Organic Matter: The Whole Truth and Nothing but the Truth

Abstract

Carbon is an essential element and building block of all living organisms. The carbon cycle is the key element to soil function and ultimately soil health. Understanding the intricacies of soil health requires a rudimentary knowledge of the carbon cycle. Other nutrient cycles, nitrogen, phosphorus, etc. are closely interconnected with organic matter and its disposition in the environment. Discussions and publications on organic matter in the popular literature abound and include articles about the value of cover crops, manuring, and rebuilding soil health. Unfortunately, some popular articles overstate or exaggerate some of the facts concerning the restoration of soil organic matter in depleted soils. In this chapter, we will explore the carbon cycle with examples of losses and gains and carbon balance examples for cropland, range, and garden settings.

Organic Matter Fact Sheet 

Global Carbon

• The carbon cycle involves four basic carbon reservoirs, the atmosphere, oceans, terrestrial biosphere, and fossil carbon.

• The decline of SOM is one of the largest inputs of CO₂ to the atmosphere, and about 27% (535 × 1015 g) of the SOC existing from prehistoric times (2014 × 1015 g) has been lost in the last two millennia.

• Anthropogenic carbon emissions of CO₂ are not balanced by CO₂ consumption. About half of the CO₂ emitted to the atmosphere by fossil fuel burning, open burning (fire), and terrestrial processes (mainly deforestation) is absorbed by terrestrial and marine environments; however, long-term trends are uncertain (Schimel et al. 2001).

• More carbon is leaving the soil reservoir (62 Pg) than entering the soil (59–60 Pg) (Battin et al. 2009; Weil and Brady 2017).

• Bicarbonates occupy the largest dissolved inorganic reservoir in the oceans and calcium carbonate (major mineral in limestone) in terrestrial and ocean ecosystems.

• Nonliving soil organic matter constitutes the largest pool of terrestrial organic carbon (Jobbagy and Jackson 2000).

• “Globally, soil contains a large amount of carbon—twice that in the atmosphere and more than carbon in vegetation and 2the atmosphere combined” (Liang et al. 2017).

• Long-term carbon storage in terrestrial ecosystems occurs primarily when plant biomass is stabilized in soils as SOM (Liang et al. 2017).

• “SOM is thus one of the key factors controlling the CO₂ concentration in the atmosphere and proper management of this resource can help mitigate global climate change and maintain or even enhance global food security” (Liang et al. 2017 citing Lal 2004).

• “Soil carbon sequestration can be viewed as a key mitigation strategy for rising CO2 concentration in the atmosphere but should also be recognized for its important role in improving the fertility and quality of soil, especially at a moment in history when agricultural lands have been seriously degraded by widespread, unsustainable management” (Franzluebbers 2012).

• “Soil organic matter enhances soil fertility and therefore increases net primary production and photosynthetic CO₂ fixation by plants” (Miltner et al. 2012).

• The terrestrial carbon cycle is a “balancing act” where plants take up inorganic carbon as CO₂ and synthesize organic compounds during the photosynthetic process.

• Fire represents a large and highly variable part of the US carbon budget, and the amount of CO₂ emitted from fires in the United States is equivalent to 4–6% of anthropogenic emissions at the continental scale. At a state level, fire emissions of CO₂ can, in some cases, exceed annual CO₂ emissions from fossil fuel usage.

• Deforestation is a major contributor to climate change—20% of anthropogenic emissions of CO₂ emanating from tropical forests (NIACS 2007). Tropical forests sequester 46% of the world’s living terrestrial carbon pool and 11% of the global soil carbon pool (Brown and Lugo 2018).

• Grasslands contain 10–30% of the world’s soil organic carbon and is sequestered by plants through photosynthesis and carbon loss by decomposition of organic matter. Carbon input from grasslands is mediated by plant life forms, climate, temperature, and precipitation regimes (Hewins et al. 2018).

• “Proper grazing management has been estimated to increase soil carbon storage on US rangelands from 0.1 to 0.3 Mg C ha^-1year^-1 and new grasslands have been shown to store as much as 0.6 Mg C ha^-1year^-1. Grazing lands are estimated to contain 10–30 % of the world’s soil organic carbon” (Schuman et al. 2002).

