Biocrusts are a critical surface cover in global drylands, but knowledge about their influences on surface soil thermal properties are still lacking because it is quite challenging to make accurate thermal property measurements for biocrust layers, which are only millimetres thick. In this study, we repacked biocrust layers (moss‐ and cyanobacteria‐dominated, respectively) that had the same material as the original intact biocrusts but was more homogeneous and thicker. The thermal conductivity (λ), heat capacity (C) and thermal diffusivity (k) of the repacked and intact biocrusts were measured by the heat pulse (HP) technique at different mass water contents (θm) and mass ratios (Wt), and the differences between repacked and intact biocrusts were analysed. Our results show that biocrusts substantially alter the thermal properties of the soil surface. The average λ of moss (0.37 W m−1 K−1) and cyanobacteria biocrusts (0.90 W m−1 K−1) were reduced by 63.0% and 10.3% compared with bare soil (1.00 W m−1 K−1), respectively. Edge effects including heat loss and water evaporation caused the λ and k of the biocrusts to be underestimated, but the C to be overestimated. The differences in thermal properties were significant (p <0.001), except for the differences in thermal conductivity between repacked and intact cyanobacteria biocrusts, which were not significant (p = 0.379). Specifically, in the volumetric water content (θv) range of 0 to 20%, the λ and k of the repacked moss biocrusts were underestimated by 59.1% and 61.8%, respectively, and the C was overestimated by 23.9% compared with the intact moss biocrusts. The λ and k of the repacked cyanobacteria biocrusts were underestimated by 15.8% and 79.2%, respectively, and the C was overestimated by 34.8% compared with the intact cyanobacteria biocrusts at the θv range of 0 to 30%. Typically, this difference increased as the θv rises between repacked and intact biocrusts. Our new measurements provide evidence that the thermal properties of biocrusts were previously misjudged due to the measurement limitations imposed by their limited thickness when measured in situ. Biocrusts are likely more significant in regulating soil heat and temperature in drylands than was previously assumed.

中文翻译：

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被忽视的生物结皮对表层土壤热特性的影响：来自大体积样品热脉冲传感的证据


生物结皮是全球旱地的关键表面覆盖物，但仍然缺乏关于它们对表层土壤热特性影响的知识，因为对只有毫米厚的生物结皮层进行准确的热特性测量是相当具有挑战性的。在这项研究中，我们重新填充了生物结皮层（分别以苔藓和蓝藻为主），其材料与原始完整生物结皮相同，但更均匀、更厚。采用热脉冲 （HP） 技术测量了不同质量含水量 （θm） 和质量比 （Wt） 下重新填充和完整生物结皮的热导率 （λ）、热容 （C） 和热扩散率 （k），并分析了重新填充和完整生物结皮之间的差异。我们的结果表明，生物结皮极大地改变了土壤表面的热特性。与裸露土壤 （1.00 W m-1 K-1） 相比，苔藓 （0.37 W m-1 K-1） 和蓝藻生物结皮 （0.90 W m-1 K-1） 的平均 λ 分别降低了 63.0% 和 10.3%。包括热损失和水蒸发在内的边缘效应导致生物结皮的 λ 和 k 被低估，但 C 被高估。热特性的差异很大 （p <0.001），除了重新填充和完整蓝藻生物结皮之间的热导率差异不显著 （p = 0.379）。具体而言，在 0 至 20% 的体积含水量 （θv） 范围内，与完整的苔藓生物结皮相比，重新填充的苔藓生物结皮的 λ 和 k 分别被低估了 59.1% 和 61.8%，C 被高估了 23.9%。重新包装的蓝藻生物结皮的 λ 和 k 分别被低估了 15.8% 和 79.2%，C 被高估了 34%。与 θv 范围内 0 至 30% 的完整蓝藻生物结壳相比，为 8%。通常，这种差异随着重新堆积和完整生物结壳之间 θv 的升高而增加。我们的新测量结果提供了证据，证明生物结皮的热特性以前是错误的判断，因为在原位测量时，其厚度有限会带来测量限制。生物结皮在调节旱地土壤热量和温度方面可能比以前假设的更重要。