of conservation agriculture practices (e.g., no‐till, permanent soil cover, and crop rotation) have increased SOC. These and other advancements in crop and soil management practices (e.g., drought and insect resistance) have the potential to restore and improve soil health. A reduction in tillage intensity, with no‐tillage being the least disruptive, allows macroaggregate formation and reduces degradation of SOC. Minimum‐ or conservation‐tillage also conserves soil C through decreased oxidation and less soil erosion than with more intensive tillage practices. Cropping intensity and diversification can also impact SOC levels by varying the amount and quality (e.g., C/N ratio) of plant residue added to the soil. Likewise, the presence of cover crop roots during the intercrop period can release exudates into the soil and contribute to greater macroaggregate formation by increasing microbial biomass and fungal networks.
In agriculture, high residue producing crops, such as corn, wheat, and sorghum tend to sustain or increase soil C. Eliminating summer fallow, a practice traditionally used in the U.S. Great Plains, can increase soil C by providing more plant material on an annual basis. In those areas where sufficient soil water is available, double cropping to produce two crops or more crops per year may be a viable practice. In grassland systems, perennial crops also tend to increase SOC because there is no‐tillage, and organic C is added through root turnover.
Functions of SOC
Soil carbon supports essential soil and associated ecosystem functions. Concentrations of SOM range from 0.2% in mineral soils to over 80% in peat soils (Smith et al., 1993). Albrecht (1938) stated “soil organic matter is one of our most important national resources… and it must be given its proper rank in any conservation policy”. Larson and Pierce (1994) listed SOC as one of five key measures of soil that included soil cation exchange capacity (CEC), bulk density (BD), water retention, and aeration.
Furthermore, although SOC comprises a small portion of agricultural soils, it significantly affects soil health (Rice et al., 1996; Doran and Zeiss, 2000; Kibblewhite et al., 2008, Bünemann et al., 2018). Currently, the USDA Natural Resource Conservation Service (NRCS) defines soil health as the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation. Soil organic matter imparts many beneficial biological, chemical, and physical properties to soil, specifically improving its structure (Six et al., 2000; Dexter et al., 2008); supporting water infiltration and retention (Boyle et al., 1989; Yang et al., 2014); reducing erosion through increased infiltration, decreased runoff, and more large aggregates (King et al., 2019; Barthès and Roose, 2002); increasing crop yield through water and nutrient supply (Cambardella and Elliot (1992; Pan et al., 2009; Oldfield et al., 2018); and storing C for climate change mitigation (Paustian et al., 2016; Lal and Follett, 2009).
Biological Effects
The biological benefits of SOC primarily relate to nutrient cycling by soil microorganisms for carbon and energy. Soil microorganisms convert complex plant and animal materials into simpler compounds. The primary SOM decomposers (i.e., consumers) include bacteria, fungi, earthworms, insects, protozoa, and nematodes. All are influenced by soil microbial diversity, which is affected by SOC and the associated land management decisions. For example, no‐till results in SOC being concentrated in the topsoil surface (Doran, 1980; Lynch and Panting, 1980; Carter and Rennie, 1982; Balesdent et al., 2000). The combination of higher SOC and greater biological activity results in greater organism diversity, and potentially to greater biological control of plant diseases and pests.
Chemical Effects
Chemical benefits of SOC relate to the transformation and flow of soil nutrients, such as mineralization and immobilization of N, P, and S. The nine essential macronutrients for plant growth are C, O, H, P, K, S, Ca, and Mg. The first three are in greatest abundance within the SOM structure (Schnitzer and Khan, 1975). However, plants need N for enzymes, proteins, and chlorophyll, which are mainly derived from SOM (Schnitzer and Khan, 1975). Nearly all N in SOM is inaccessible to plants until it is converted by microbes into ammonium (NH4) and nitrate (NO3). The SOM is also a source of P, generally making up 15 to 80% of total soil P (Mortensen and Himes, 1964). Soil S is estimated to be 50 to 70% in or adsorbed onto SOM (Schnitzer and Khan, 1975). Conversely, K, Ca, and Mg are primarily derived from insoluble inorganic compounds and have not been demonstrated to be obtained from SOM (Schnitzer and Khan, 1975). However, K, Ca, and Mg can form complexes with SOM (Broadbent and Ott, 1957; Schnitzer and Skinner, 1963) and the capacity of soils to store mineral nutrients (i.e., CEC), increases with SOM.
Physical Effects
The physical benefits of SOC relate to the formation and stabilization of soil aggregates. Several studies have reported high correlation between soil aggregation and SOC (Wilson et al., 2009; McVay et al., 2006; Six et al., 2002). Degradation of organic materials by soil organisms leads to the formation of humified materials associated primarily with mucilage that surrounds and binds to clay particles, thus developing and binding the particles into microaggregates (Balesdent et al., 2000). The decomposition of protected SOC can become slow due to the clay barrier, thus promoting soil carbon sequestration. Soil health benefits of greater soil aggregation include less crusting, compaction, and bulk density (Diaz‐Zorita and Grosso, 2000); enhanced soil structure for greater water infiltration and water holding capacity (Hudson, 1994; Emerson, 1995; Gupta and Larson, 1979, Yang et al., 2014); decreased soil erosion (Schertz et al., 1994; Benito and Diaz‐Fierros, 1992); and improved aeration for root growth and microbial activity. As tillage intensity increases, soil microbial activity increases right after tillage and microaggregates are dispersed, thus releasing SOM from protection (Puget et al., 1995, 2000).
Crop Productivity
In addition to supporting biological control of crop pests and disease, SOM contributes to agricultural crop yields in various ways by overcoming negative soil conditions. Adequate amounts of SOM can enhance: (i) release of nutrients from decaying organic materials and thus reduce commercial fertilizer requirements; (ii) soil porosity which increases plant available water retention and aeration for root development; (iii) soil structure which reduces soil erosion potential and increases aggregation; and (iv) storage of other nutrients, including an increase in cation exchange sites for plant nutrient retention.
Measurement of SOC
Recognizing there are multiple methods for measuring SOC, dry combustion, loss‐on‐ignition (LOI) (Schulte and Hopkins, 1996), and the Walkley–Black method (Nelson and Sommers, 1996) were evaluated for SOM are evaluated to examine the advantages and disadvantages of each method (Table 3.1). The traditional method for SOM analysis was wet oxidation in potassium dichromate (K2Cr2O7), better known as the Walkley–Black method. The potassium dichromate solution oxidizes soil organic material through a chemical reaction that generates heat when two volumes of sulfuric acid are mixed with one volume of dichromate. The remaining dichromate is titrated with ferrous sulfate, with the titer being inversely related to the amount of C present in the soil sample (Nelson and Sommers, 1996; Meersmans et al., 2009).
Although Walkley‐Black was the standard method for SOM analysis for decades, its use has diminished due to the high potential for environmental pollution during disposal and exposure of personnel to hazardous chemicals, such as potassium dichromate and sulfuric acid. In contrast, the LOI method is a simple, inexpensive method for SOM estimation that involves the combustion of samples at high temperatures and measuring weight loss after ignition. The ability of LOI to quantify SOM content has
