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

微生物分解枯死的植物物质会导致大量的二氧化碳从土壤呼吸到大气中。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个微生物群参与了这一过程。我们研究的目标是(1)检验真菌物种在分解中扮演不同角色的可能性，以及(2)将这些差异纳入森林火灾和N沉积后大规模分解估计中。具体地说，我们将检验这样一个假设，即腐生型真菌物种对碳源的吸收速率不同，因此“鲁德勒”迅速获得不稳定的化合物，而“竞争性”主要依赖于对顽固化合物的缓慢、持续的吸收。如果这一假设得到支持，它将提出一种在真菌物种之间进行资源分配的机制。我们还预计，在氮素有效性较高的情况下，竞争性物质对分解的贡献将受到抑制，这种减少将在年轻的火灾伤疤中最为明显，在这些地方，顽固的底物作为木质碎屑大量存在。为了实现这些目标，拟议的工作包括三个主要方法，这些方法将以阿拉斯加北部的森林为基础。首先，我们将在田间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。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将被应用于土壤，并将测量从已知真菌物种的蘑菇中呼吸的二氧化碳的同位素特征的时间线。14C标记的二氧化碳与13C标记的二氧化碳的释放将表明不同物种使用顽固性碳与不稳定碳的程度。13CO2呼吸的时间将表明不同物种开发新的不稳定碳源的速度。其次，我们将检查真菌的天然14C特征，以估计单个物种分解化合物的年龄。我们预计，如果竞争者专注于更顽固的化合物，那么竞争者将拥有比鲁德尔人更古老的C。第三，我们将把关于真菌物种功能作用的信息与阿拉斯加火灾时间序列沿线自然和氮肥地区蘑菇丰度的调查数据结合起来，以估计真菌群落的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者分解森林火灾产生的木质碎屑的能力。这项拟议工作的学术价值包括审查全球变化下微生物群落组成变化的大规模后果，潜在地提高我们预测生态系统对环境反应的能力。更广泛的影响包括开发场标记技术，该技术利用加速器质谱学测量14C的灵敏度，以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.