Biofertilization: Increased soil nitrogen availability

Crops rely on nutrients to thrive; they fail or grow more slowly when certain nutrients are deficient.

Nitrogen is one of the top three essential plant nutrients for crop growth, along with potassium and phosphorus. It is responsible for photosynthesis and chlorophyll concentration, which supplies the green color to plants, allowing farmers to monitor crop health more quickly and easily. Vibrant deep green indicates healthy plants with a lot of chlorophyll. Yellowing (chlorosis) and light green, however, show a lack of chlorophyll and plant health issues, potentially due to a lack of nitrogen.

Our atmosphere is abundant in nitrogen — 79% of nitrogen is in the form of N2 gas. Nonetheless, this is unavailable to crops unless it is “fixed” (combined) in the form of ammonium (NH4) or nitrate (NO) ions to be used for plant development. In this sense, nitrogen is often a limiting factor for optimal crop growth, even under ideal climate and water supply conditions.

Overuse of synthetic nitrogen

Fertilizers are essential for the production of food worldwide. The invention of synthetic nitrogen at the turn of the 20th century changed how we dealt with the availability of nitrogen in soil — we no longer had to rely on the limited amount of nitrogen found naturally in soils across the world but could produce and feed it to plants. Therefore, fertilizers allow us to obtain better agricultural yields with a favorable overall impact: farmers profitably produce more on less land.

But there are some severe drawbacks. In theory, utilizing more fertilizer would not necessarily be problematic if the crops used all that was supplied. However, when we look at the ratio of nitrogen in harvested crops compared to nitrogen inputs (through fertilizers or manure), we can clearly see an unbalanced output. Our crops take up less than half of the nitrogen we apply.

Let us take a step back and explore what this data means.

The comparison between nitrogen input and output is called “nitrogen use efficiency” (NUE). The higher this number is, the better the plants are uptaking and utilizing the nutrient provided. An NUE of 80% means that the quantity of nitrogen in crops equals 80% of the nitrogen supplied as inputs, with the plants not utilizing the remaining 20% of nitrogen.

A low NUE is undesirable, as it means that very little of the nitrogen applied gets absorbed by crops, and the rest becomes a pollutant. Since 1980, global NUE has remained at a low level of 40-50%. The remaining nitrogen is waste that seeps into the natural environment, flowing off the soils and polluting rivers and lakes, disturbing ecosystems and causing biodiversity loss.

Consequences of a low NUE include:

• Groundwater pollution: Nitrates lost outside the root zone pollute groundwater.
• Eutrophication: Harmful algal growth that depletes oxygen and harms aquatic organisms.
• Nitrogen deposition: Ammonia released into the air through volatilization comes back to the surface as sulfur dioxide gas.
• Greenhouse effect: Nitrous oxide formed through denitrification is responsible for 5% global climatic change (Shoji et al., 2001).

The lack of uptake leads to the need to apply more and more nitrogen fertilizers to continue growing crops at an increasing rate to feed the ever-growing population. Farmers boost nitrogen application, prolonging the cycle while further depleting the soil and raising crop costs. This regular application increase means that it now takes significantly higher doses of nitrogen to produce the same amount of crops as in the 1960s.

Soil and plant nitrogen losses harm the ecosystem, soil fertility and plant production. Ammonia emissions cause acid rain and nitrous oxide emissions. Eutrophication — when nitrate leaches into aquifers and promotes an overgrowth of aquatic plants and algae — threatens fish populations, water quality, and human and animal health. Overuse of synthetic nitrogen fertilizers has generated over 500 dead zones at a global level. As a result of nitrate leaching regulations, agricultural land usage has been restricted in several nations. 

Increasing yields require increased inputs, which in turn increase pollution. We seem to be closed in this continuous cycle, where many people think that additional fertilizer use for crop yields is an unquestionable trade-off. Yet, we are not limited to this compromise, and microorganisms play a key role in the emerging solution.

