Plants absorb nitrogen in the form of amino acids. To make the pool of organic nitrogen from soil organic matter available to plants, soil proteases, produced by micro-organisms and potentially excreted by plants, release amino acids. These are used by the plant's metabolism and could be a potential source of nitrogen.
We know plants absorb inorganic nitrogen from the soil in the form of ammonium, nitrate and urea. These elements come from the nitrogen cycle, the nitrification of soil organic matter by micro-organisms and, in the case of cultivated plants, from fertilizer inputs. Plants can also use organic nitrogen in the form of amino acids.
A study by a laboratory at the University of Queensland, Australia, published in PNAS in 2008, showed that small proteins, previously considered unusable in their present state, could be internalized in plant roots and cells and used as sources of nitrogen.
However, the mechanisms by which such protein forms are acquired are still poorly understood. Conversely, the transporters that enable plants to absorb amino acids at root level and transport them to all organs are beginning to be known.
This discovery raises a number of questions about the role of these transporters in the adaptation of plants to sources of nitrogen available in the soil. It offers new prospects for improving plants in the face of diversification of nitrogenous substrates in agriculture.
Bacteria, primary source of nitrogen for plants
“Plants grow with forms of nitrogen that are not commonly known, as our focus tends to be on nitrates and synthetic fertilizers,” points out Konrad Schreiber, a French independent agronomist and co-founder of Ver de terre Production, emphasizing that “bacteria are the primary form of nitrogen that plants consume.”
He says that in the fall, nature produces a lot of carbon, with a high C/N ratio, loaded with lactic acid bacteria and fungi of the Fusarium genus. To break down the straw and dead leaves, these fungi take nitrogen from the soil and denitrify it. There are many free nitrogen-fixing bacteria in the soil (Azotobacter, Nitrobacter, Azospirillum...) “All these bacteria are literally gobbled up by plant roots to provide themselves with proteins,” Schreiber underlines.
The protozoa then become part of the soil food chain. They consume Fusarium and protein-rich bacteria. In this way, they release amino acids that the plant absorbs through its roots.
Autumn is also a good time for urea production. This urea, soluble in water, diffuses into the soil solution. “When stored, urea forms a reserve that is available to the biological community and crop roots over the winter and throughout the growing season that follows,” he explains, adding that “in spring, plants pump out the urea-laden water to grow. The soil is gradually depleted of urea, water and nitrogen, which mineralizes into the ammonium ion NH4+. Then, in the presence of oxygen, the remaining urea and ammonium nitrogen oxidizes to produce nitrates, which can be washed away.”
Plants know how to feed themselves
To speed up the digestion of organic matter, some practices involve spreading proteases on the soil. Researchers have discovered that cereals secrete sugar and proteins from their coronal roots, which are rich in nutrients and airborne bacteria. “This operating principle is only possible if the plant finds sources of nitrogen in the form of ammonium, amino acids and urea in the soil to produce these sugars and proteins,” Schreiber points out.
The nitrogen supplied to plants comes from two possible sources. One is linked to symbiosis with a rhizobium, as in the case of legumes, while the other is due to mycorhization. Mycorhization provides the plant with nitrogen in the form of ammonium, peptides and amino acids.
Amino acids: source of nitrogen or biostimulants?
Biostimulants on the market contain amino acids that could be used by plants as a source of nitrogen. However, recommended application rates in the field are low, suggesting that the amino acids in these commercial solutions primarily function as stimulators of nitrogen metabolism for both conventional and organic farming.
The effects on the plant could be direct or indirect, involving soil micro-organisms, for example. Manon Lardos, a PhD student at Inrae (French National Institute for Agricultural, Food and Environmental Research), is interested in the biostimulant nature of different amino acids and their effect on plant growth. If the results of her research confirm a positive effect of some amino acids on the efficiency of nitrogen use by plants, their use in agriculture could make it possible to reduce the costly inputs of synthetic fertilizers, thereby diversifying the nitrogen supply and stimulating growth.
Schreiber thus believes that “a revolution in plant nutrition is in the offing”. He points out that fall applications of amino acids to crops such as alfalfa, maize and rape have produced positive results. “The measured yields reach the same level as crops grown using conventional cultivation techniques. And these innovative practices bring other benefits: a plant that has amino acids at its disposal uses less energy to feed itself and consumes less water to absorb them,” he explains.