Soil organic carbon sequestration: scoping study

10 Novel technologies

FTIR (Fourier Transform Infrared) Spectroscopy is a non-destructive analysis technique, using an infrared sensor (from the mid infrared region), which can be used to characterise a wide range of materials, including soil. Infrared radiation is absorbed at specific frequencies depending on the chemical composition of the sample which allows producing a chemical “fingerprint” or chemical profile (IR spectrum). For soil analysis the overall chemical profile includes information about both the organic and mineral components. Therefore, the IR spectrum provides an instant insight into the proportion and nature of clay minerals and allows a rapid assessment of the relative amount and nature of the soil organic matter.

FTIR spectra can provide an effective method for prediction of soil parameters using statistical methods that correlate IR spectra to soil properties. Multiple soil properties can be predicted from a single spectrum (for example, %C, %N, pH, bulk density) and therefore the method allows rapid and economical monitoring of soil. In the Lab, prediction of dried milled soil by FTIR has been found to be very accurate when compared to the usual analytical methods. It has been shown to be more accurate than the more widely used technique, NIR spectroscopy, which uses a slightly different frequency range.

A field-based FTIR analysis method, using a handheld instrument, could potentially be very useful for on-farm SOC analysis. Such a method, using a novel sampling accessory, is under development at the James Hutton Institute. However, there remain some challenges to overcome such as being able to account for soil spatial heterogeneity (a problem for all soil sampling), variable moisture content and differing soil textures. In addition, there is a requirement for good spectral libraries (datasets) to get sufficient representation for reliable predictions. Once operational this method could be used both for the soil analysis/ monitoring of SOC but also as a management tool – deployed on farms to assess how current practices are impacting on SOC.

Development of reliable spectroscopic analysis of soil using the mid infrared region is the subject of a global working group on soil spectroscopy recently set up by the FAO under their GLOSOLAN umbrella http://www.fao.org/global-soil-partnership/glosolan/soil-analysis/dry-chemistry-spectroscopy/en/

Although with the handheld FTIR method, samples could be analysed in situ, there is still a requirement to dig a pit to assess SOC % below the topsoil. However, one of the other advantages of using the FTIR technique is that prediction of undisturbed bulk density, needed for calculating C stocks, can also be done reliably by FTIR on the same IR spectrum.

There are also wider benefits to FTIR spectral analysis of soils in that information on the chemical composition the SOM can be gained from interpretation of the IR spectrum of a soil and FTIR spectra can also be linked to soil properties/functions such as available water capacity.