Introduction
Carbon and nitrogen are two elements in soils and are required by most biology for energy. Carbon and nitrogen occur in the soil as both organic and inorganic forms. The inorganic carbon in the soil has minimal effect on soil biochemical activity, whereas the organic forms of carbon are essential for biological activity. Inorganic carbon in the soil is primarily present as carbonates, whereas organic carbon is present in many forms, including live and dead plant materials and microorganisms; some are more labile and therefore, can be easily decomposed, such as sugars, amino acids, and root exudates; while others are more recalcitrant, such as lignin, humin, and humic acids. Soil nitrogen is mostly present in organic forms (usually more than 95 % of the total soil nitrogen), but also in inorganic forms, such as nitrate and ammonium. Soil biology prefers a certain ratio of carbon to nitrogen (C:N). Amino acids make up proteins and are one of the nitrogen-containing compounds in the soil that are essential for biological energy. The C:N ratio of soil microbes is about 10:1, whereas the preferred C:N ratio of their food is 24:1 (USDA Natural Resource Conservation Service 2011). Soil bacteria (3-10:1 C:N ratio) generally have a lower C:N ratio than soil fungi (4-18:1 C:N ratio) (Hoorman & Islam 2010; Zhang and Elser 2017). It is also important to mention that the ratio of carbon to other nutrients, such as sulfur (S) and phosphorous (P) also are relevant to determine net mineralization/immobilization. For example, plant material with C:S ratio smaller than 200:1 will promote mineralization of sulfate, while C:S ratio higher than 400:1 will promote immobilization (Scherer 2001).
Carbon: Nitrogen Ratio of Materials
The C:N ratio of organic materials is important when making compost. The C:N ratio of organic materials in the soil or compost affect the rate at which microbes process carbon, make nutrients in the organic matter available to plants, and form stabilized soil carbon compounds. If the organic matter contained in the soil has a high C:N ratio, microbes are inefficient at digesting the material due to a lack of nitrogen. Nutrients in organic matter that are mineralized can become available to plants when broken down. If organic materials contain a low C:N ratio they are rapidly broken down. As organic materials are broken down, the C:N ratio increases from organic carbon being converted to carbon dioxide, as long as denitrification does not occur. If the C:N ratio of the soil is low (<10) bacteria prefer the environment, whereas fungal communities prefer a soil C:N ratio closer to about 24:1. Natural grassland and forest soils generally contain more fungi than bacteria in the microbial community. Healthy agricultural soils typically have a higher fungal:bacteria ratio also.
Organic materials contained in the soil and added to composts have a wide range of C:N ratios. Examples of C:N ratios of different compost materials, soils, and soil components are listed in Table 1. Some materials, such as sawdust, paper clippings, corn, and wheat straw, have high C:N ratios. However other materials, such as coffee grounds, manures, and grass clippings have lower C:N ratios. Values in Table 1 are examples of percent carbon and nitrogen in organic materials, but these values can vary between samples. Publications that contain C:N ratios for organic materials are listed in Wilson and Rynjk (1991) and Wortman and Shapiro (2012) and are included in the references at the end of this article.
| Material | Percent Carbon | Percent Nitrogen | Carbon: Nitrogen Ratio | Source |
|---|---|---|---|---|
| Coffee Grounds | 49.7 | 2.31 | 21.5 | Liu and Price 2011 |
| Alfalfa Hay | 44.4 | 3.03 | 14.7 | Mayland 1968 |
| Corn Straw | 51.9 | 0.84 | 61.8 | Wang et al., 2011 |
| Cardboard | 42.1 | 0.11 | 383 | Liu and Price 2011 |
| Sawdust | 46.5 | 0.07 | 664 | Huang et al., 2004 |
| Vegetable Scraps | 37.8 | 2.94 | 12.9 | Abdullah and Chin 2010 |
| Fresh Cattle Manure | 39.3 | 3.33 | 11.8 | Wang et al., 2011 |
| Liquid Dairy Manure | 3.70 | 0.375 | 9.99 | Marino et al., 2008 |
| Pig Manure | 11.6 | 1.02 | 11.3 | Huang et al., 2004 |
| Poultry Manure | 5.8 | 0.611 | 9.6 | Wang et al., 2014 |
| Wheat Straw | 45.0 | 0.34 | 132.3 | Curtin et al., 1998 |
| 0-6 in soil near Brookings South Dakota | 2.17 | 1.83 | 11.6 | Chalise et al., 2019; Maiga et al., 2019; Sangotayo et al., 2023 |
| 0-6 in soil at Dakota Lakes Farm in South Dakota | 1.84 | 0.18 | 10.3 | Strum 2022 |
| 0-6 in soil near Beresford South Dakota | 27.2 | 2.18 | 12.5 | Maiga et al., 2019; Sangotayo et al., 2023 |
| Soil Fungi | 44 | 4.0 | 18.2 | Zhang and Elser 2017 |
| Soil Microbial Biomass | NA | NA | 10:1 | USDA Natural Resources Conservation Service 2011 |
| Soil Particulate Organic Matter | 23.1 | 1.18 | 20.4 | Nichols and Wright 2006 |
| Glomalin | 39.6 | 2.75 | 15.1 | Nichols and Wright 2006 |
| Humic Acid | 53.2 | 3.9 | 14.4 | Nichols and Wright 2006 |
| Fulvic Acid | 26.1 | 2.3 | 11.8 | Nichols and Wright 2006 |
Factors that can affect the carbon and nitrogen content include the amount of liquid in the material, stage of decomposition, portion of or growth stage of the plant, and carbon types contained in the organic materials. Manures that are mixed with high C:N ratio beddings, such as sawdust, wheat straw or corn stover, generally will have a higher carbon content and C:N ratio. However, liquid manures generally have a lower percentage of carbon and nitrogen and C:N ratio. Many online references list the C:N ratio of organic materials; however, knowing the actual nitrogen content is important if you plan on using the material as a fertilizer. When making composts, high C:N ratio materials are blended with low C:N ratio materials to form a C:N ratio of around 30:1. The ideal C:N ratio of compost is 30:1 (Wilson & Rynjk, 1991). If you have the nitrogen content and C:N ratio, you can multiply the percent nitrogen times the ratio of C:N to get the percent carbon. When making compost blends, the moisture content of the material also needs to be accounted for when calculating the carbon and nitrogen content of the compost. Other than the percent carbon and nitrogen values of the manure types, values presented in Table 1 are on a dry matter basis. However, most materials contain some moisture in them prior to composting, which needs to be accounted for. Higher C:N compost ratios are more desirable too, because this will reduce the amount of denitrification that could occur from organic nitrogen being converted to inorganic nitrogen then to nitrogen gas (N2). When the C:N ratio of the compost is higher, it keeps the nitrogen present in the compost as an organic form instead of an inorganic form.
