Summary:

• The decomposition of cover crop residues is regulated by soil microbes, which require a diet of approximately 24 parts carbon to 1 part nitrogen.
• The balance of carbon to nitrogen in cover crop biomass will affect how quickly nitrogen is released by a decomposing material, with low carbon-to-nitrogen cover crops releasing nitrogen and high carbon-to-nitrogen cover crops tying up nitrogen.
• Management of cover crops including species selection, planting rate, termination date, and fertility management can mitigate potential nitrogen tie-up from decomposing cover crops.

Cover crops can greatly affect the amount of nitrogen in a cropping system. For instance, the legumes hairy vetch, red clover, and field peas can fix up to 90, 100, and 174 lbs/ac of atmospheric nitrogen, respectively (1, 2). While cover crops can fix atmospheric nitrogen or capture excess nutrients after a cash crop, they can also tie up nutrients, potentially leading to negative effects on the following crop (Figure 1). Many of these effects are regulated by the balance of carbon and nitrogen in cover crop residues, and the proper management of cover crops can help to maximize cover crop benefits and lessen risks.

Figure 1. Nitrogen deficiency in corn at V8 development stage following cereal rye cover crop terminated one week before corn planting. Pictures were taken on July 1st 2022 at the Bayer Water Utilization Learning Center, in Gothenburg, Nebraska. Courtesy of Alex Rosa.

The decomposition of organic matter in most agricultural soils is a function of microbial oxidation. The process beings with the addition of plant residues, manure, or any carbon-containing material. In an aerobic (oxygenated) environment, additions of carbon-containing material awaken dormant soil microbes and increase their populations exponentially. These microbial organisms are varied in type and function, with different species specializing in breaking down different compounds in the decaying material.

Soil microbes, like all living organims, are comprised of nitrogen, a component of proteins, and carbon, as well as other trace elements. The carbon-nitrogen ratio of most common soil microbes falls between 5 to 10 parts carbon to 1 part nitrogen. Like other living organisms, soil microbes require a “diet” of energy (carbon) and protein (nitrogen). These microbes require carbon at a rate of about 24 parts to every 1 part nitrogen. (The organic matter of most cultivated soils ranges from about 8:1 to 15:1.) As with allThe respiration of soil microbes releases carbon dioxide, which means that soils are constantly releasing CO2, with increased temperature and moisture increasing the rate of microbial respiration and organic matter decomposition. This means that organic material must be constantly added to soils to maintain any given level of organic matter.

As soil microbes continue with the process of organic matter oxidation, the easily-decomposed materials are exhausted and portions of the microbial community begin to die, providing a feedstock for the remaining microbes and releasing nitrogen from the dead microbes. The decomposition of organic materials and other microbes continues until the microbial populations dwindle, leaving those organisms that can slowly decompose the most resistant material. Ultimately, approximately two-thirds of the organic material in the soil is broken down by microbes and released as carbon dioxide, while the remainder is converted to humus or a stabilized source of soil organic matter.

Table 1. C:N ratios of common cover crops and crop residues. Adapted from Carbon to Nitrogen Ratios in Cropping Systems, by NRCS East National Technology Supprot Center, 2011 (https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcseprd331820.pdf).

The carbon-to-nitrogen ratio (C:N) will influence the rate of carbon decomposition and the release of nitrogen. As noted above, soil microbes require a diet of approximately 24:1 C:N. Carbon-based crop residues and materials have a much wider range of carbon to nitrogen, extending from 10:1 for legume cover crops to 80:1 for wheat straw (Table 1). When organic material has a C:N ratio of greater than 24:1, free nitrogen in the soil will be consumed by soil microbes to break down the material, “stealing” nitrogen that would otherwise be available to plants. If not enough soil nitrogen is present, the nitrogen from decaying microbes becomes available to decompose the remaining carbon, and this occurs until enough carbon leaves the soil as carbon dioxide and the organic residues reach a C:N ratio of approximately 20:1 (Figure 2). The time in which soil nitrogen is limited by microbial activity is known as the nitrate depression period and the process can take from days to months, with large quantities of easily decomposable, high C:N feedstocks creating the most severe depression, and harder to decompose, lower C:N materials making for a less severe nitrate depression period.

Figure 2. Nitrate depression pool after the addition of high-carbon materials. Adapted from The Nature and Properties of Soils, 15th Edition (p. 540) by R. R. Weil & R. C. Brady, 2017, Pearson. Adapted with permission.

