Soil microbial carbon use efficiency (CUE) is a combination of growth and respiration, which may respond differently to climate change depending on physical protection of soil carbon (C) and its availability to microbes. In a mid-latitude hardwood forest in central Massachusetts, 27 years of soil warming (+5 °C) has resulted in C loss and altered soil organic matter (SOM) quality, yet the underlying mechanisms remain unclear. Here, we hypothesized that long-term warming reduces physical aggregate protection of SOM, microbial CUE, and its temperature sensitivity. Soil was separated into macroaggregate (250–2000 μm) and microaggregate (<250 μm) fractions, and CUE was measured with ¹⁸O enriched water (H₂¹⁸O) in samples incubated at 15 and 25 °C for 24 h. We found that long-term warming reduced soil C and N concentrations and extracellular enzyme activity in macroaggregates, but did not affect physical protection of SOM. Long-term warming showed little effect on CUE or biomass turnover time because it reduced both growth and respiration. However, CUE was less temperature sensitive in macroaggregates from the warmed compared to the control plots. Our findings suggest that microbial thermal responses to long-term warming occur mostly in soil compartments where SOM is less physically protected and thus more vulnerable to microbial degradation.

土壤微生物碳利用效率（CUE）是生长和呼吸的结合，取决于土壤碳（C）的物理保护及其对微生物的可用性，其对气候变化的反应可能不同。在马萨诸塞州中部的一个中纬度硬木森林中，土壤变暖（+5°C）的27年已经导致了碳损失和土壤有机质（SOM）质量的改变，但是其潜在机制仍不清楚。在这里，我们假设长期变暖会降低SOM的物理聚集保护，微生物CUE及其温度敏感性。将土壤分为大骨料（250-2000μm）和微骨料（<250μm）部分，并用¹⁸ O富水（H ₂ ¹⁸O）在15和25°C下孵育24小时的样品。我们发现，长期变暖减少了大型骨料中土壤碳和氮的浓度以及胞外酶的活性，但并未影响SOM的物理保护。长期变暖对CUE或生物质周转时间几乎没有影响，因为它同时减少了生长和呼吸。但是，与对照样区相比，在加热后的大型骨料中，CUE对温度的敏感性较低。我们的发现表明，微生物对长期变暖的热响应主要发生在土壤隔间，在土壤隔间中，SOM受到的物理保护较少，因此更容易受到微生物降解的影响。