it with an artificial soil cover for simulating moss effects on soil temperature and moisture. We combined this with a fertilization experiment with 15N-labelled N for analyzing the effects of moss N-sequestration on soil processes. Our results demonstrate the capacity of mosses to reduce soil N availability and retard N cycling. The comparison with artificial soil cover plots suggests that the effect of mosses on N cycling is linked to the thermal insulation capacity of mosses causing low average soil temperature in summer and strongly reduced soil temperature fluctuations, the latter also leading to a decreased frequency of freeze-thaw events in autumn and spring. Our results also showed, however, that the negative temperature effect of mosses on soil microbial activity was in part compensated by stimulatory effects of the moss layer, possibly linked to leaching of labile substrates from the moss. Furthermore, our results revealed that bryophytes efficiently sequester added N from wet deposition and thus prevent effects of increased atmospheric N deposition on soil N availability and soil processes.

Synthesis. Our study emphasizes the important role of mosses in carbon and nutrient cycling in high latitude ecosystems and the potential strong impacts of reductions in moss abundance on microbial decomposition processes and nutrient availability in subarctic and boreal forests.

在高纬度生态系统中，苔藓植物（Bryophytes）是驱动生态系统功能的核心类群。全球变化引发的苔藓丰度改变，可能显著影响碳（C）与氮（N）循环过程，进而对气候系统形成反馈。然而，目前学界对苔藓调控土壤微生物活性的具体机制仍不甚明晰。本研究旨在阐明苔藓植物通过何种途径与机制，影响土壤有机质的微生物分解过程，进而调控土壤养分有效性。我们在瑞典北部的亚北极桦木林中开展了野外操控实验：一方面通过移除部分苔藓覆盖物并替换为人工土壤覆盖层，模拟苔藓对土壤温度与湿度的调控效应；另一方面结合15N标记氮（15N-labelled N）的施肥实验，解析苔藓氮固持（N-sequestration）对土壤过程的影响。研究结果显示，苔藓能够降低土壤氮有效性并延缓氮循环进程。通过与人工土壤覆盖样地的对照分析发现，苔藓对氮循环的调控效应与其隔热性能密切相关：该性能可降低夏季土壤平均温度，并大幅抑制土壤温度波动；而温度波动的减弱同时会减少春秋两季的冻融事件发生频率。此外，本研究还发现，苔藓对土壤微生物活性的负向温度效应，可部分被苔藓层的刺激作用所抵消，这一补偿效应可能与苔藓组织中易分解底物的淋溶作用有关。进一步分析表明，苔藓植物能够高效固持湿沉降中的外源氮素，从而规避大气氮沉降增加对土壤氮有效性及土壤过程的负面影响。

综合来看，本研究凸显了苔藓在高纬度生态系统碳循环与养分循环中的关键作用，同时揭示了苔藓丰度下降可能对亚北极与寒温带森林的土壤微生物分解过程及养分有效性产生强烈冲击。