Soil Moisture and Temperature Response Differences in Bare Soil and Perennial Grasses, Central North Carolina, USA

In temperate climate regions, significant heterogeneity exists in regard to the spatial variation of soil texture and predominant land cover type. This heterogeneity inherently complicates monitoring and analysis of soil moisture and temperature, in turn making quality control of these parameters a challenge. While rudimentary quality control processes involving saturation index and porosity ranges are currently employed, no land cover or soil type characteristics are assimilated into the process. Since these quality control measures are implemented within a variety of land cover types, a better understanding of soil moisture and temperature behavior within various categories of land cover is vital to maintaining quality assurance of soil data and further investigating the interactivity between pedospheric and atmospheric processes. Different rates of soil moisture loss would be expected between a plot of bare soil with no rooted vegetation as compared to a vegetation-rich plot with numerous, evaporation-inhibiting roots, for soil-water relationships are often contingent on land cover type. A field experiment was conducted at a research farm located in central North Carolina, USA, where a coarse, loamy sand is the predominant soil characteristic. Two, one square meter plots of land were marked for experimentation; one with only bare, loamy sand exposed after existing vegetation was cleared, and a second plot with perennial grasses left to naturally redevelop. Three sensors were installed in each plot: a multilevel soil temperature sensor designed in-house at the State Climate Office of North Carolina, a CS-109 soil temperature probe, and a Delta-T ML3 soil moisture probe. Soil moisture and temperature data were measured at a one-minute resolution, resulting in a detailed and continuous time series spanning June 2017 through August 2018. Analysis of the data showed that the vegetation-rich plot exhibited a well-defined, diurnal evaporation cycle not present within the bare soil plot. Mean soil moisture in the covered plot was observed to be 0.031 m³m^-3, or 15.1%, greater than the mean bare soil moisture. Response curves for soil moisture in both plots responded to precipitation as expected, with bare soil more quickly absorbing water and drying out at a faster rate than that of the covered soil. Soil temperature measurements responded as expected with the bare soil exhibiting a greater overall temperature range and higher mean temperature than the soil in the covered plot. The range between maximum and minimum temperature observed in the bare soil plot was 24.67℃ compared to only 20.91℃ in the covered soil plot. Bare soil temperature was consistently warmer during the daytime hours and cooler during nighttime hours, averaging 0.47℃ warmer than the covered soil throughout the experiment. Given that soil moisture sensors are installed at a 20cm standard at various research farms across North Carolina, further analysis may support the need for installation of additional probes at varying depths to observe different rates and behaviors of soil moisture loss. Multilevel soil temperature sensors are already commonplace and are often collocated with soil moisture probes, but the scope of interdependence between land cover and soil temperature requires further analysis. With a multitude of soil texture combinations and vegetative root systems present in the pedosphere, multiple levels of soil moisture monitoring and further research into soil temperature behavior would yield a better understanding of the relationship between land cover, soil temperature, and soil moisture.