• Terrestrial carbon pools in cropland, rangeland, forests, and wetlands can act as a sink for sequestering atmospheric CO₂ (as much as 50 ppm of CO₂ for 100–150 years). In the United States, the sink capacity of sequestering additional carbon ranges from 0.2 to 0.48 Pg C yr⁻¹ when all land uses are tallied. Forests (0.03–0.05 Pg C yr⁻¹) and cropland (0.144–0.432 Pg C yr⁻¹) have the largest potentials for sequestering carbon, although grazinglands (range and pasturelands) can contribute up to 10% of the sink capacity (Lal 2010).

Soils

• Carbon is the 15th most abundant element in the Earth’s crust and 4th most abundant in the universe.

• Soil organic matter (SOM) is an integration of biologic and mineral components.

• Additions of organic matter to the soil may not supply the nutrient needs of vegetable plants in newly established gardens. Soil organic matter and nutrients accumulate over time. Additional nutrients from soil amendments may be needed to supply macronutrients for plants in new garden soils.

• In average soil, about 50% of the mass is carbon (C).

• SOM on the average contains 5% nitrogen (N).

• Decomposed soil organic matter, or humus, contains about 58% carbon, 4.8–5% N, 1.2% P, and 0.8% S and an array of micronutrients.

• Organic compounds have a tenuous hold on soil N (90–95%), which is unavailable to plants.

• Plants can uptake soluble organic compounds (mostly soluble proteins and amino acids), which comprise about 0.3–1.5% of the total organic N in soils.

• SOM is the primary source of nitrogen, phosphorus, and sulfur. Carbon ratios are 12:1 C:N, 50:1 C:P, and 70:1 C:S.

• Potential SOM content and the end point in the soil are dependent upon the soil parent materials, soil environment, climate, and native vegetation.

• Forests and cropland have the largest potentials for sequestering carbon, although grazinglands (range- and pasturelands) can contribute up to 10% of the sink capacity. Currently, grazinglands contain 10–30% of the world’s soil organic carbon.

• SOM accumulates over long periods (centuries) and eventually reaches an equilibrium level (levels off) that is correlated with the particular environment.

• The decline of soil health is due to many factors; however, the loss of soil organic carbon from tillage practices over time is the most significant factor as it affects the chemical, physical, and biological properties of soils.

• Cultivation of native grasslands to cropland results in rapid depreciation of soil organic matter, up to 40% loss in five years (Davidson and Ackerman 1993).

• In non-till cropping systems, soil organic carbon may be close to a steady-state level.

• On the average, one tillage operation with a one-way plow buries about 60% of surface crop residue.

• The decline of soil organic carbon can be slowed, and a degree of restoration is possible with cover crops and manure applications; however, the reality is that restoration of soil organic carbon may be limited, especially in more dry environments.

• Once optimum soil organic carbon levels are reached (equilibrium), the annual increase in SOM fluctuates between positive and negative values, but the long-term average is approximately a zero gain (Larcher 1983).

• Organic matter development is greater (1.5–3 times) in the surface soil layer for grassland plant communities compared to forests.

• During soil development, nutrients in the soil are derived from specific minerals in the parent material. As soils develop and become older, nutrients are supplied by the organic fraction of the soil.

• On the average, after 1 year, most of the carbon from soil organic matter added to the soil returns to the atmosphere as carbon dioxide (CO₂). About 25–33% remains in the soil (~5% in organism biomass, ~20% humic substances, and ~5% nonhumic fractions).

• SOM buildup in the soil is a slow process.

• Decomposition of organic matter is a complex process and is affected by certain carbon dynamics such as the makeup of carbon compounds in plant cells, linkages to other nutrient cycles (N, P, and S), and diversity of microorganisms.

• The principal elements of SOM are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, and sulfur.

• SOM enhances soil physical properties such as soil structure, aggregate stability, infiltration capacity, porosity, water holding capacity, granulation, and friability—ease of working the surface.

• At 5% soil organic matter, water field capacity is two times greater compared to an identical soil with 1% soil organic matter.

• SOM is linked directly to soil health and the productivity of a soil.

• SOM generally decreases with soil depth.

• SOM varies in content in soils; soils in desert and arid regions contain on the average less than 1.5% organic matter. Soils developed with prairie grasses generally contain more organic matter (3–5%) than forested soils.

