Non-invasive Measurements of Oilseed Temperature in Soil and Soil Thermal Diffusivity Using Time-Domain NMR Relaxometry

Global warming is threatening food production in tropical areas, because the increase of soil temperature may limit seed germination and plant growth. Soil temperature and thermal diffusivity (λ) have been measured using the conventional thermometry. In this study, we are demonstrating that time-domain nuclear magnetic resonance (TD-NMR) relaxometry can be a non-invasive method to determine oilseed temperature in soils and soil thermal diffusivity (λ). The correlation between oilseed transverse relaxation times (T2) and seed temperature has been used to measure the temperatures of intact oilseed in soil samples. To calculate soil thermal diffusivity, spherical soil samples with 7 cm in diameter containing a macadamia nut in the center were heated to 70 °C and then placed in an air bath at room temperature. λ values were calculated using the time constant of oilseed temperature decay, measured by TD-NMR and sample dimensions. The λ values of dry entisol, yellow oxisol, and red oxisol soils were 1.89 × 10−7, 1.52 × 10−7, and 1.03 × 10−7 m2 s−1, respectively. These values were within the same order of magnitude range observed for the values measured by both thermocouple and Dickerson methods. The λ values of dry and moist typic hapludox were 1.16 × 10−7 and 2.29 × 10−7 m2 s−1, respectively. Therefore, TD-NMR is shown to be a feasible method to measure seed temperature in soils and soil thermal diffusivity, and is a potential non-invasive tool to investigate the effect of temperature on seed germination and seedling.