Once the soil C:N ratio is approximately 20:1, nitrogen can become available to plants as additional organic matter is consumed. Conversely, when high-nitrogen materials are added to the soil (C:N is less than 20:1), free nitrogen is quickly released into the soil solution and is available for plant uptake, generally with no nitrate depression period after the addition of residues. An example of this is with the addition of hairy vetch residue, which has been shown to release both carbon and nitrogen at rates 1.6 and 2.1 times faster, respectively, than cereal rye (3). Therefore, nitrogen-hungry cash crops, such as corn, are less likely to be nitrogen deficient following hairy vetch cover crops than higher C:N cover crops.

The tie-up and release of nitrogen through the decomposition of cover crops is generally the most challenging when cover crops are followed by nitrogen hungry cash crops, such as corn. Corn’s demand for nitrogen is the greatest from around V8 (8 visible collars on the plant) through tassel, with additional nitrogen required through grain fill, although at a lower rate. Therefore, it is imperative to understand nitrogen tie-up from cover crops, and how they should be managed through species selection, termination timing, and fertility programs to reduce nitrogen tie-up at the following cash crop’s highest nitrogen demand period.

Cover crop species selection and management

Changing species or the components of a cover crop mix is one of the most effective ways of lowering the C:N ratio of a cover crop. Crop residues having C:N ratios of 20:1 or less, such as vetches and clovers, will release nitrogen as they decompose as opposed to temporarily tying up nitrogen, and they can be used to offset fertilizer inputs (Figure 3). In Kentucky, a hairy vetch cover crop with no applied nitrogen yielded similar to a no cover crop treatment with 150 lbs/ac of applied nitrogen (4). When 150 lbs/ac N was applied in the vetch treatment, the corn yielded over 20 bu/ac more than the no vetch treatment. Low C:N cover crops can also release nitrogen at times more closely aligned with crop needs. In southern Illinois, nearly all of the nitrogen release from hairy vetch occurred before corn growth stage V8, with over ¾ occurring before V15.

Figure 3. Hairy vetch (A), rapeseed (B), and winter peas (C) are examples of low C:N ratio cover crops. Pictures were taken on July 1st 2022 at the Bayer Water Utilization Learning Center, in Gothenburg, Nebraska. Courtesy of Alex Rosa.

Replacing cereal grains with legumesmay be a challenge in certain crop rotations. Management options for introducing legumes in place of cereal grains include planting shorter maturity hybrids, interseeding cover crops into a prior year’s cash crop, or adding a small grain to the rotation to allow for the planting of a legume cover crop in a timely manner.

Another way to reduce the potential nitrogen tie-up from cover crops is to lower the seeding rate of grasses and cereal grains, to lower the amount of total carbon to convert to humus. Lower seeding rates generally result in lower overall biomass, although some plants will tiller more under lower populations so it is not necessarily a one-to-one relationship. However, seeding at a lower seeding rate may come with tradeoffs, such as reduced fall canopy cover, which may limit a cover crop’s ability to suppress weeds.

Cover crop termination timing

Termination timing can also impact the C:N ratio of crop residues, with earlier termination resulting in a lower risk of nitrogen immobilization from decomposing material (Figure 4). Termination of cereal cover crops two weeks prior to planting or during the early jointing stage or will generally result in reduced nitrogen immobilization, which will minimize subsequent yield drags. Additionally, in limited rainfall areas, termination of winter hardy species should be performed in early spring to minimize soil water depletion and, consequently, corn yield loss (6). Utilizing winterkilled cover crops, such as oats, is also a way to reduce nitrogen immobilization in the following crop, and it will eliminate unwanted soil moisture depletion in the spring.

Figure 4. A cereal rye cover crop in Pennsylvania photographed at the same location on April 9th (left), April 25th (middle), and May 10th (right) showing growth over one month. The biomass and higher C:N ratio of the May 10th photo will pose the greatest risk for N tie-up in corn. Zachary Larson photos.

Corn fertility management

Nitrogen immobilization from cover crops can also be mitigated with changes to the nitrogen management program. When cover crops with high C:N ratios are present, additional nitrogen applied ahead of, or at, planting may help to offset the N tied up by decomposing cover crops. Estimating exactly how much N is necessary is not very straightforward, so adaptive nutrient management practices are the best option in such scenarios. In general, for those that apply a larger percentage of their nitrogen as a sidedress, more of the N should be shifted to an at-planting application, with up to 50 lbs/ac N up front. This should be followed by pre-sidedress soil nitrate testing or the use of a chlorophyll meter with a high-N reference strip to estimate demand at sidedressing.

