1. Introduction
Agricultural site categorizations or environmental evaluations tend to include a measurement of total organic carbon because its presence or absence can significantly affect how compounds react in the soil or sediment. The amount of soil organic carbon (SOC) in soils and sediments can be determined using a variety of techniques. The typical origins of carbon include weathering of the parent rock or geology, breakdown of plant and animal materials, and human activity. The SOC can be identified and quantified using a wide variety of approaches and modifications of those methods. Soil organic carbon (OC) serves as an invaluable indicator of soil health, and plays a pivotal role in mitigating climate emissions through crop uptake and sequestration in agricultural soils. Agriculture and Agri-Food Canada data have demonstrated that Canadian agricultural soils possess the potential to annually capture 11.9 million tonnes of CO
2 emissions from the atmosphere [
1,
2,
3,
4]. According to the latest data from the 2021 Census of Agriculture by the Government of Canada, the provinces of Alberta and Ontario stand out as the epicenter of Canadian agriculture, serving as the areas for analysis and investigation. Both provinces comprise nearly 40% of Canada’s total farm area, which makes them an exceptional setting for assessing carbon sequestration processes through stable SOC.
Accurate, rapid, and cost-effective estimation of TOC in soil is vital for farmers looking to address climate change mitigation strategies and claim future carbon credits. To this end, the Pan-Canadian Framework on Clean Growth and Climate Change emphasizes the emissions reporting options that allow traders to trade verified carbon credits both domestically and internationally [
5].
Currently, soil OC can be measured either destructively or nondestructively using methods such as the Walkley–Black method and dry combustion (DC) [
6,
7,
8,
9,
10,
11,
12]. The Walkley–Black method is an efficient and widely employed technique for measuring soil organic carbon (SOC). Based on the principle of oxidative digestion, organic matter in soil is decomposed and converted into carbon dioxide via this approach. The procedure involved mixing potassium dichromate and concentrated sulfuric acid in a digestion flask containing soil samples for testing, and then heating this solution until the organic carbon in the soil was oxidized by dichromate ions. As part of this oxidation process, the color of the solution changes from orange to green due to dichromate ion reduction, and the solution should be allowed to react for an appropriate length of time to ensure the complete digestion of organic carbon. After digestion, any excess dichromate is titrated against a ferrous ammonium sulfate solution to determine its consumption; this consumption is proportional to the organic carbon levels in the soil samples. The SOC content was calculated using a calibration factor determined from standard solutions with known concentrations of organic carbon. The Walkley–Black method offers an effective yet simple means for estimating soil organic carbon, making it popularly utilized by soil analysis laboratories and research studies alike [
6,
7,
8,
9,
10,
11,
12]. The DC method has long been recognized and widely adopted for evaluating the organic carbon content in soil. This technique involves subjecting soil samples to elevated temperatures within a furnace where organic carbon can be converted to carbon dioxide; the technique has proven to be accurate and reliable for measuring this form of organic matter in testing laboratories worldwide [
6,
7,
8,
9,
10,
11,
12].
However, these methods of analysis can be slow, difficult, and expensive, or require toxic chemicals for analysis. To provide an alternative solution, MANTECH Inc. (Guelph, ON, Canada) offers PeCOD analyzers (that use photochemical oxidation of organic carbon catalyzed by titanium dioxide as an analytical technique. As the oxidation reaction progresses, the excess electrical charge increases proportionately with increasing organic carbon consumption. An analyzer measures this generated charge by plotting the output current (I) against time (t), with its area under the curve directly related to the TOC content in a sample. Although our software has been configured to convert the integrated area into chemical oxygen demand (COD) values and estimate the organic matter content based on these COD values [
13], there is no reliable method for analyzing soils or interpreting soil-derived data. The success of the research project would be determined if the TOC analysis protocol developed could accurately and precisely identify soil TOC content with at least 95% accuracy and 90% precision, such as standard TOC methods [
14,
15,
16,
17,
18]. As part of its validation process, its results should be compared with those generated from standard methods such as the Walkley–Black method, DC, and spectral analyses for comparison purposes.
