Wildfires have the potential to dramatically alter the carbon (C) storage potential, ecological function, and the fundamental mechanisms that control the C balance of Pacific Northwest (PNW) forested ecosystems. In this study, we explored how wildfire influences processes that control soil C stabilization and the consequent soil C persistence, and the role of previous fire history in determining soil C fire response dynamics. We collected mineral soils at four depth increments from burned (low, moderate, and high soil burn severity classes) and unburned areas and surveyed coarse woody debris (CWD) in sites within the footprint of the 2020 Holiday Farm Fire and in surrounding Willamette National Forest and the H.J. Andrews Experimental Forest. We found few changes in overall soil C pools as a function of fire severity; we instead found that unburned sites contained high levels of pyrogenic C (PyC) that were commensurate with PyC concentrations in the high severity burn sites—pointing to the high background rate of fire in these ecosystems. An analysis of historical fire events lends additional support, where increasing fire count is loosely correlated with increasing PyC concentration. An unexpected finding was that PyC concentration was lower in low soil burn severity sites than in control sites, which we attribute to fundamental ecological differences in regions that repeatedly burn at high severity compared with those that burn at low severity. Our CWD analysis showed that high mean fire return interval (decades between fire events) was strongly correlated with low annual CWD accumulation rate; whereas areas that burn frequently had a high annual CWD accumulation rate. Within the first year postfire, trends in soil density fractions demonstrated no significant response to fire for the mineral-associated organic matter pool but slight increases in the particulate pool with increasing soil burn severity—likely a function of increased charcoal additions. Overall, our results suggest that these PNW forest soils display complex responses to wildfire with feedbacks between CWD pools that provide varying fuel loads and a mosaic fire regime across the landscape. Microclimate and historic fire events are likely important determinants of soil C persistence in these systems.

野火有可能显着改变太平洋西北地区 (PNW) 森林生态系统的碳 (C) 储存潜力、生态功能以及控制碳平衡的基本机制。在这项研究中，我们探讨了野火如何影响控制土壤碳稳定和随之而来的土壤碳持久性的过程，以及以前的火灾历史在确定土壤碳火灾响应动态中的作用。我们从烧毁区域（低、中和高土壤烧伤严重程度等级）和未烧毁区域收集了四个深度增量的矿质土壤，并在 2020 年假日农场火灾覆盖范围内和威拉米特国家森林周围的地点调查了粗木本碎片 (CWD)和 HJ 安德鲁斯实验森林。我们发现总体土壤碳库随火灾严重程度变化不大；相反，我们发现未燃烧地点含有高水平的热原碳 (PyC)，与高严重度燃烧地点的 PyC 浓度相当，这表明这些生态系统中的背景火灾率很高。对历史火灾事件的分析提供了额外的支持，其中火灾数量的增加与 PyC 浓度的增加呈松散相关。一个意外的发现是，土壤烧伤严重程度较低的地点的 PyC 浓度低于对照地点，我们将其归因于反复烧伤严重程度较高的地区与烧伤程度较低的地区之间存在根本的生态差异。我们的 CWD 分析表明，较高的平均火灾重现间隔（火灾事件间隔数十年）与较低的年度 CWD 累积率密切相关；而燃烧频繁的地区，CWD 的年累积率较高。在火灾后的第一年内，土壤密度分数的趋势表明，与矿物相关的有机物库对火灾没有显着的反应，但随着土壤烧伤严重程度的增加，颗粒物库略有增加，这可能是木炭添加量增加的结果。总体而言，我们的结果表明，这些 PNW 森林土壤对野火表现出复杂的反应，并提供不同燃料负载的 CWD 池之间的反馈和整个景观的马赛克火灾状况。小气候和历史火灾事件可能是这些系统中土壤碳持久性的重要决定因素。