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Agriculture

Biological fertilizer

Bacteria replace nitrogen fertilizers as instigators of sugarcane growth

embrapa agrobiologia Result in the field: sugarcane with biological fertilizer (to the left) and without the inoculantembrapa agrobiologia

Five species of nitrogen fixing bacteria make up the base of a new product, a biological fertilizer that replaces nitrogen fertilizers, used to foster sugarcane growth. Application of this biological inoculant, developed by researchers at Embrapa Agrobiologia (the agro-biology branch of Embrapa, the Brazilian agricultural research company) in Seropédica, in Rio de Janeiro, will lead to a clear reduction in the level of spending on nitrogen fertilizers in Brazil. Initially, research focused on replacing nitrogen in the sugarcane from the first crop, which is also known as plant-cane. With an area under cultivation in excess of 6 million hectares and a total production of 426 million tons a year, Brazil is the world’s largest sugarcane producer. On account of the drop in the plant’s productivity after it has been harvested four times, about 20% of the area under cultivation is renewed annually, i.e., 1.2 million hectares that could potentially receive the biological fertilizer. “If the dose is of 30 kg of nitrogen per hectare, which is the minimum amount used, then we would save a total of 50 thousand tons of nitrogen fertilizer a year, with no fall in productivity”, says researcher Verônica Massena Reis, the coordinator of the Embrapa Agrobiologia group that is studying the bacteria-based inoculant.

The estimated savings only take into account first-year sugarcane, which requires much less nitrogen than the 80 kilos for hectare required for stubble cane – replantings from the first harvest that can be cut up to four times. This significant difference occurs because when the ground is plowed and leveled for planting, the soil is turned over and as a result releases this chemical element, which is then available for the first year’s crop. When the sugarcane is in the ground and is cut in order to produce new shoots, all the available nitrogen is shifted to the so-called aerial part of the plant. Unlike carbon dioxide and oxygen, nitrogen is relatively nonreactive from the chemical point of view, and only a few bacteria and blue algae can assimilate it from the atmosphere and transform it so that it can be used by the cells of the plants.

In many cases the lack of nitrogen is the main factor limiting vegetable growth. For grass-like plants such as sugarcane, corn and sorghum, all of which are fast-growing, nitrogen is the chief element required. However, estimates indicated that about 50% of the nitrogen added to the soil is lost: as nitrogen is a highly mobile element, when it is not absorbed by the plant it can either be carried by the rain to the streams and rivers, thus causing contamination, or it can return to the air in the form of ammonia. As the process of producing fertilizers demands large amounts of oil-derived fossil fuels, a product that was just recently setting a series of price records on the international market, its replacement by the inoculant made of nitrogen fixing bacteria will mean a significant reduction of sugarcane production costs.

That was the path taken by Brazil’s soybean industry, which only became competitive after Embrapa researchers developed strains of the Rhizobium bacteria, which extract nitrogen from the air and transfer it to soybean roots (see Pesquisa FAPESP nº 85). The original studies from which the technique is derived were begun in the 1950s by the researcher Johanna Dobereiner (1924-2000), at the Ministry of Agriculture’s former National Center of Agronomy Teaching and Research, which in turn gave rise to Embrapa Agrobiologia. Verônica Reis began research in this area in 1982, when she joined the institution to work with grasses and learned the technique from Johanna herself. The difficulty in using inoculants with this type of plant is that the bacteria are distributed throughout the plant, being found mainly in the spaces between cells and in the tissues of the vascular system, whereas in the case of legumes, such as soy and other beans, they are located in a specific part of the root.

embrapa agrobiologia Prior to planting, the stems are dipped in the inoculant for one hourembrapa agrobiologia

Absorption of nitrogen
“At present our collection contains more than eight thousand bacteria”, explains Veronica. To arrive at the ideal mixture, the researchers initially selected the most promising nitrogen fixing bacteria isolated from sugarcane. “We began testing each one of them individually and soon we produced one mixture with three bacteria and another with five bacteria. The biological process’ contribution was assessed using a device that measures the degree to which the microorganism absorbs nitrogen from the air”, she explains. The mixture made up of five bacteria strains – Gluconacetobacter diazotrophicus, Herbaspirillum seropedicae, Herbaspirillum rubrisubalbicans, Azospirillum amazonense and Burkholderia tropica – was the one that contributed the most to the biological fixation of the nitrogen. These bacteria were isolated from the tissue of sugarcane planted in the area of the Mata Atlântica (Atlantic Seaboard Forest) region.

In order to obtain the inoculant it is first necessary to grow all the bacteria separately in the appropriate growing environment, as each has a different physiology. Once past this stage, the bacteria are inoculated into sterile peat, resulting from the decomposition of organic matter in flooded areas, and distributed in 250 g plastic bags. The peat is a means of transporting the product to the countryside. The five packages of the black colored microbial inoculant, which has a viscous consistency, are then mixed with 100 liters of water. All that must be done to use the product is to dip the stems used for planting – normally the producers use stems with three shoots/buds – in the bacterial broth for one hour. Once this has been done, they can be planted.

Each dose of the inoculant has an estimated cost of R$ 15.00 to R$ 20.00. Given that for the first year planting one to two doses of inoculant are needed per hectare, the cost of the biological fertilizer will total about R$ 30.00 to R$ 40.00 per hectare at most. This reflects significant savings versus the cost of nitrogen fertilizers. “The 30 kg of urea needed per hectare correspond, in reality, to 60 kilos, since half of it gets lost”, explains Veronica. A kilogram of urea, one of the cheaper nitrogen fertilizers, costs R$ 0.80 to R$ 1.00, resulting in a total cost per hectare of R$ 48.00 to R$ 60.00, without taking into account the cost of applying the fertilizer.

At present, there are 11 field studies in progress, in partnership with sugar mills in the states of São Paulo, Rio de Janeiro, Alagoas, Sergipe, Pernambuco and Piauí. The estimate is that in two years’ time the biological fertilizer will be available on the market. For this to occur, the next stage is the transfer of the technology to the industrial sector, given that the product consists of microorganisms that cannot be patented. One of the industrial challenges of these inoculants is the lack of specific legislation for their quality control.

The researchers also want to extend this technology so that the effect obtained with plant-cane continues throughout the entire production cycle, including stubble-cane. “We also have a product for corn and another for rice and are working on an inoculant for sorghum”, says Veronica. Regarding corn, since a great deal of research has already been done, the technology has been passed on to two companies, which for the time being prefer to keep their name confidential.

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