Monitoring soil organic carbon levels is essential for ensuring that agricultural practices either maintain current levels or contribute to increasing them and sequestering more carbon [
19]. Certain agricultural practices, including tillage, the removal of plant residues, and a lack of soil coverage between crop seasons, are known to help lower TOC in soil. In contrast, climate-smart approaches that aim at increasing soil TOC through no-till agriculture, cover crop use, and soil amendment with organic or biochar-derived materials such as biochar and humic substances have become popular among farmers [
20]. Research currently being undertaken is exploring the effects of approaches that result in soil inorganic carbon sequestration, such as enhanced rock weathering and soil remineralization, on organic carbon pools [
21]. Soil organic carbon can also be considered meta-stable, as its degradation could release greenhouse gases such as CH
4 and N
2O into the environment. With all these research topics—many already adopted by farmers—it is imperative that reliable, cost-effective, and swift methods of measuring TOC levels across spatial and temporal scales consider both soil heterogeneity in terms of area and depth [
19].
For this project, the research team introduced the fractionation of soils and used geographic information system (GIS) method to geo-reference soil sampling locations for use as a viable aid to determine the organic carbon content for current and future prediction purposes. Physical fractionation has become an effective method to gain greater insight into soil behavior and assess organic carbon content. Essentially, this technique involves segregating soil samples according to particle size, such as sand, silt, and clay, and then analyzing their organic carbon content within each fraction. Studies have demonstrated that soils with high clay content typically have relatively high organic carbon concentrations, likely attributable to their increased surface area and cation exchange capacity, enabling the retention and stabilization of organic matter. Fractionation has long been employed as a way of investigating organic matter distribution within soils, as well as in developing effective management practices to promote soil health and sequestration of carbon dioxide emissions [
22].
GIS software has become increasingly popular as an approach for georeferencing soil sampling locations and producing spatially explicit maps depicting organic carbon content. GIS allows researchers to visualize and analyze soil data, making it easier to identify spatial patterns and trends related to organic carbon levels in various land use types, such as croplands, forests, or grasslands, as well as to investigate the potential effects of land use changes and management practices on carbon dynamics. It has even been employed in studies that aim at mapping and quantifying organic carbon levels within specific land uses, such as cropland forests and grasslands, creating maps of organic carbon content within various land use types that investigate impacts from land use changes and soil management practices on soil carbon dynamics [
23]. Finally, the upcoming sections of this paper demonstrate the unique on-site approach to determining the organic carbon content in soils.
2. Materials and Methods
Measuring soil organic carbon (SOC) content using the PeCOD “Photocatalytic Chemical Oxygen Demand” analyzer involves some specific adaptations that must be made for each instrument being utilized. Sampling soil samples that accurately represent the characteristics of the study area is an essential step in soil analysis, as their accuracy plays an integral role in producing quality results. As soon as a sampling area is identified, it is ensured that all samples taken represent the larger study area by considering variables such as texture, color, vegetation density, and topography, as well as land use and topography. The sampling area is subdivided according to any variations observed in soil properties; the exact number of subareas depends upon both its size and the extent of variability present. Sampling points are chosen at patterns within each subarea for fair representation; alternatively, systematic sampling is used if a specific distribution pattern is needed.
At each sampling point, an auger was used to collect soil samples at various depths (for instance, 0–10 cm, 10–20 cm, and 20–30 cm) to produce a vertical profile of the soil properties. Composite samples are created by combining soil samples collected at each depth at each sampling point into one composite sample that represents the soil properties in their subarea. All the composite samples were identified by providing information such as the sampling point location, depth, date, and collection time. Furthermore, they are stored in airtight and moisture-proof containers to avoid contamination or moisture loss. The sampling process was repeated until representative soil samples from across the entire study area were obtained.
After collection, the samples were prepared by removing visible roots or stones and performing sieve analysis following ASTM C-136–01 standards [
24]. Sieve analysis is an efficient method for estimating the particle size distribution of soil. Sets of sieves featuring mesh sizes of 2.0 mm, 250 µm, 75 µm, and 50 µm were used, along with weighing balances, ovens, cleaning brushes, containers for collecting the soil passing through the finest sieve, gloves, and safety glasses. The soil samples were prepared by clearing away any large stones or plant material and breaking up any clumps. Each soil sample was weighed to the nearest milligram and then heated in an oven to 105 °C for 24 h to remove moisture. The temperature gradient (°C/min) was approximately 3.33 °C/min for the oven used. It should be noted that the oven was preheated to the desired temperature, after which the soil samples were placed in the oven. The sieves were assembled for use and carefully cleaned using a cleaning brush to ensure that they were completely dry before use. Furthermore, the sieves were arranged in order of decreasing mesh size, beginning with the largest size and ending with the smallest size, starting with the collection pan at the base. The soil samples were placed onto the top sieve and shaken vigorously horizontally and vertically for 10–15 min until no more soil passed through the finest sieve. The soil retained on each sieve was weighed with a precision of up to one milligram, and was recorded for further analysis. The percentage of soil retained on each sieve was calculated by dividing its weight by its initial weight and multiplying by 100 (Equation (1)).
