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June 7, 2018
By Steve Werblow for Conservation Technology Information Center (CTIC)
Cycling nitrogen in the soil is a lot like managing money. There’s investment, returns, bonuses and most of all, strategy. Cover crops can be a great strategy for building reserves of soil nitrogen, says soil scientist Eileen Kladivko at Purdue University—but mostly a long-term one.
“It’s kind of like putting money into a CD,” Kladivko explains. “You can’t withdraw it immediately. You let it build up.”
The alternative—leaving fields fallow for seven months between harvest and planting in a typical corn/soybean rotation—leads to a steady drain of N from the soil bank. Applied fertilizer, and nitrogen released from organic matter by soil microbes, leaches into the soil, drains through tile lines, or volatilizes from the surface.
Planting cover crops uses that fallow period to build the nitrogen account instead.
Grasses and cereals provide more plant-available N if they are terminated in their vegetative stages. By the time the flag leaf emerges, plant-available N is approaching zero, and when seed heads form, the residue will actually tie up N before it is eventually converted into soil organic matter.
Just as there are differences between making money and saving money, different cover crops contribute to nitrogen-building strategies in different ways:
Legumes like vetch, Austrian winter peas, and clovers capture nitrogen from the air and transform it into soil N. That’s like earning a salary.
Other crops like grasses or brassicas—radish or rape—scavenge nutrients from the soil and sequester them in the root zone. That’s saving nitrogen from loss and putting it in the bank.
Nitrogen transactions take place on the microscopic and molecular levels, driven by soil microbes. The difference between a positive or negative balance is driven by the carbon:nitrogen (C:N) ratio. Microbes use nitrogen in the process of breaking down carbon-based plant matter. If the residue is tough to digest and low in nitrogen content, the microbes will source N from the soil around them to fuel the reaction. If the decomposing plant material contains a surplus of nitrogen, some will be left over for the subsequent cash crop.
Deep-rooted radishes are extremely effective at capturing N in the fall. But winter kill can release that nitrogen before crops can use it, so radishes work best in blends that allow grasses to keep their N from leaching.
“If organic matter has a carbon:nitrogen ratio around 25:1, it means there is just enough nitrogen for organisms to decompose the material and blow off CO2,” Kladivko explains. “The nitrogen is now in their bodies. As they die, some of their bodies get eaten by other bacteria, which release nitrogen, and some of their bodies become soil organic matter.
“If the material has more nitrogen than the microbes need, they release the extra nitrogen as ammonium,” she adds. So, in the case of legumes—which contain more nitrogen than the microbes need for the breakdown process—about half of that N is available about one to two months after the cover crop dies.
Wheat straw has a C:N ratio of 80:1 and corn stover’s C:N ratio is about 57:1, which is why fields experience a significant tie-up of nitrogen as mature crop residues decompose. A cereal rye cover crop in its vegetative stage is about the nitrogen-neutral level of 26:1. When that same rye stand begins to flower, the plants increase their carbon content, increasing the C:N ratio to a more difficult-to-decompose level of 37:1. Meanwhile, legumes are very rich in nitrogen. Hairy vetch has a C:N ratio of 11:1, which prompts soil microbes to deposit excess N in the soil as they quickly break down the soft stems and leaves.
Nitrogen-fixing legumes can be a powerful tool for injecting new N into the soil nitrogen bank. Hairy vetch can fix 38 to 170 pounds of nitrogen per acre, for instance. But Kladivko is quick to caution that only about half of that amount will be available to the next crop, and that legume cover crops must be managed carefully to get the N-fixing benefit—especially in northern areas where the growing season is relatively short.
“You need to get enough growth of the legume to start fixing nitrogen. That’s really tricky to do in a corn/soybean rotation,” she explains. “A lot of legumes don’t do much in a typical fall. It’s a combination of time and temperature. Where most of the legume nitrogen fixation comes into fruition is in the spring, but a lot of farmers don’t leave them long enough. They would need to grow ‘till mid-May or the end of May. If you’re going to kill it in late April, you’re probably not going to get much of it.”
To try to give legumes more of a chance to start fixing nitrogen before winter, some farmers interseed their cover crops into standing cash crops.
