Parameters that determine soil organic matter (SOM) status, soil health, and functions are generally difficult to measure directly. Therefore, they are evaluated by deriving indicators that correlate with soil conditions. Soil condition indicators may be chemical, physical, or biological, and can be either descriptive or quantitative. Descriptive indicators are qualitative and are used in the field, while quantitative indicators are assessed by laboratory analytical procedures. Because total soil organic matter is often not sensitive enough to small and short-term changes due to its complexity levels and background, some studies have recommended using soil organic matter fractions (sub-pools) as more sensitive indicators to detect even small changes over a short period of time. These fractions or sub-pools have been classified by various researchers based on their formation, levels, and ease of decomposition. They include labile, less-stable, and stable fractions. The most labile fraction can decompose in less than a year or two, while the actively decomposing fraction, including partially stabilized organic material from plants and microbial metabolites, may have a turnover of up to 26 years. There is also a chemically stabilized and resistant fraction with a radiocarbon age of up to 2500 years. Quantitative analysis of SOM can be performed using various parameters, including oxidation kinetics, lability, carbon management index, humification degree, humification index, and humification ratio. On the other hand, qualitative evaluation of SOM can involve techniques such as oxidizability, high-performance size-exclusion chromatography, electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, visual examination, smell, assessment of microorganism content, plant growth, cation exchange capacity, type of organic material, and decomposition. These techniques and parameters provide valuable insights into the characteristics and transformation of SOM, enabling a comprehensive understanding of its dynamics. Evaluating SOM dynamics is of utmost importance as it is a determining factor for soil health, fertility, organic matter stability, and sustainability. Therefore, developing SOM models and other assessment techniques based on soil properties, environmental factors, and management practices can serve as a tool for sustainable management. Long-term or extensive short-term experimental data should be used for modeling to obtain reliable results, especially for quantitative SOM transformation analysis, and changes in the quality and quantity of SOM should be considered when developing sustainable soil management strategies.
1. SOM Fractionation
Soil organic matter can be physically, chemically, and biologically fractionated to help in better understanding its composition, properties, and functions in soil 
. Physical fractionation involves separating SOM based on its particle size, where smaller particles usually have higher decomposition rates and contain more labile SOM fractions. One of the commonly used physical fractionation methods is density fractionation, which separates SOM into different fractions based on their densities using a heavy liquid (e.g., sodium polytungstate) 
. Chemical fractionation separates SOM based on its chemical properties, where different fractions are at different degrees of decomposition, stability, and reactivity. The most common chemical fractionation method is the acid hydrolysis or oxidation method, which separates SOM into different fractions based on their oxidation or solubility in acid solutions of varying strengths 
. Biological fractionation separates SOM based on its microbial accessibility, where different fractions have different microbial decomposability and utilization. One of the commonly used biological fractionation methods is the substrate-induced respiration (SIR) method, which measures the microbial respiration rate of SOM fractions incubated with a specific substrate (e.g., glucose). This method measures fungal, bacterial, and total microbial contributions to glucose-induced respiration and the potentially active microbial biomass on decaying plant residues of differing composition 
2. Quantitative Techniques for SOM Measurement
Quantitative soil organic matter measurement techniques can be broadly classified into two categories, dry combustion methods and wet oxidation methods 
. Dry combustion methods include thermal exchange, loss on ignition (LOI), and Walkley–Black (WB) methods. These techniques rely on the complete combustion of soil samples to determine the organic carbon content. Thermal exchange involves heating a soil sample in a furnace under an inert atmosphere and measuring the evolved CO2
. The LOI method involves heating a soil sample to a high temperature to burn off organic matter, and measuring the weight loss. The WB method involves adding a dichromate-sulfuric acid reagent to a soil sample, which oxidizes the organic matter and releases CO2
, which is then measured 
Wet oxidation methods involve the oxidation of dissolved organic material with dissolved oxygen at high temperatures. A strong oxidizing agent is used to release carbon dioxide or to change the absorbance properties of the soil sample. Wet oxidation methods include wet digestion and near-infrared reflectance (NIR) 
. Digestion methods breakdown soil samples using acid (such as hydrochloric acid and hydrogen peroxide) digestion to release soil organic matter, while the NIR is a non-destructive method that uses the interaction of infrared light with organic matter to estimate the quantity and quality of organic matter by measuring using a spectrophotometer the reflectance or absorbance of near-infrared light by the soil sample. The choice between dry and wet methods depends on the accuracy and precision needed, time, resources, and all organic matter properties needing to be measured 
3. Qualitative Techniques for SOM Measurement
Qualitative techniques used to assess SOM are descriptive and involve visual examination. These techniques entail assessing physical properties of the soil such as color, texture, and structure 
. A positive correlation has been observed between soils with high organic matter content and darker color, crumbly texture, and granular structure. Additionally, the smell test can be employed to identify organic matter, as soils with high organic matter content often have a rich, earthy smell. The number of earthworms, the soil crumb test, infiltration rate, plant growth, organic matter color, and crop residue decomposition can all serve as qualitative indicators of soil organic matter quality. Although cation exchange capacity (CEC) can also be used, other factors such as soil texture, pH, and mineral content can influence its accuracy 