It should be noted that while particle size classification (e.g., sands, silts, and clays) plays a part in soil organic carbon content, other factors, such as climate, vegetation, soil type, and management practices, also impact how organic carbon is distributed throughout soil layers and managed [
25]. Soils with higher clay content typically exhibit higher organic carbon content due to the large surface area and cation exchange capacity of clay particles, which help protect organic matter retention and increase organic matter protection [
26]. Sandy soils tend to have lower organic carbon content due to their reduced cation exchange capacity, while silt soils fall somewhere between sandy and clayey soils in terms of organic carbon content. After sieve analysis was performed, the samples were segregated, and an analysis method was used. The three (3) main organic carbon analysis methods carried out in this project were the Walkley–Black, LOI, and photocatalytic COD analyzer methods. The purpose of this study was to investigate the validity and reliability of the photocatalytic COD analyzer method compared to currently accepted methods.
First, the loss on ignition (LOI) method can be used to measure organic carbon levels in soils by measuring mass differences before and after combustion. This process typically entails several steps. First, the crucible and sample are weighed to establish an initial weight, and they are placed into a muffle furnace for exposure to high temperatures for a designated timeframe [
27]. Soil is placed in a furnace at temperatures ranging between 350 and 440 °C [
27], but the research team investigated temperatures ranging between 400 and 600 °C to explore the different possibilities resulting in higher temperatures. At this stage, organic matter found in the soil sample is burned away through combustion, leaving behind only inorganic matter. When complete, the remaining soil residue and the crucible are allowed to cool before being weighed once more to determine their weight. By subtracting that figure from that of the initial weight of the soil sample and subtracting the weight of the remaining residue from the initial weight of the sample, the organic carbon content can then be calculated and expressed as a percentage (Equation (2)).
This method using mass differences offers an efficient and cost-effective method for estimating soil organic carbon content. However, it should be noted that this approach does not distinguish between types of organic matter or inorganic carbon, and may introduce some inaccuracies in its results. Precise control over the combustion temperature and duration is crucial for obtaining accurate outcomes with this approach. Although the LOI method provides an estimate of total organic carbon based on its high ratio to other constituents in organic matter, when applying this method, it is essential to consider certain factors. Structural water loss during heating could result in weight loss due to structural water evaporation; this may misrepresent the organic matter content of the soil sample. Treating with hydrochloric acid prior to ignition could mitigate this effect; however, the presence of HCl could result in some organic material dissolving into the solution. LOI methods are well known for being simple, affordable, and low-risk; no harmful chemicals are necessary [
27]. Although the LOI method estimates organic carbon content accurately, given its diverse nature, more investigations are needed to fully comprehend how organic matter differs among soil types to ensure accurate estimations.
One of the other current methods investigated in this project is the Walkley–Black method. The Walkley–Black method is a wet oxidation procedure used to determine total organic carbon levels in soil samples [
28]. While it is considered to be semiquantitative due to requiring a correction factor specific to each sample being examined, this approach remains viable in many situations. This method assumes that organic carbon is the main reducing agent in soil organic matter and that the proportions of hydrogen and nitrogen are directly related to its carbon content. Oxidation reactions involve the use of a concentrated solution of sulfuric acid and potassium dichromate (K
2Cr
2O
7) for organic carbon conversion to carbon dioxide (CO
2) [
28]. This results in the release of carbon dioxide gas as a product. Reduced dichromate ions are then back-titrated using ferrous sulfate (FeSO
4) to determine any excess dichromate, while the organic carbon content is calculated by comparing the volume of titrant used during soil sample titration to that used during blank titration, typically with an approximate correction factor of 1.3 being applied, as this helps account for incomplete combustion and recovery of organic carbon using this method. The calculation of the organic carbon content in the soil samples was performed by using an equation (Equations (3) and (4) [
28]) that considers both the volumes of the titrant used during the soil sample titrations and those used during the blank titrations. The organic carbon content of the soil samples was calculated and utilized to establish their final organic carbon content [
29].