Immobilization
To get an optimal amount of energy from carbon-containing food sources, there needs to be a certain amount of nitrogen in the food. If the food material has a C:N ratio of 24:1 or less, the microbes do not need to use any of the nitrogen in an inorganic form in the soil to digest the food source for energy. However, if the material has a high C:N ratio, microbes will use nitrogen as ammonium or nitrate, which is present in the soil to help them digest the material and is known as N immobilization. This phenomenon most commonly occurs when crops are grown with high C:N ratio residues, such as corn or wheat, or when high C:N mulches are applied on soil surfaces, such as straw and sawdust. This process will take urea or nitrate that is present in an inorganic form and convert the nitrogen into an organic form in the soil microbes. If a crop that requires nitrogen fertilization is grown in soil that has too high of a C:N ratio, immobilization can make it more difficult for the crop to take up nitrogen and reduce the yield and fertilizer uptake efficiency. Based on data from previous studies in eastern South Dakota, the C:N ratio of field soils was in the 10-13:1, range which is below the C:N ratio of 24:1. These studies were conducted in crop rotations commonly grown in the region, including corn-soybean, corn-soybean-wheat, and corn-soybean-wheat-oat rotations. This means the soil environment is not nitrogen limiting, even when multiple crops are grown in the rotation that produce high C:N ratio crop residues. However, if the soil C:N ratio is greater than 24:1, microbes will start to immobilize inorganic N to help it digest the carbon sources that it eats.
Management to Optimize C:N Ratio
Agricultural management practices can affect the C:N ratio of the soil environment. Some examples of management practices that can be used to influence the soil C:N ratio are: crop rotation, cover crop species selection, nitrogen fertilization, and application of animal manure or other organic materials. Adding legume crops with a low C:N ratio, such as soybeans, peas or alfalfa, to your crop rotation can help to balance the high C:N crop residues produced from crops, such as corn, wheat, or rice. Also, some cover crops can be planted specifically to fix nitrogen ,such as hairy vetch, field pea and various types of clovers and legumes. If you expect a cover crop to produce a high amount of biomass (>2000 lbs./acre), grass cover crops can produce a high C:N ratio and biomass, which is deposited on the soil surface. If you plan on planting crops that require high amounts of nitrogen following the cover crop there may be reduced soil inorganic nitrogen concentration available initially when certain cover crops are grown. However, this nitrogen will slowly release when dead cover crop materials are broken down. Having a mix of cover crop species that produce both high and low C:N ratio biomass, such as rye and vetch, can prevent the C:N ratio from getting too high for soil microbes. How much biomass is produced by the cover crop also affects the C:N ratio of the cover crop material, with the C:N ratio increasing as the cover crop gets closer to maturity and produces more biomass. Materials can also be added to the soil, affecting the C:N ratio. Materials such as manure can be added to the soil, which can reduce the soil C:N ratio. Also, high C:N ratio materials can be added to the soil, such as wheat or corn straw, biochar, or sawdust, to increase the C:N ratio. Additionally, synthetic nitrogen fertilizers can be added to the soil, such as anhydrous ammonia, urea, and diammonium phosphate, which will reduce the C:N ratio. Although materials with high C:N ratio can tie up nitrogen, they can provide other ecosystem services to the soil. High C:N ratio materials are frequently used as mulches for weed control in gardens or specialty crops. Also, materials, such as wheat straw or corn stover, remain on the soil surface for long periods of time, which provides soil and water conservation benefits. The return of these residues is also important to maintain soil carbon concentration. If the C:N ratio near the soil surface is high, it is ok, as long as the crop can get the nitrogen elsewhere in the soil and the C:N ratio in the crop rooting depth is below 24:1.
Summary
The ratio of carbon to nitrogen in the soil is essential for soil biochemical functioning. Having too high of a C:N ratio decreases the rate at which organic matter is cycled and may immobilize inorganic nitrogen. The ideal C:N ratio for soil microbes is 24:1, although most soils have a C:N ratio less than that. Agricultural management practices, such as crop rotation, residue management, cover crop selection, organic matter additions to soil, and nitrogen fertilization, can affect the soil C:N ratio. Having a C:N ratio that is too high or too low may cause the loss of nitrate from the soil and cause a decrease in agricultural productivity.
References
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