Soil nitrogen-fixing bacteria and nitrogen cycle

Bacteria are the only known microorganisms capable of converting nitrogen gas into the plant-available organic compound ammonia. Before commercial nitrogen fixation methods were developed, plants relied only on microorganisms to provide useable nitrogen.

Soil bacteria play a crucial role in practically all elements of nitrogen availability, supporting the formation and growth of both underground ecosystems and plants through:

• Conversion of N2 into ammonia through nitrogen fixation. These bacteria are either free-living (they live independently of other organisms) or form symbiotic associations with plants or other organisms (e.g., termites, protozoa).
• Transformation of ammonia to nitrate and of nitrate to N2 or other nitrogen gases.
• Degradation of organic matter, releasing fixed nitrogen for reuse by other organisms.

Soil bacteria are responsible for the major conversion of N2 into ammonia and subsequently into proteins in the process called nitrogen fixation (or dinitrogen fixation).

Nitrogen fixation is the process of converting relatively non-reactive atmospheric N2 into more reactive molecules (nitrates, nitrites or ammonia). These reactive forms are essential for crops, helping them thrive. Nitrogen shortage, on the other hand, stunts crop growth and healthy development.

Nitrogen-fixing bacteria, such as Rhizobium, Azospirillum or Rhodobacter, manufacture a unique enzyme responsible for nitrogen fixation, which accounts for about 90% of natural nitrogen fixation on our planet.

In short, nitrogen-fixing bacteria transform atmospheric nitrogen into inorganic chemicals. Nitrogen-fixing bacteria accomplish what crops cannot themselves: they get assimilative nitrogen. Bacteria absorb it as a gas from the air and release it to the soil, typically as ammonia. It is the only viable alternative for plants since they can only ingest nitrogen from the soil as nitrogenous inorganic molecules, emphasizing the need for nitrogen fixation.

This ready-to-use nitrogen that bacteria provide to the crops is a much-needed component of chlorophyll molecules. Chlorophyll is essential for photosynthesis, which converts sunlight energy into the chemical energy that plants need.

Furthermore, plants need nitrogen as a component of amino acids in order to construct proteins that function in metabolism and energy storage. A lack of nitrogen fixation causes yellowing, thinning, withering, general growth delay and decay.

Nitrogen-fixing soil bacteria ultimately provide the ground with inorganic nitrogen-containing compounds that are essential crop nutrients. Upon their death, these nitrogen-fixing bacteria release the nitrogen stored in their biomass into the soil, naturally increasing soil fertility and enabling farmers to save money on synthetic fertilizers.

“Nitrogen bacteria teach us that nature, with her sophisticated forms of the chemistry of living matter, still understands and utilizes methods, which we do not as yet know how to imitate.”

Fritz Haber, Nobel Lectures, Chemistry 1901-1921, The Synthesis of Ammonia from Its Elements

Biofertilization supports sustainable crop production

Plants benefit from biofertilization because it encourages the use of inputs containing a range of microorganisms capable of populating the rhizosphere and making nutrients more easily accessible to plant root hairs through bioavailability. Including, but not limited to, symbiotic and free-living nitrogen-fixers, biofertilizers are cost-effective and environmentally sustainable, aside from being effective alternatives to synthetic fertilizers.

Biofertilizers can help sustain agricultural productivity and fulfil the rising demand for crop products while conserving and preserving natural resources for future generations. Numerous studies throughout the world demonstrate the value of biofertilizers in boosting crop yields and improving the quality of agricultural products by:

• Enhancing soil content with nutrients and useful microorganisms.
• Promoting soil fertility and health.
• Preserving natural resources.
• Improving productivity and cost-benefit ratios to achieve better agricultural sustainability.

Alltech Crop Science, a global leader in microbial fermentation and the utilization of their metabolites, continues to research and innovate to assist farmers worldwide in moving toward more sustainable, productive and profitable crop production.