Cover Crops and Nitrogen Recovery
October 02, 2023 | By Zachary Larson, ForGround by Bayer Sustainable Systems Agronomist
While critical for plant growth, the behavior of nitrogen in the soil is fraught with potential transformations and losses, which ultimately reduce the amount of nitrogen that reaches a plant. The two dominant sources of nitrogen in synthetic fertilizers are the ammonium (NH4+) and nitrate (NO3-) ions, and of the two, nitrate is taken up by plants in the largest quantity. In the soil, both forms can be tied up in organic matter or lost to the atmosphere through the processes of volatilization (ammonium) and denitrification (nitrate).
Ammonium is converted to nitrate through a bacterial process known as nitrification. However, if not quickly taken up by plants, nitrate can easily leach through the soil profile as there are few places for the negatively charged ion to attach. This contrasts with base cations, such as calcium, magnesium, and ammonium, the positive charges of which easily bind to clays. Nitrogen losses through leaching can exceed 50 lbs per acre and some studies have shown losses of over 75 lbs per acre. (1, 2)
Nitrate is contained within soil water and is affected by precipitation and soil drainage. Sandy soils with high permeability can lose nitrate quickly, while the process moves more slowly in clayey soils. Additionally, rainfall amount and timing affect nitrate leaching. Times of high precipitation and slow crop growth promote leaching, typically in the fall through early spring, while the process slows in the summer. This creates a problem for farmers as by late spring nitrate may have leached below the rooting depth of cash crops, making it effectively lost from the soil.
How can cover crops capture soil nitrate? (1)
With cover crops, living roots are present during times of peak nitrate leaching. These roots can capture a large portion of soil nitrate before it leaches past the rooting zone. As the cover crops are terminated and decomposed, the nitrogen within is converted to more stable forms contained within organic matter. The amount of nitrogen recovered can be substantial. A study in Iowa and another study in Ohio covering 40 sites both showed an approximately 50% reduction in nitrate leaching when cover crops were used (3, 4). A study in Maryland and Pennsylvania and another study in Michigan showed planting cover crops resulted in a 66% and over 80% reduction in nitrate leaching, respectively. (5, 6)
Figure 1. Brassica cover crops such as radish, which is prominent in this field, can capture nitrogen below the rooting zone of cash crops, such as corn, and bring it to the surface where it can be utilized by subsequent crops. Zach Larson photo.
Can cover crops capture nitrogen located deep in the soil profile? (1)
Deep-rooted cover crops such as rapeseed and radish can capture soil nitrate that would otherwise leach though the soil profile (Figure 1). Observations of both species in Maryland demonstrated that planting rapeseed or radish cover crops reduced fall nitrate leaching to 70 inches below the soil surface by two-thirds or more compared to no cover crops (7). This nitrogen would have been lost to groundwater long before corn roots could reach it the following summer, and none of it would have been recovered if corn roots didn’t reach that depth.
Can cover crops help to retain fall-applied nitrogen?
In addition to trapping residual nutrients, cover crops can also retain nitrogen in fall-applied fertilizer and manure. This may be especially important in regions with fluctuating winter temperatures and ample rainfall where fall-applied ammonium nitrogen may be converted to nitrate and leach. For instance, radish cover crops have been shown to retain up to 91% of fall-applied ammonia in the 0-8 in soil depth to the following spring, compared to a no cover-crop rate of only 57% retention (8). Cover crops are also effective in fields receiving fall-applied manure, where treatments with cover crops have been shown to reduce soil nitrate the following spring (9).
What cover crops would be best for recovery of soil N? (1)
Generally, grasses and brassicas are the best species for capturing residual soil nitrate. The fibrous root system of grasses scavenges nitrogen very well and much of the nitrogen is held in the leaf tissue until it decomposes. A study in Nebraska found that winter cereal contained 30 to over 50 lbs of nitrogen per acre, while a Maryland study found that winter cereals contained over 75 lbs of nitrogen per acre in the spring (10, 6). For nitrogen that is located deeper in the soil profile, deep-rooted grasses such as sorghum-sudangrass or annual ryegrass excel at capturing nitrogen (Figure 2). Additionally, brassicas such as radish and rapeseed, which have been shown to accumulate over 130 lbs/ac of nitrogen, are quite effective (11). While legumes will scavenge some soil nitrate, their fixation of atmospheric nitrogen will likely result in a net release of nitrate if planted in a solid stand.
What management tactics maximize recovery of soil N?
Practices that maximize cover crop biomass production will generally maximize nitrogen recovery. This includes planting cover crops as early as possible and terminating as late as possible. Additionally, increasing seeding rates will result in greater nitrogen capture, as increased seeding creates more roots to exploit the soil. Increased seeding is especially effective when planting late in the fall in regions where winter cover crop growth is limited. Additionally, cover crops that die off in the winter (e.g., tillage radish) or that are killed early in the spring are the most effective in providing N to the succeeding crop.
Figure 3: This mixture of radish, oats, and winter pea utilizes a variety of nutrient acquisition strategies. Radish roots will scavenge nitrate deeper in the soil profile while oats will pull nitrogen from near the surface. The pea will fix atmospheric nitrogen, with the greatest fixation occurring in the spring. Zach Larson photo.
