Local Calibration of TDR Measurements for Determining Water and Organic Carbon Contents of Peaty Soils
2023
Claudia Kalla Nielsen | Anton Gårde Thomsen
Time domain reflectometry (TDR) measurements of the volumetric water content (&theta:) of soils are based on the dielectric permittivity (&epsilon:), relating &epsilon: to &theta:, using an empirical calibration function. Accurate determination of &theta: for peaty soils is vital but complicated by the complexity of organic soils and the lack of a general calibration model. Site-specific calibration models were developed to determine &theta: from TDR measurements for a heterogenous peatland across gradients of peat decomposition and organic carbon (OC) content: derived by soil organic matter conversion. The possibility of predicting OC contents based on the corrected &theta: (&theta:cor): &epsilon:: electrical impedance (&Zeta:): and a categorical predictor variable was explored. The application of plot-specific and local area calibration models resulted in similar results. Compared to common calibrations, the threshold for accurate determination of &theta: was at &epsilon: = 5: with higher &epsilon: underestimating &theta: by up to 25%. Including the von Post degree of peat humification as a bioindicator, the OC content could be modelled across the area and the full range of &theta: with an accuracy of ±:1.2% for 496 measurements. In conclusion, a strong indication was found for determining OC in peatlands in situ using TDR and a site-specific calibration model for &theta: together with indices of peat decomposition.
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