The natural cohesion between soil structure, soil organic matter (SOM) and plant and animal life living within soil (soil biota) is fundamental for the process of carbon cycling, and therefore it has a vital impact on future climate. Although there's been decades of research into the carbon cycle within soil, definitive descriptions of microbiological activity that occurs in soil, and, by extension, what happens to the carbon at this stage of the cycle, are still missing. Crucially it is not understood how the complex pore structure found in soil and the distribution of the organic carbon within it affects the soil biota and its activity. This could be because there is no current method that allows reliable three-dimensional mapping of organic matter in intact soil at scales larger than a few millimetres.
Scientists from the Swedish University of Agricultural sciences (Uppsala, Sweden), University of Manchester (UK), Agroscope Reckenholz-Tanikon (Zurich, Switzerland), CLF-Scitech Precision Ltd (UK), Diamond Light Source (UK) and ISIS Neutron and Muon Source (UK) worked in collaboration to investigate the potential of combined neutron and X-ray imaging to quantify local carbon contents in soil.
Soil samples were collected from the topsoil of four long term field experiments in southern Sweden, and two in eastern Sweden. These sites were all under crop rotations that are in line with the typical local climate conditions. The samples each had different amounts of SOM within them. As well as the samples taken, the scientists made composite samples from illite clay powder, apatite and quartz sands as well as sawdust in order to create samples of different mineral composition and known organic matter content to benchmark against.
To analyse the samples, the cross-disciplinary team used bothX-ray and time-of-flight (TOF) neutron imaging to map the 3-dimensional organic carbon distribution in soil. It is well known that neutron and X-ray beams have complementary attenuation properties. Soil minerals are largely made up from silicon and aluminium, which attenuate X-rays but not neutrons. However, the attenuation of neutrons is strong for hydrogen, which is extremely plentiful within the SOM.
The researchers collected TOF neutron imaging data at the IMAT beamline at ISIS, synchrotron X-ray computed tomography at the I12 beamline at the Diamond Light source, X-ray radiographies at CLF-Scitech Precision Ltd and X-ray diffraction at the ISIS Materials Characterization Lab, alllocated within the Rutherford Appleton Laboratory.
The study of the soil conducted by the scientists, published in European Journal of Soil Science, shows that a combination of neutron and X-ray imaging does enable the identification of different materials within soils.
The data collected by TOF neutron imaging have the potential to quantify the amount of specific minerals in the matrix regions of the soil. These regions are made up from pores, various minerals and SOM. Nevertheless, there are limitations on these data. The current level of TOF neutron technology means that the data collected has too much background interference or 'noise', resulting in the conversion of this data into 3-dimensional modelling extremely difficult. To get high enough resolution, the image acquisition times for a single sample would need to be increased to several weeks.
However, the scientists did find that the combination of X-ray and TOF neutron imaging demonstrated the ability to identify quartz grains within the soil as well as distinguish between plastics and plant seeds.
“By combining complementary advanced characterization techniques available at ISIS, CLF and Diamond in our research, the investigation of the organic carbon distribution in soil reached a whole new level and this could help the scientific community to develop more projects in this direction," says ISIS scientist Dr Genoveva Burca.
The full paper can be found online at https://doi.org/10.1111/ejss.13178