Estimation of soil organic carbon as a function of soil pretreatment and spectral features of radiometers within the visible and near-infrared spectra

Authors: Fortuna, Ann-Marie; STARKS, PATRICK - Retired ARS Employee; Moriasi, Daniel

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2025
Publication Date: 11/3/2025
Citation: Fortuna, A., Starks, P.J., Moriasi, D.N. 2025. Estimation of soil organic carbon as a function of soil pretreatment and spectral features of radiometers within the visible and near-infrared spectra. Journal of Soil and Water Conservation. 80(4):476-490. https://doi.org/10.1080/00224561.2025.2528348.
DOI: https://doi.org/10.1080/00224561.2025.2528348

Interpretive Summary: Soil health is defined as the “continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”. Scientists at the United States Department of Agriculture are involved in applying measurements of soil health to assess the effect of conservation practices on the Nation’s soil resources. One of the most critical gaps of soil health assessments is the volume of soil samples needed. Hundreds of samples are typically required to evaluate a single farm (10s to 100s of acres or hectares), and thousands of samples are needed for assessment of a watershed (miles or kilometers). Standard methods used to measure total soil carbon are destructive and require extensive preparation that includes sieving, drying, grinding, and weighing prior to analyses. Additionally, the process of analyzing soil samples for total soil carbon generates waste. Low-cost alternatives to laboratory-based methods of measuring total soil carbon using portable radiometers have been explored. Radiometers are instruments used to detect and monitor radiant energy, a proxy for various physical and chemical characteristics of soil. Estimates of total soil carbon concentrations are estimated by measuring reflected and emitted radiation at a distance rather than destructively sampling and measuring total soil carbon directly in the laboratory. Radiometers must be calibrated and validated against laboratory measurements of total soil carbon prior to application of radiometry. Radiometer scans can be used across a range of environments, replace preparation required to analyze samples for direct chemical measurements, and reduce the number of direct chemical measurements of total soil carbon required to one tenth of the total soil samples collected. Stored scans can be used to develop a library of total soil carbon measurements used to standardize multiple radiometers across the United States and globally. Application of the information derived from this study enables research at the Southern Plains Long-Term Agriculture Research (LTAR) site to build calibrations for use in monitoring carbon seasonally, annually and in perpetuity. The LTAR Network combines strategic research projects with common measurements on multiple agroecosystems (croplands, rangelands, and pasturelands) and provides common measurements and data streams that complement other federally funded national networks. Future research will apply this approach across multiple LTAR sites. USDA is an equal opportunity provider and employer.

Technical Abstract: Soil health assessments have become one of the primary measures of successful implementation of the United States Department of Agriculture, Natural Resource Conservation Service’s conservation practices codes and programs. Despite national and global implementation of soil health standards there is a gap in adoption and standardization of rapid high throughput proximal measures of soil health. This research focuses on a single indicator of soil health, soil organic carbon (SOC) measured via proximal sensing using visible-near infrared (VIS-NIR) sensors in the wavelength region of 350 – 2500 nm(nanometres). Soil organic carbon is correlated with other chemical, physical and biological indicators of soil health and is part of the efforts being made globally to assess carbon stocks. Our objectives were to determine the minimum pretreatment(s) required to obtain accurate estimates of SOC concentrations via several radiometers within the 350 -2500 nm range, compare lower cost spectroradiometers in the 400 – 1000 nm range used for rapid assessment of SOC to the higher cost wider ranging radiometers, and to address some of the contradictory results of soil sample treatment effects existing in the current literature. The calibration equations developed using soil spectra did not necessarily improve if pre-treatment included sieving and/ or air-drying of soil suggesting that direct in field scanning of soils may be a viable option when large numbers of samples are being collected in a time-limited situation. We recommend baseline evaluation of soil organic carbon be conducted with a field radiometer in the range of 350-2500 nm due to the overlap of SOC with carbon constituents that are of plant, animal or microbial origin and soil pre-treatment that initially includes sieving and air drying. After which, rapid assessment and monitoring of SOC can be accomplished using cheaper, easier to operate radiometers within the 300-1000 nm range that can undergo calibration transfer to transform spectra prior to use on the secondary instrument located in a different environment. Future research will apply this approach across multiple Long-Term Agriculture Research (LTAR ) sites located across 19 locations within the United States representing variable soil, agronomic land use, and climate. USDA is an equal opportunity provider and employer.