Quantifying bulk density of boreal peat using X-ray computed tomography

Peatlands play a crucial role in carbon storage and climate regulation. Traditional gravity-based and loss-on-ignition methods have been widely used to acquire bulk density and thus organic carbon estimates in peat sequences. However, these methods are time-consuming, and the measurement resolution frequently ranges from half to a few centimetres, hampering the understanding of carbon accumulation history at finer temporal resolution. Here, we explore the potential of non-destructive X-ray computed tomography (XCT), a method for analyzing 3D material structure and mass density, for obtaining proxy measurements for bulk density parameters using peat cores collected in eastern boreal Quebec, Canada. We find that the Hounsfield Unit (HU) of medical XCT scans is a robust surrogate for the bulk density of wet peat (BD wet ). A universal linear model can be applied to calibrate HU values for a wide range of peat stratigraphy from different microforms: Sphagnum hummock, lichen hummock, lawn, and hollow. Moreover, HU of dry peat is indicative of both dry and organic matter bulk density (BD dry and BD om ). It is possible to develop case-specific logarithmic models to calibrate HU with BD dry . In addition, the precise measurement of the peat sample volumes using XCT suggests that traditional methods can be subject to substantial uncertainties when estimating bulk density and carbon content. Medical XCT can be applied to quantify bulk density in peat soils in a more time-efficient manner, with a resolution up to 0.6 mm, approximately equivalent to the yearly accumulation rate.