RAPID: Effects of changing wildfire regimes on soil carbon fluxes during and following fire

北方森林的土壤含有大量的碳(C)。由于最近的野火模式也与气候变化有关，这些碳有可能流失到大气中并导致气候变化。确定野火如何影响土壤中储存的碳是很困难的，因为研究人员很少知道野火发生之前会发生在哪里，所以通常不会测量野火发生前的碳含量。除了评估在野火中有机物燃烧时向大气中损失了多少碳之外，火灾还可以改变剩余土壤中碳的转化方式，因为剩余的有机物分解并将碳返回到大气中。对于这个RAPID项目，2023年加拿大广泛的野火季节为该团队提供了一个独特的机会，可以在他们之前研究的地点测量火灾引起的土壤C损失，并比较火灾期间燃烧与之后分解产生的C排放的大小。在这个项目中，研究人员将测试火灾前的样本，并返回获取和测试火灾后的样本，以确定火灾如何影响碳循环。这项研究提供了一个重要的机会，利用罕见的火灾前数据来调查野火对全球相关生态系统中土壤C含量影响的紧迫问题。这些发现将有助于了解野火后的生态以及火灾严重程度与碳循环之间的相互作用。此外，初级研究人员将接受培训，项目团队将编写新的宣传材料，让公众参与到火灾和土壤的主题中来。作为2019年和2022年在加拿大AB和NWT的伍德布法罗国家公园（WBNP）及周边地区取样活动的一部分，研究小组从至少30年没有经历过火灾的地点收集了大量土壤样本。他们表征了燃烧前土壤C和N浓度、pH、微生物群落组成和呼吸（分解）。2023年，WBNP火灾综合体烧毁了70个采样点中的32个，代表了一系列土壤类型，包括富含有机物的泥炭地和沙质酸性土壤，以及优势树种。在这个项目中，研究人员将燃烧后的样本与现有的燃烧前样本配对，以回答两个问题：(1)火灾严重程度与燃烧后土壤C分解速率有何关系？(2)随着火灾严重程度的增加，燃烧造成的碳损失的相对重要性与碳分解率的变化是如何随时间变化的？在每个地点，研究人员将估计野火造成的碳损失，收集土壤芯并量化土壤C，并使用土壤孵育测量潜在的分解。他们将使用简单的生物地球化学模型来推断通过分解产生的潜在净碳损失，量化这两种主要碳损失途径的相对量级——通过燃烧产生的直接损失与野火后碳分解率的变化。进一步利用这个数据集，研究人员将收集和保存土壤的亚样本，以便将来努力表征微生物群落的组成。这些样本将与以前采样活动中燃烧前（和实验室燃烧）的微生物群落进行比较，这是利用这些不寻常的燃烧前数据集的另一个机会。样品也将被保存下来，以进一步进行化学表征，以支持未来的建议，以探索有机物质化学在确定火灾后土壤C分解速率方面的潜在作用。该研究项目将为一名研究生、一名本科生现场助理和一名研究技术人员提供培训，并加强野火和气候变化的公共教育。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The soil of northern forests holds massive reservoirs of carbon (C). This carbon is at risk of being lost to the atmosphere and contributing to climate change, due to recent patterns of wildfire that are also related to climate change. Determining how wildfires affect C stored in the soil is difficult, because it is rare that researchers know where a wildfire will take place before it happens, so there are not often measurements of C content from before the wildfire. In addition to assessing how much C is lost to the atmosphere when organic matter is combusted during a wildfire, fires can change how the remaining soil C is transformed, as remaining organic matter decomposes and returns C to the atmosphere. For this RAPID project, the extensive wildfire season in Canada in 2023 offers this team a unique chance to measure fire-induced soil C losses at their previously studied sites and to compare the size of C emissions derived from combustion during the fire vs. decomposition afterwards. In this project, the researchers will test pre-fire samples and return to to obtain and test post-fire samples in order to determine how fires affected carbon cycling. This research offers a critical opportunity to leverage rare pre-fire data to investigate pressing questions about wildfire effects on soil C content in a globally-relevant ecosystem. The findings will inform understanding of post-wildfire ecology and interactions between fire severity and C cycling. In addition, junior researchers will be trained, and the project team will develop new outreach materials to engage the public on the topic of fires and soils.As part of sampling campaigns in 2019 and 2022 in Wood Buffalo National Park (WBNP), AB and NWT, Canada, and the surrounding region, the research group collected a large number of soil samples from sites that had not experienced fire for at least 30 years. They characterized pre-burn soil C and N concentrations, pH, microbial community composition, and respiration (decomposition). The 2023 WBNP fire complex burned 32 out of 70 of the previously sampled sites, representing a range of soil types, including organic-rich peatlands and sandy acidic soils, and dominant tree species. In this project, researchers are pairing post-burn sampling with the existing pre-burn samples to answer two questions: (1) How does fire severity relate to post-burn soil C decomposition rates? (2) How does the relative importance of C losses due to combustion vs. changes in C decomposition rates change over time with increasing fire severity? At each site, researchers will estimate C losses due to wildfire, collect soil cores and quantify soil C, and measure potential decomposition using soil incubations. They will use simple biogeochemical models to extrapolate potential net C losses through decomposition, to quantify the relative magnitude of these two major C loss pathways – direct losses through combustion vs. changes in C decomposition rates after the wildfires. Further leveraging this dataset, researchers will collect and preserve subsamples of soil for future efforts to characterize microbial community composition. These samples will be compared with pre-burn (and lab-burned) microbial communities from previous sampling campaigns – another opportunity to take advantage of these unusual pre-fire datasets. Samples will also be preserved for further chemical characterization supported by future proposals, in order to explore the potential role that organic matter chemistry plays in determining post-fire soil C decomposition rates. The research project will provide training for a graduate student, undergraduate field assistants, and a research technician, as well as enhance wildfire and climate change public education.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.