Scavenger cover crops are efficient gatherers of nutrients that are already present in the soil. “Scavengers are keeping 20 to 30 pounds of nitrogen out of the tile drain, so that’s going into your nitrogen bank account,” says Kladivko.
In fact, for fields where manure has been applied, she notes, cover crops can scavenge 80 to 100 pounds of N per acre—nitrogen that would have been lost to cash crops and could have contributed to pollution downstream.
With deep roots and fast growth, some covers are even more effective at scavenging. For instance, a Cornell study found that a tillage radish cover crop captured 172 pounds of N per acre. However, Kladivko points out that because radish is typically winter-killed and decomposes quickly in the early spring, it releases nitrogen before the cash crop needs it. Planting radish in a blend with grass cover crops like spring oats that are winter-killed can help keep captured N in the soil long enough for cash crops to access it, she says.
However, a just-released, three-year University of Wisconsin study found that the way radishes decompose, they don’t release nitrogen for the next crop. What happens to the N remains a mystery.
Just like a sound investment strategy, diversification is important when building a nitrogen account in the soil. Cover crop blends tap into the benefits of a range of plant species and characteristics. A mix of covers can combine N fixers and scavengers, deep- and shallow-rooted plants, low and high C:N ratios, and even hosts of different kinds of microbes.
“By increasing the diversity of plant materials, it leads to increasing diversity of microbial populations,” says Kladivko.
Many soil scientists estimate that about two percent of the nitrogen tied up in soil organic matter in a field is converted to plant-available forms every year. The steady deposit of carbon and nitrogen into the soil organic matter account in the soil builds a bigger principal, which in turn yields a bigger return.
“If we have a bigger bank account and draw out two percent a year, we’re getting a bigger amount,” Kladivko notes. “We can’t say it happens in two or three or four years, but we know it can happen.”
When it comes to nitrogen from cover crops, most of the discussion revolves around the decay of above-ground biomass like shoots and leaves. Those tender tissues tend to be relatively high in nitrogen and easy for microbes to decompose, contributing to soil nitrogen in plant-available or organic forms.
Below ground, roots represent a significant amount of biomass, but because they tend to be harder to decompose than leaf tissue and soft stems, their nitrogen is often not available to cash crops for years.
However, cover crop roots play a major role in building soil organic matter. A study of cereal rye cover crops in continuous corn led by Emily Austin of the University of New Hampshire found that most contributions of carbon to soil organic matter by the cover crop was from decaying root biomass and rhizodeposits, which included sloughed-off root cells, mycorrhizal hyphae associated with the roots, and secretions and exudates from the plants.
That is a significant contribution by cover crops that literally goes unseen. Carbon is a vital component of soil organic matter—in fact, it’s the “organic” in “organic matter”—so it is a key currency in a farm’s bank account.
Because plant materials become more difficult to break down as they get more mature, terminating cover crops in their vegetative stage helps deliver more plant-available nitrogen to the soil within four to six weeks of termination, and minimize N tie-up.
An Oregon State University bulletin by D.M. Sullivan and N.D. Andrews (PNW 636) notes that plant-available nitrogen levels from a good stand of legumes peaks at the budding growth stage and declines as growth continues. The balance of plant-available nitrogen from cereal cover crop residues is positive through tillering in the early spring, but by the time the flag leaf emerges, plant-available nitrogen contributions from the residue are approaching zero.
By the time seed heads are visible, the balance shifts to the negative—decomposing cereal cover crop residue will tie up N. That nitrogen is still in the soil, but it is not available to plants because it is in use by the microbes. In the long term, it will build soil N levels and ultimately become available to future crops but won’t be immediately available for use.
As a result, they write, terminate cereal cover crops early to maximize plant-available nitrogen, but wait until bud stage to terminate legumes. Blends of legumes and grasses can help maintain positive levels of plant-available nitrogen through the cereals’ boot stage, they add.
The amount of N available for cash crops 4 to 6 weeks after cover crop termination depends in part on the maturity of the cover crop when it is killed. Oregon State University researchers estimated a cereal rye cover crop yielded 20 pounds of plant-available N per acre when terminated during its vegetative growth stage and tied up 27 pounds of N per acre when terminated at heading. Source: D.M. Sullivan and N.D. Andrews, Oregon State University (2012): PNW 636
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