In order to increase the quantity of nitrogen in soil, chemical fertilizers and cover crops can both be used. In addition to improving soil structure, cover crops also contribute to soil health by stimulating beneficial microbes.
The Cornell University department of agronomy is looking at ways of developing more efficient cover crops, or green manures, that can help farmers to grow crops in a more sustainable manner. Crop Science, a journal of the Crop Science Society of America, was the forum where their results were published.
In her current research and development effort, Katherine Muller and her team are aiming to devise strategies for determining the amount of nitrogen. The studies are addressing two common cover crops, crimson clover and hairy vetch. This process, known as nitrogen fixation, is used by both crops to assist in their growth.
In simple terms, “green manures” are crops used to enhance soil fertility, according to Muller. By adding nutrients to the soil, they assist in building healthy soil. For example, legumes are nitrogen-rich soil improvers due to their symbiotic relationship with bacteria.
Plants such as legumes have been used in green manures for thousands of years. Nevertheless, chemical fertilizers became increasingly important for farmers in developed countries after the 1950s. This is because two scientists, Haber and Bosch, developed a technique to pull nitrogen from the air and formulate chemical fertilizers.
It is important to remember, however, that although this type of fertilizer is productive, it also consumes energy to make it – and, if not managed correctly, can easily enter water bodies.
Muller believes that cover crops are important ecological management tools. They provide nutrients to the soil and foster microbial communities. They improve soil fertility so that plants have access to nutrients when they are needed.
Because farmers are unable to determine the exact amount of nitrogen supplied to their soil by cover crops, the use of cover crops can be risky for them. It is possible to calculate the precise amount of nitrogen used to fertilize a crop using chemical fertilizers. Although there is no accurate indication of how much nitrogen is provided by each type of cover crop.
An individual legume cover crop can provide varying amounts of nitrogen depending on how well it grows and how much of its nitrogen is fixed and how much is taken up from the soil. As of today, there are no cover crop seeds available that are bred for nitrogen fixation, a valuable characteristic.
In an effort to avoid risks associated with cover crops and increase benefits to farmers, plant breeders are developing cover crop varieties. In order to improve the use of cover crops as an alternative to chemical fertilizers, they are developing new varieties. In terms of legume green manure, nitrogen fixation is one of their primary concerns.
According to Muller, “We are attempting to assist plant breeders in the development of nitrogen fixation strategies in cover crops.” Since nitrogen fixation is a complex trait that varies with plant growth, it is important to measure it at the right time.”
In agriculture, it is the end of the harvest that is the most significant indicator of nitrogen fixation. A legume green manure is generally terminated when it has reached the late flowering stage. In the event of an earlier termination, the crop has a higher probability of resprouting and becoming a weed. As a result, breeding programs for hairy vetch and crimson clover are not able to measure cross-pollination, since the plants need to be removed before cross-pollination occurs.
“Our team conducted a field experiment involving an active breeding program,” explains Muller. During the experiment, plant tissues were collected and nitrogen fixation was assessed. It was possible to determine how much of the plant’s nitrogen is fixed as opposed to being absorbed from the soil.”.
To compare the data with that relevant to farmers, the team tested three types of samples that plant breeders might use. The samples were then sent to a lab that measures total nitrogen content as well as an abundance of a naturally occurring stable isotope to determine nitrogen fixation.
Normally, soil nitrogen contains a higher concentration of nitrogen stable isotopes compared to nitrogen fixed by bacteria. Scientists can use this method to determine what proportion of nitrogen plants obtain from soil in comparison to that which they fix.
The recommendation from Muller is to collect stems from each plant at the start of flowering in order to determine the nitrogen fixation via stable isotopes. A good proxy for nitrogen fixation in whole plants can be obtained by observing nitrogen fixation in the late flowering phase, which is more relevant to farmers.
Muller maintains that breeders should add one additional measurement if they are to do so. There is often no correlation between the proportion of nitrogen obtained by fixation and the size of the plant.
Müller notes that measuring the amount of nitrogen fixed by a cover crop is crucial because it is subject to fluctuations. “Farmers are interested in knowing how much nitrogen they are loading onto their fields. It is important that this information is accurately measured and communicated to farmers in order to enable them to make informed decisions. “We sincerely hope that our research will lead more farmers to use legumes as nitrogen sources in their cover crops.”
Katherine Muller works at Cornell University as a postdoctoral researcher. Researchers carried out this study in conjunction with the US Department of Agriculture’s Legume Cover Crop Breeding Project.