In some instances, additional nitrogen may be necessary to break down residues following a high C:N cover crop. However, that extra nitrogen will not go to waste; when the nitrogen is combined with the carbon from a decomposing cover crop, additional organic matter will be assimilated by microbes, which will slowly release the nitrogen back into your soil system as it breaks down.

One tool that is currently available to show the relationship between cover crops and nitrogen availability ahead of corn is the Cover Crop Nitrogen Calculator by Precision Sustainable Agriculture. The tool utilizes soil survey data, user inputs of cover crop and cash crop management, measurements of cover crop biomass, and data from lab-sumbitted cover crop samples to estimate the nitrogen release from decomposing residues, and recommended nitrogen application rates.

Cover crop – cash crop relationships

Another option for managing high C:N cover crops is to prioritize planting cereal cover crops ahead of soybean. Since soybean have a low demand for nitrogen, they generally are much less sensitive to high-carbon residues than other cash crops. Planting soybean allows for a cereal cover crop to be planted with minimal risk from immobilizing nitrogen, and allows one to let cover crops mature to maximize their function, including the building of soil carbon. Soybean can even be planted “green” into mature, standing cover crops without showing a yield decline compared to planting after early terminated cover crops (7), and in some cases planting a cereal cover crop prior to soybean can provide a yield benefit.

Cover crops can create management challenges due to their effects on soil carbon-nitrogen ratios, especially when planted ahead of high nitrogen-demand cash crops. However, many of the benefits associated with cover crops can be achieved by adjusting your cover crop management system. Utilizing alternate cover crop species, adjusting termination timing and nitrogen management practices, or planting cover crops ahead of soybean can mitigate potential problems and allow cover crops to fit in your management system.

Sources cited:

1. Brady, N.C. & Weil, R.R. (2002). The nature and properties of soils (13th Ed.). Prentice
2. Hall. Heichel, G.H. (1987). Legume nitrogen: Symbiotic fixation and recovery by subsequent crops. In Energy in plant nutrition and pest control; Helsel, Z.R., ed.. Elsevier Scientific Publishers: Amsterdam, 63–80.
3. Lacey, C., Nevins, C., Camberato, J., Kladivko, E., Sadeghpour, A., & Armstrong, S. (2020). Carbon and nitrogen release from cover crop residues and implications for cropping systems management. Journal of Soil and Water Conservation, 75, 505-514. Doi:10.2489/jswc.2020.00102
4. Utomo, M., Frye, W.W., & Blevins, R.L. (1990). Sustaining soil nitrogen for corn using hairy vetch cover crop. Agronomy Journal, 82, 979-983. Doi: 10.2134/agronj1990.00021962008200050028x
5. Singh, G., Dhakal, M., Yang, L., Kaur, G., Williard, K., Schoonover, J.E. & Sadeghour, A. (2020). Decompostion and nitrogen release of cover crops in reduced- and no-tillage systems. Agronomy Journal, 112, 3605-3618. Doi:10.1002/agj2.20268.
6. Rosa, A.T., Creech, C.F., Elmore, R.W., Rudnick, D.R., Lindquist, J.L., Fudolig, M., Butts, L., & Werle, R. (2021). Implications of cover crop planting and termination timing on rainfed maize production in semi-arid cropping systems. Field Crops Research, 271, 108251. Doi:10.1016/j.fcr.2021.108251
7. Reed, H.K., Karsten, H.D., Curran, W.S., Tooker, J.F. & Duiker, S.W. (2019). Planting green effects on corn and soybean production. Agronomy Journal, 111, 2314-2325. Doi:10.2134/agronj2018.11.0711.
8. USDA NRCS East National Technology Support Center (2011). Carbon to Nitrogen Ratios in Cropping Systems [Fact sheet]. U.S. Department of Agriculture, Natural Resources Conservation Service. https://marionswcd.org/wp-content/uploads/C_N_ratios_cropping_systems.pdf
9. Weil, R.R. & Brady, N.C. (2017). The nature and properties of soils (15th Ed.). Prentice Hall.