Root influences on soil bulk density measurements with thermo-time domain reflectometry

Abstract

The thermo-TDR (time domain reflectometry) technique has been applied for measuring soil bulk density (ρb) in-situ. However, the accuracy of thermo-TDR measured ρb data, as influenced by plant roots, has not been studied. In this study, we applied the extended de Vries heat capacity model to examine the influences of roots on thermo-TDR sensor performance for measuring ρb dynamics in the root zone. Soil samples were collected at multiple depths and horizontal positions over time during a maize growth period, and ρb values were determined gravimetrically and indirectly from thermo-TDR measurements. Results showed that by using the extended de Vries model, the thermo-TDR measured ρb agreed well with the gravimetric values. Ignoring root contribution to bulk soil heat capacity introduced 6.7%, 13.8% and 13.9% errors in thermo-TDR measured ρb data on the loamy sand, sandy loam, and clay loam soils, respectively. A critical root density of 0.037 g cm−3 was determined beyond which roots may induce ρb errors greater than 0.1 g cm−3 with the thermo-TDR technique.