• The average carbon-to-nitrogen (C/N) ratio of undisturbed soils is relatively stable at 12:1.

• When native soils are first cultivated, about 20–40% of the SOM is lost over a 5–20 year period.

• Soil erosion has and is a major contributor to carbon losses, which ultimately affects hydrologic function and biotic integrity.

• Carbon storage on rangelands can reduce soil productivity in source areas and potentially increase it in depositional areas where carbon is redistributed.

• SOM affects chemical processes in the soil, increases cation exchange capacity (CEC), and acts as a buffer to maintain soil pH.

• Organic residues on the soil surface decompose more slowly than if incorporated into the soil because aboveground material is beyond the reach of soil microorganisms.

• Organic matter content is dependent upon plant productivity. Grasslands develop higher organic matter content than forest soils. As soil organic matter increases, plant productivity increases.

• Plant nutrients are retained by soil organic matter; thus nutrients do not leach readily into subsurface soil layers (horizons).

• Soil pathogens are suppressed to a large extent in natural ecosystems and undisturbed soils. Non-pathogenic and pathogenic microorganisms coexist.

• The incorporation of organic materials (compost, decomposed manure, cover crops) can stimulate non-pathogenic antagonist microorganisms and suppress pathogenic microorganisms.

• Hydrophilic and hydrophobic aspects of SOM and inherent minerals adsorb pesticides and other toxic compounds in the soil.

• For each 1% organic matter, on the average there is 435.5 kg (1000 pounds) of N, 45.35 kg (100 pounds) of P, and K in the upper 15 cm (6 inches) of soil.

• In plant ecosystems, the primary source of SOM is derived from plant tissues; animal waste products are secondary.

• SOM sustains and regulates nutrient cycling in soils.

• About 60–90% of carbon is in upper 1 m of soil profile, although stored carbon is also significant below 1 m in histosols and gelisols (Weil and Brady 2017).

Microbial Roles in SOM Formation

• SOM enhances microbial biodiversity in soils.

• One gram of soil can contain up to 1010–1011 bacteria, 6000–50,000 bacteria species, and up to 200 m of fungal hyphae.

• Microorganisms in the soil mineralize nitrogen, phosphorus, and sulfur as they decompose organic matter. Nutrients are released slowly, over years.

• Soil pathogens are suppressed to a large extent in natural ecosystems and undisturbed soils. Non-pathogenic and pathogenic microorganisms coexist.

• SOM enhances microbial biodiversity in soils.

• Organic residues on the soil surface decompose more slowly than if incorporated into the soil because aboveground material is beyond the reach of soil microorganisms.

• Soil organic matter and soil microorganisms function together to improve soil physical properties that are related to greater soil resilience and resistance to erosion. As organic matter is added to the soil, biological activity increases, soil porosity is greater, and more stable soil aggregates or formed, which is related to greater infiltration capacity .

• “Microorganisms have two critical, contrasting roles in controlling terrestrial carbon fluxes: promoting release of carbon to the atmosphere through their catabolic activities, but also preventing release by stabilizing carbon into a form that is not easily decomposed” (Liang et al. 2017 citing Schimel and Schaeffer 2012).

• Miltner et al. (2012): “Although most soil carbon ultimately derives from plant material (Kögel-Knabner 2002), a large proportion may pass through microbial biomass before being transformed to SOM. Microbes grow on plant residues and utilise plant-derived carbon to build their biomass, and after cell death, part of this carbon is transformed into nonliving SOM (Kindler et al. 2006, 2009; Miltner et al. 2009).”

• The living part of soil organic matter includes a wide variety of microorganisms, such as bacteria, viruses, fungi, protozoa, and algae. It also includes plant roots and the insects, earthworms, and larger animals, such as moles, woodchucks, and rabbits that spend some of their time in the soil. The living portion represents about 15% of the total soil organic matter (Magdoff and van Es 2009).

• Microbes contribute to more than half of global respiration , but precise estimates are difficult in various terrestrial plant communities.

• Bacteria and fungi provide more than 95% of the biotic contribution to organic matter decomposition (Persson et al. 1980).

• “Fungal and bacterial necromass are the primary carbon-containing constituents contributing to the stable soil organic matter (SOM) pool” (Liang et al. 2017 citing Kindler et al. 2006; Schweigert et al. 2015).

