Radiocarbon Experiments in Boreal Forests to Assess Roles of Fungal Species in Decomposition

微生物分解死去的植物材料会导致大量呼吸二氧化碳从土壤排放到大气中。在不断变化的环境条件下，呼吸速率通常很难预测，可能是因为包括真菌在内的多种微生物群参与了这一过程。我们研究的目的是（1）检查真菌物种在分解中发挥不同作用的潜力，以及（2）将这些差异纳入森林火灾和氮沉降后分解的大规模估计中。具体来说，我们将研究这样的假设：腐养真菌物种对碳源的吸收率不同，因此“Ruderals”快速获取不稳定的化合物，而“Competitives”主要依赖于缓慢、持续地吸收顽固化合物。如果这一假设得到支持，则将提出一种真菌物种之间的资源分配机制。我们还预计，在更高的氮利用率下，竞争者对分解的贡献将受到抑制，这种减少在年轻的火疤中最为明显，那里的顽固基质像木质碎片一样丰富。为了实现这些目标，拟议的工作包括以阿拉斯加北方森林为基础的三种主要方法。首先，我们将在现场条件下对蘑菇进行双同位素标记，以检查可能影响真菌之间底物使用分配的权衡。将放射性碳 (14C) 标记的顽固底物和 13C 标记的不稳定底物的混合物应用于土壤，并测量从已知真菌物种的蘑菇中呼吸的二氧化碳同位素特征的时间线。 14C 与 13C 标记的 CO2 的释放将表明不同物种使用顽固碳与不稳定碳的程度。 13CO2 呼吸的时间将表明不同物种利用新的不稳定碳源的速率。其次，我们将检查真菌的天然 14C 特征，以估计各个物种分解的化合物的年龄。如果竞争对手专注于更顽固的化合物，我们预计竞争对手将拥有比 Ruderals 更旧的 C。第三，我们将把有关真菌物种功能作用的信息与阿拉斯加火灾年代沿线的自然和氮肥地区蘑菇丰度调查所获得的数据结合起来，以估计真菌群落变化对土壤碳转化的影响。我们预计氮的添加将降低木质纤维素降解剂分解森林火灾产生的木质碎片的能力。拟议工作的智力价值包括检查全球变化下微生物群落组成变化的大规模后果，可能提高我们预测生态系统对环境的反应的能力。更广泛的影响包括现场标记技术的发展，该技术利用加速器质谱测量 14C 的灵敏度来最大限度地减少实验伪影。

Decomposition of dead plant material by microbes elicits a large flux of respired CO2 from soils to the atmosphere. Respiration rates are typically challenging to predict under changing environmental conditions, potentially because multiple microbial groups, including fungi, contribute to this process. The objectives of our study are (1) to examine the potential for fungal species to perform different roles in decomposition, and (2) to incorporate these differences in large scale estimates of decomposition following forest fires and N deposition. Specifically, we will examine the hypothesis that saprotrophic fungal species differ in uptake rates of carbon sources, so that "Ruderals" quickly acquire labile compounds, while "Competitives" primarily rely upon slow, constant uptake of recalcitrant compounds. If this hypothesis is supported, it would suggest a mechanism for resource partitioning among fungal species. We also expect that contributions to decomposition by Competitives will be inhibited under greater N availability, and this reduction will be most pronounced in young fire scars, where recalcitrant substrates are abundant as woody debris. To address these goals, the proposed work encompasses three major approaches that will be based in boreal forests of Alaska. First, we will perform dual-isotope labeling of mushrooms under field conditions to examine trade-offs that may influence partitioning of substrate use among fungi. A mix of radiocarbon (14C) labeled recalcitrant substrates and 13C labeled labile substrates will be applied to the soil, and a timeline of isotope signatures of CO2 respired from mushrooms of known fungal species will be measured. The release of 14C- versus 13C-labeled CO2 will indicate the extent to which different species use recalcitrant versus labile carbon. The timing of 13CO2 respiration will indicate the rate at which different species can exploit new labile C sources. Second, we will examine natural 14C signatures of fungi to estimate the ages of compounds decomposed by individual species. We expect that Competitives will possess older C than do Ruderals, if Competitives are specializing on more recalcitrant compounds. Third, we will combine information regarding functional roles of fungal species with data derived from surveys of mushroom abundance in natural and nitrogen-fertilized areas along a fire chronosequence in Alaska, in order to estimate effects of shifts in fungal communities on carbon transformations in the soil. We expect that nitrogen additions will reduce the ability of lignocellulose degraders to decompose woody debris generated by forest fires. The intellectual merit of the proposed work includes an examination of large-scale consequences of shifts in microbial community composition under global change, potentially improving our ability to predict ecosystem responses to the environment. The broader impacts include the development of field-labeling techniques that take advantage of the sensitivity of accelerator mass spectrometry measurements of 14C in order to minimize experimental artifacts.