Other important qualitative techniques include using infrared spectroscopy where infrared (IR) radiation identifies and quantifies functional groups in SOM. It can provide information on the composition and structure of soil organic matter as it measures the absorption and transmission of infrared light by soil organic matter functional groups, providing information on SOM quality, quantity, and composition. IR spectra can be collected from bulk soil samples, or from specific SOM fractions obtained by soil fractionation 
. Fluorescence spectroscopy is another technique that uses the fluorescence properties of soil organic matter to characterize its composition and structure. It can provide information on the humification degree, aromaticity, and molecular weight of soil organic matter. It measures the emission of light from soil organic matter after excitation with ultraviolet or visible light. Fluorescence spectra are sensitive to SOM quality and can be used to assess changes in SOM quantity and quality due to management practices or environmental factors 
. Pyrolysis mass spectroscopy (PyMS) is a technique that uses high temperatures to decompose soil organic matter into smaller fragments, which are then analyzed using mass spectrometry. It measures the mass and abundance of pyrolysis products generated from SOM upon heating to high temperatures in the absence of oxygen. PyMS provides information on SOM functional groups and the distribution of carbon and nitrogen within SOM molecules 
. Nuclear magnetic resonance (NMR) is a technique that uses magnetic fields to analyze the structure and composition of soil organic matter. It can provide information on the molecular structure, functional groups, and chemical bonding of soil organic matter. It measures the relaxation times of nuclei within SOM molecules in response to a magnetic field 
. High-performance size-exclusion chromatography is another technique that separates soil organic matter into fractions based on their size and chemical composition. It can provide information on the molecular weight, size distribution, and chemical composition of soil 
. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) is a technique that measures the mass-to-charge ratios of SOM molecules. It combines electrospray ionization with Fourier transform ion cyclotron resonance mass spectrometry. It informs us about the molecular weight, elemental composition, and functional group composition of individual organic molecules, which helps in processes that control the SOM dynamics 
. Likewise, it should be noted that the use of both quantitative and qualitative methods is often advised for a more comprehensive assessment of soil health and organic matter content.
4. Lability and Stability of Organic Matter in Soils
SOM is a fraction of soil composed of a heterogeneous mixture of plants and macro- and micro-organisms at different stages of decomposition. Their quality, quantity, and decomposition are essential for soil health, nutrients, and carbon cycling. Therefore, understanding the relationships between SOM fractions (labile fraction, stable fractions, and total carbon stock) can help to express the relative degree of lability, stability, and humification of overall soil organic matter, hence carbon management index development. It should also be added that variations in these sub-pools have been used in developing and understanding SOM dynamics models 
. The constant supply of organic matter carbon depends on the available stock and estimated turnover speed. Hence, quantitative assessment of soil organic matter dynamics and its lability and stability could be monitored early using parameters such as carbon pool index (CPI), lability (L), lability index (LI), carbon management index (CMI) 
, carbon stock 
, humification index (HI), humification degree (HD), and humification rate (HR) 
to be able to manage the available SOM changes.
where TOC is total organic carbon, TC is total carbon, Ds is soil bulk density (g. cm−3
), e is the thickness of the layer (cm), CPI is carbon pool index, L is lability, LI is lability index, CMI is carbon management index.
where HI is humification index, HD is humification degree, HR is humification ratio, NH is non-humified (labile) fraction, H is humified (Humic acid + Fulvic acid = non-labile) fraction, TOC is total organic carbon, and TC is total extracted carbon.
The assessment of soil organic matter (SOM) is crucial for understanding soil health, fertility, and sustainability in agricultural systems. Various parameters and indicators have been developed to evaluate SOM status, including quantitative and qualitative techniques. These techniques provide valuable insights into the composition, properties, and functions of SOM.
Quantitative techniques such as dry combustion and wet oxidation methods offer precise measurements of organic carbon content in soil samples. These methods rely on complete combustion or oxidation to determine SOM levels accurately. On the other hand, qualitative techniques involve descriptive evaluation through visual examination, smell tests, and assessments of physical properties like color, texture, and structure. These qualitative indicators can provide valuable information about the quality and characteristics of SOM.
Fractionation of SOM into labile, less-stable, and stable fractions allows for a better understanding of its composition and decomposition rates. Physical, chemical, and biological fractionation methods help in distinguishing different fractions based on their properties and microbial accessibility.
Parameters such as humification index, humification degree, humification ratio, carbon pool index, lability, and carbon management index serve as indicators of SOM stability, humification levels, and carbon cycling. Monitoring changes in these indicators can provide insights into the dynamics of SOM and its response to management practices and environmental factors.
Developing models and assessment techniques based on soil properties, environmental conditions, and management practices can contribute to sustainable soil management. Long-term experimental data and consideration of changes in SOM quality and quantity are crucial for reliable and accurate modeling and analysis of SOM transformation.