Is it better to use a mixture of cover crop species or a single cover crop? (1)
Mixtures can be an effective way to maximize nitrogen retention or combine the function with other ones, such as suppressing weeds or building soil carbon. However, single species are still capable of performing multiple functions. As noted, grasses and brassicas excel at nitrogen recovery, with deep-rooted species demonstrating the ability to recover nutrients from deep in the soil profile, and a combination of the two can be effective in capturing excess nitrogen. Additionally, overlapping winter-hardy species, such as cereal rye, with winter-killed species, such as forage radish, can retain nitrogen for longer periods as the rye can help to recover nitrogen from the decomposing radish.
While they are typically a source of nitrogen, legumes can be a component of cover crop mixes, allowing for the supply of nitrogen with minimal impacts to nitrate leaching. Effectively utilizing legumes in a cover crop mixture is best accomplished by planting winter-hardy legumes such as hairy vetch, winter pea or clovers that fix most of their nitrogen in the spring (12). For example, in a mix of winter pea, forage radish, and a small grain, the radish will capture nitrogen in the fall and release it after it winterkills, the small grain will continue to capture nitrogen into the spring, and the pea will fix small amounts of atmospheric nitrogen in the fall and larger amounts as the plant grows in the spring, peaking during flowering, around the time of cash crop planting (Figure 3).
Can I reduce my fertilizer rates, since more nitrogen is being captured with cover crops?
Because the nitrogen captured by cover crops is tied up in plant biomass, its slow release may not coincide well with the nitrogen demand curves of crops such as corn. However, the nitrogen captured by cover crops is held in organic matter, which results in a slower release that coincides with warmer soil temperatures and good soil moisture, typically at a rate of 30 to 40 lbs per every 1% of soil organic matter. The additional cover crop biomass ultimately results in greater soil organic matter, which in turn releases more nitrogen as organic matter levels climb, while providing additional soil health benefits such as improved soil structure and water holding capacity.
Figure 4.: A cereal grain cover crop, such as wheat, can scavenge excess nitrogen from a corn crop. While high C:N ratios commonly associated with small grains can negatively impact corn crops, soybeans can often be seeded into small grains with little potential yield loss. Zach Larson photo.
How will planting cover crops affect my cash crops?
A cover crop’s impact on cash crop yield is affected by the cash crop grown and the rate of decay of the terminated cover crop, which is influenced by the residue’s ratio of carbon to nitrogen (C:N ratio). The C:N ratio can predict how fast the residue will break down and release the nitrogen back into the soil. Residues with higher C:N ratios (above 24:1) require soil microbes to consume nitrogen as they break down residues, while residues with lower C:N ratios (below 20:1) decompose quickly, making nitrogen available for the next crop. (13)
Grasses and small grains have high C:N ratios relative to most other cover crops, resulting in a slower rate of decay. Therefore, such cover crops are more likely to reduce the yield of a following high-nitrogen cash crop, such as corn. This can be managed by terminating the cover crop at an earlier growth stage when the biomass has a lower C:N ratio, or by applying at least 40 lbs of the crop’s nitrogen needs at planting. Conversely, grasses and small grains present fewer issues ahead of legumes, such as soybeans, and they provide additional benefits such as weed suppression (Figure 4).
Brassicas, such as forage radish, have a lower C:N ratio than overwintering grasses and cereal grains. The low C:N ratio of brassicas, combined with their propensity to winterkill, tends to reduce any negative impacts on the following cash crop. Most studies report slight declines to slight improvements in cash crop yield following brassica cover crops, and many studies report no yield difference in cash crops following radish cover crops and cash crops following no cover crops.
Putting it all together
Scavenging soil nitrate is one of many functions of cover crops. While the process is dependent on precipitation and soil drainage, there are many places where this function is well-served. As with any system, the desired function needs to be matched with the right crop and management, but when done correctly many long-term benefits are possible. With that, here are a few key takeaways:
- Cover crops can scavenge soil nitrogen that would otherwise not be available to cash crops. Once contained in the biomass of cover crops, this nitrogen will help to increase the pool of soil organic matter, which slowly returns to crops as it mineralizes.
- Ahead of corn, lower C:N ratio cover crops such as brassicas (rapeseed, radish, others) can capture nitrogen deep in the soil. Winter termination of some of these crops often results in no negative impact to cash crop yields. Brassicas can be mixed with winter-hardy legumes, such as vetch, peas, and clovers to provide additional benefits to cash crops.
- Cereal grains or grasses such as cereal rye, barley, and annual ryegrass are good options for nitrogen scavenging ahead of soybeans. These higher C:N ratio crops, when managed correctly, generally do not pose a high yield risk and provide other agronomic benefits.
- Maximizing biomass production will maximize nitrogen capture. Planting cover crops as early as possible and terminating as late as possible will tie up the most nitrogen. Brassicas should generally be planted ahead of small grains, and interseeding brassicas into a standing crop or planting earlier-maturing crops can help to facilitate early planting.
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