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Abstract 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 averageλof 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 theλandkof the biocrusts to be underestimated, but theCto 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%, theλandkof the repacked moss biocrusts were underestimated by 59.1% and 61.8%, respectively, and theCwas overestimated by 23.9% compared with the intact moss biocrusts. Theλandkof the repacked cyanobacteria biocrusts were underestimated by 15.8% and 79.2%, respectively, and theCwas overestimated by 34.8% compared with the intact cyanobacteria biocrusts at theθvrange of 0 to 30%. Typically, this difference increased as theθvrises 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.
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Subsampling method to eliminate wall flow from soil cores held in plastic liners
Abstract Plastic liners are sometimes used with soil samplers in order to collect and store intact soil cores. Gaps at the soil–wall interface caused by the flexibility of plastic liners can result in wall flow, preventing accurate fluid flux density measurements. A subsampling method was developed to overcome problems with wall flow from soil samples collected with plastic liners in order to measure air permeability (ka) and saturated hydraulic conductivity (Ksat) on the intact cores. Subsamples were obtained after first immobilizing the soil within plastic liners by injecting expanding foam into the gaps between the soil and the liners. Once the soil was fixed in place, the soil samples were cut to the desired length, and sharpened metal rings were inserted into the original soil sample with a vise. With the metal ring at the desired depth, the subsample was removed from the original soil sample by cutting the liner and removing excess soil from the ends of the rings. Initial attempts to measurekaandKsaton samples within the original liners led to unrealistically high values because significant wall flow occurred. However, after implementing the improved subsampling approach, the measuredkaandKsatof the subsamples were within the range of expected values based on the literature. The subsampling method effectively eliminated wall flow on soil originally collected in plastic liners and is relatively easy to implement without the need for specialized tools.
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- Award ID(s):
- 2037504
- PAR ID:
- 10584808
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Soil Science Society of America Journal
- Volume:
- 89
- Issue:
- 2
- ISSN:
- 0361-5995
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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