24 May 2024
 | 24 May 2024
Status: this preprint is open for discussion.

Large errors in common soil carbon measurements due to sample processing

Rebecca J. Even, Megan B. Machmuller, Jocelyn M. Lavallee, Tamara J. Zelikova, and M. Francesca Cotrufo

Abstract. To build confidence in the efficacy of soil carbon (C) crediting programs, precise quantification of soil organic carbon C (SOC) is critical. Detecting a true change in SOC after a management shift has occurred, specifically in agricultural lands, is difficult as it requires robust soil sampling and soil processing procedures. Informative and meaningful comparisons across spatial and temporal time scales can only be made with reliable soil C measurements and estimates, which begin on the ground and in soil testing facilities. To gauge soil C measurement inter-variability, we conducted a blind external service laboratory comparison across eight laboratories selected based on status and involvement in SOC quantification for C markets. To better understand how soil processing procedures and quantification methods commonly used in soil testing laboratories affect soil C concentration measurements, we designed an internal experiment assessing the individual effect of several alternative procedures (i.e., sieving, fine grinding, and drying) and quantification methods on total (TC), inorganic (SIC), and organic (SOC) soil C concentration estimates. We analyzed 12 different agricultural soils using 11 procedures that varied either in the sieving, fine grinding, drying, or quantification step. We found that a mechanical grinder, the most commonly used method for sieving in service laboratories, did not effectively remove coarse materials (i.e., roots and rocks), thus resulted in higher variability and significantly different C concentration measurements from the other sieving procedures (i.e., 8 + 2 mm, 4 mm, and 2 mm with rolling pin). A finer grind generally resulted in a lower coefficient of variance where the finest grind to < 125 µm had the lowest coefficient of variance, followed by the < 250 µm grind, and lastly the < 2000 µm grind. Not drying soils in an oven (at 105 °C) prior to elemental analysis on average resulted in a relative difference of 3.5 % lower TC, and 5 % lower SOC due to inadequate removal of moisture. Compared to the reference method used in our study where % TC was quantified by dry combustion on an elemental analyzer, % SIC was measured using a pressure transducer, and % SOC was calculated by the difference of % TC and % SIC, predictions of all three soil properties (% TC, % SIC, % SOC) using Fourier Transformed Infrared Spectroscopy were in high agreement (R2 = 0.97, 0.99, 0.90, respectively). For % SOC, quantification by loss on ignition had a low coefficient of variance (5.42 ± 3.06 %) but the least agreement (R2 = 0.83) with the reference method.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Rebecca J. Even, Megan B. Machmuller, Jocelyn M. Lavallee, Tamara J. Zelikova, and M. Francesca Cotrufo

Status: open (until 21 Jul 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
Rebecca J. Even, Megan B. Machmuller, Jocelyn M. Lavallee, Tamara J. Zelikova, and M. Francesca Cotrufo

Data sets

Soil carbon measurements R. Even

Rebecca J. Even, Megan B. Machmuller, Jocelyn M. Lavallee, Tamara J. Zelikova, and M. Francesca Cotrufo


Total article views: 601 (including HTML, PDF, and XML)
HTML PDF XML Total Supplement BibTeX EndNote
412 182 7 601 26 7 9
  • HTML: 412
  • PDF: 182
  • XML: 7
  • Total: 601
  • Supplement: 26
  • BibTeX: 7
  • EndNote: 9
Views and downloads (calculated since 24 May 2024)
Cumulative views and downloads (calculated since 24 May 2024)

Viewed (geographical distribution)

Total article views: 603 (including HTML, PDF, and XML) Thereof 603 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
Latest update: 15 Jun 2024
Short summary
We conducted a service soil laboratory comparison study and tested the individual effect of common sieving, grinding, drying and quantification methods on total, inorganic, and organic soil carbon (C) measurements. We found that inter-lab variability is large and each soil processing step impacts C measurement accuracy and/or precision. Standardizing soil processing methods is needed to ensure C measurements are accurate and precise, especially for C credit allocation and model calibration.