• “When bacteria degrade plant residues, they use low-molecular-weight compounds, nucleic acids, lipids, proteins, and carbohydrates from the plant biomass to build their own biomass. If this biomass is then incorporated into non-living SOM, this portion is structurally derived from microbial biomass components even though the carbon ultimately originates from plant residues” (Miltner and Bombach 2012).

• “Microbial communities in forests are better adapted to degrading complex carbon compounds than microorganisms in grassland. Yet grassland microorganisms degrade grass litter more effectively than forest litter, while microorganisms in forests do not preferentially degrade forest litter” (Liang et al. 2017 citing Waldrop and Firestone 2004; Strickland et al. 2009).

• “Microbes grow on plant residues and utilise plant-derived carbon to build their biomass, and after cell death, part of this carbon is transformed into nonliving SOM” (Miltner et al. 2012 citing Kindler et al. 2006, 2009; Miltner et al. 2009).

• “Microbial biomass is considered to be turned over much faster than plant residues (Kästner 2000; Schink 1999), and therefore, the microbe-derived carbon input to SOM formation may be much higher than might be expected from its small pool size” (Miltner et al. 2012).

• “The molecular imprint of SOM by molecules and fragments derived from microbial biomass is presumably much more important than previously considered” (Miltner et al. 2012).

• Soil organic matter (SOM) is composed of the “living” (microorganisms), the “dead” (fresh residues), and the “very dead” (humus) fractions. The “very dead” or humus is the long-term SOM fraction that is thousands of years old and is resistant to decomposition.

• Soil organic matter comprises the active (35%) and the passive (65%) pools. The active SOM pool is live and dead plant and animal matter and is the food and energy source for microbes. The passive SOM pool can be high in lignin and is resistant to decomposition by microbes.

Manure

• The nutrient composition of manure varies considerably among livestock classes (see Table 6.12).

• Typically, recommended manure application rates are about 11.2–22.4 Mg ha⁻¹ yr⁻¹ (5–10 tons ac⁻¹ yr⁻¹) (for corn) and 4.4–6.7 Mg ha⁻¹ yr⁻¹ (2–3 tons ac⁻¹ yr⁻¹) for grain crops. If manure is available, application rates can be increased two to three times the average recommended rates (Thorup 1984).

• Nutrient release is variable over time (see Table 6.15).

• Average water, carbon, nitrogen, and phosphorus contents of fresh and stockpiled beef cattle manure (Data from Larney et al. 2006).

Age of manure	Water (%)	Total C (%)	Total N (%)	Inorg. N (%)	Total P (%)	C/N ratio
Fresh	65.1	10.75	0.565	0.125	0.160	19.7
Stockpiled	57.15	10.6	0.660	0.190	0.225	15.85

• As with growing cover crops to increase soil organic carbon, adding manure enhances microbial populations in the soil; however, soil carbon increases are minimal for singular applications (see Text Box 6.14).

• Percent ash in beef cattle manure = 35.57% (Larney et al. 2005).

• On the average, about 75% of the N, 80% P, and 90% K ingested by animals pass through the digestive system as manure and urine (Weil and Brady 2017).

• In any given year, the USDA estimates that about 5% of US cropland receives manure applications (Weil and Brady 2017).

• Manure applications on cropland do have environmental concerns both locally and globally. Local concerns are air quality (odors from ammonia and sulfurous gases), nutrients in runoff, and fecal pathogens (see discussion on soilborne pathogens in this chapter). Global implications are related to CO₂, NO_x, NH₃, and CH₄ emissions from decomposition (Weil and Brady 2017).

• “There is a significant trend to using more natural organic nutrient sources such as manure, compost, and cover crops. Organic methods tend to be more appealing to home gardeners since they are more able to manage the addition of organic materials in their garden settings. However, in large-scale production agriculture, agricultural scientists and producers point out that to produce abundant food, both organic and synthetic fertilizers have specific and important roles” (Spaeth 2018)

• There can be potential problems associated with nutrient leaching from long-term applications of manure. Excess accumulation of nitrogen and phosphorus can occur. Organic phosphorus can be leached and transported by water runoff to streams, rivers, and lakes, causing pollution and degraded water quality.