Climate change may alter soil microbial communities and soil organic matter (SOM) composition. Soil carbon (C) cycling takes place over multiple time scales; therefore, long-term studies are essential to better understand the factors influencing C storage and help predict responses to climate change. To investigate this further, soils that were heated by 5 °C above ambient soil temperatures for 18 years were collected from the Barre Woods Soil Warming Study at the Harvard Forest Long-term Ecological Research site. This site consists of large 30 × 30 m plots (control or heated) where entire root systems are exposed to sustained warming conditions. Measurements included soil C and nitrogen concentrations, microbial biomass, and SOM chemistry using gas chromatography–mass spectrometry and solid-state ¹³C nuclear magnetic resonance spectroscopy. These complementary techniques provide a holistic overview of all SOM components and a comprehensive understanding of SOM composition at the molecular-level. Our results showed that soil C concentrations were not significantly altered with warming; however, various molecular-level alterations to SOM chemistry were observed. We found evidence for both enhanced SOM decomposition and increased above-ground plant inputs with long-term warming. We also noted shifts in microbial community composition while microbial biomass remained largely unchanged. These findings suggest that prolonged warming induced increased availability of preferred substrates, leading to shifts in the microbial community and SOM biogeochemistry. The observed increase in gram-positive bacteria indicated changes in substrate availability as gram-positive bacteria are often associated with the decomposition of complex organic matter, while gram-negative bacteria preferentially break down simpler organic compounds altering SOM composition over time. Our results also highlight that additional plant inputs do not effectively offset chronic warming-induced SOM decomposition in temperate forests.

气候变化可能会改变土壤微生物群落和土壤有机质（SOM）的组成。土壤碳 (C) 循环发生在多个时间尺度上；因此，长期研究对于更好地了解影响碳储存的因素并帮助预测对气候变化的反应至关重要。为了进一步研究这一问题，从哈佛森林长期生态研究中心的巴里伍兹土壤变暖研究中收集了18年来比环境温度高出5°C的土壤。该场地由 30 × 30 m 的大型地块（控制或加热）组成，整个根系都暴露在持续变暖的条件下。测量包括土壤碳和氮浓度、微生物生物量以及使用气相色谱-质谱法和固态¹³ C 核磁共振光谱法进行的 SOM 化学。这些互补技术提供了所有 SOM 成分的整体概述，并在分子水平上全面了解 SOM 组成。我们的结果表明，土壤碳浓度并没有随着变暖而显着改变；然而，观察到 SOM 化学性质发生了各种分子水平的变化。我们发现了长期变暖导致 SOM 分解增强和地上植物输入增加的证据。我们还注意到微生物群落组成的变化，而微生物生物量基本保持不变。这些发现表明，长期变暖导致首选底物的可用性增加，导致微生物群落和 SOM 生物地球化学的变化。 观察到的革兰氏阳性菌的增加表明底物可用性的变化，因为革兰氏阳性菌通常与复杂有机物的分解有关，而革兰氏阴性菌优先分解更简单的有机化合物，随着时间的推移改变 SOM 组成。我们的结果还强调，额外的植物投入并不能有效抵消温带森林中长期变暖引起的 SOM 分解。