Field-Calibration of a Hand-Held TDR for Determining Soil Moisture In An Irrigated and a Rainfed Biofuel Feedstock Production System.

Agricultural soil emissions of temperature-forcing greenhouse gases such as carbon dioxide, methane, and nitrous oxide are tightly coupled to soil moisture status.  To determine the suitability of a rapid measurement method for soil volumetric water content (VWC; m³ m^-3), a hand-held time-domain reflectometry (TDR) device was field-calibrated over the 2010 growing season at an irrigated and rainfed site in Nebraska used for biofuel feedstock production (corn stover, switchgrass).  TDR measurements were calibrated against VWCs calculated from gravimetric water content and bulk density values measured on soil cores (0-15 cm) sampled next to TDR reading locations.  From April to September 2010, 466 soil moisture readings and cores were taken from weekly samplings across both sites.  Soil moisture data were collected in conjunction with soil greenhouse gas emissions for USDA-ARS’s GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network).  The two available factory calibrations (standard, high-clay) substantially overestimated measured soil VWC by an average of 90% and 47% (or 0.25 and 0.14 m³ m^-3), respectively.  A single field-calibration was then calculated across both study sites using the unadjusted TDR output data (i.e. measurement period in microseconds).  Field-calibrated TDR values resulted in estimated VWC values that differed an average of 3% ± 1% from measured VWC (mean ± standard error, n= 466), but failed to predict soil VWC values > 0.37 m³ m^-3.  Field data were then supplemented with a TDR measurement of an air-dried laboratory soil of similar texture and organic matter content as field soils (i.e. silt loams).  The recalculated field calibration including the dry-soil value resulted in TDR-estimated VWC values that were -0.5% ± 0.3% different than measured values and significantly improved the estimated range of soil VWC.  The use of both field data and a laboratory-determined air-dry soil value is recommended for proper calibration of this hand-held TDR device.