Abbreviations/definitions:
%WBC: Organic carbon in the soil sample measured in the reaction.
VBlank: Volume of titrant in blank titration.
VSample: Volume of titrant in sample titration.
M_(Fe2+): Concentration of ferrous sulfate.
mcf: Moisture correction factor.
W: Weight of the soil sample.
For the utilization of PeCOD, organic carbon in the soils was extracted in liquid form from these samples to be fed directly into a photocatalytic COD analyzer for further evaluation. To prepare the soil sample for analysis using the COD analyzer, 0.25 g of soil was weighed, and 50 mL of deionized water was added to a 100 mL cup with a sealable lid. The mixture was mixed manually for one minute and was allowed to settle for 30 min before being transferred to a 50 mL test tube and centrifuged for 15 min. Finally, using a syringe with a 45 µm filter tip, 20 mL of this solution was removed and then transferred to another 50 mL test tube, as this solution became the sample solution used for further analyses.
For the PeCOD sample setup, 15 mL of green range electrolyte was added to a test tube containing 15 mL of soil sample and mixed well using a vortex mixer. As an additional step in the preparation of the PeCOD samples, a blank sample was prepared by mixing 15 mL of Milli-Q water with 15 mL of the green range electrolyte in separate test tubes. Next, the PeCOD was calibrated by following the software setup steps, using a 1:1 ratio between the calibrant and electrolyte solutions for the green range in Port A and deionized water in Port B for calibration of the green range electrolytes. To run the PeCOD analysis, the prepared soil sample was placed in Port A, and the blank solution was placed in Port B. The analysis was started by choosing the desired number of replicates, and the data sets were collected/downloaded after every run. At the conclusion of each run, both ports were flushed with deionized water to ensure adequate cleaning between samples. The PeCOD analyzer measures the amount of oxygen necessary to oxidize organic carbon present in samples. Photocatalysis involves ultraviolet (UV) radiation stimulating a catalyst in the presence of oxygen and excitation, which generates free holes and electrons that then initiate oxidation reactions. Titanium dioxide (TiO
2) has proven to be an efficient catalyst for these reactions because of its excellent efficiency, stability, low toxicity, and cost-effectiveness. For optimal photocatalysis with TiO
2, wavelengths under 400 nm are necessary (approx. (λ = 354 nm)). PeCOD relies on titanium dioxide as its photocatalyst, whereby UV light exposure excites valence electrons to release free holes and electrons that then create oxygen radicals that react with organic material, breaking it down to carbon dioxide and water. As oxygen radicals react with the organics present, more free holes are freed, creating a photocurrent that measures the sample oxidation demand. Research shows that TiO
2 photocatalysis is a powerful tool for removing soil polluted by pesticides, PAHs, or petroleum hydrocarbons. It seems that the speed of these clean-up reactions is closely related to how quickly organic matter breaks down and how efficient the whole process is. Multiple scientists have also investigated the UV absorbance levels and concentrations of COD and SOC in new photocatalyst solutions that are currently available on the market. Experiments examining total organic carbon (TOC) in water have some serious potential, especially in regard to identifying places with TOC contamination. Getting these soils back to their original state is not as easy; it could become tricky because each site has its own set of challenges. Therefore, studies have focused on using TiO
2 photocatalysis specifically for tackling issues caused by pesticides and PAHs in soils [
30,
31,
32].
To calculate the organic carbon content, the COD value obtained during analysis was used. The conversion factors varied according to the soil type, and could be obtained either through the literature sources or calibration curves using known organic carbon standards. Additionally, soil samples collected for sampling were georeferenced with GIS technology using GPS devices, and then were processed through GIS software to generate maps that depict the organic carbon distribution across their study areas. Interpretation of the results obtained was accomplished using spreadsheet software. Statistical measures such as the mean and standard deviation were computed to summarize the organic carbon content data, while GIS maps generated earlier allowed for the identification of areas with high or low organic carbon concentrations for spatial analysis and interpretation. To ensure the accuracy and reliability of the results, it was crucial to follow standard protocols and implement quality control procedures at each step of the process. It was also necessary to consider factors such as sample representativeness, COD analyzer calibration, and GPS device quality when interpreting the results.