Nitrogen fixing bacteria such as Azospirillum brasilense and Azotobacter chroococcum found in Aspen™ manufactured in Australia by Novum Lifesciences are root surface colonising and free-living Plant Growth Promoting Bacteria (PGPB) that convert atmospheric nitrogen into ammonium via the nitrogenase pathway. Under certain environmental and soil circumstances its inoculation of soil and plants can stimulate a positive influence on crop growth, agricultural productivity as well as the nitrogen content in plant tissue. 
What are nitrogen fixing bacteria?  

Nitrogen fixing bacteria, also known as diazotrophs, fix atmospheric nitrogen (N2) and convert it into a more plant available form such as ammonium (NH4+). There are numerous strains of diazotrophs, both free-living and endophytic. The Azospirillum genus contain several aerobic strains that has been well researched, Azospirillum brasilense in particular. Azospirilla, including A.brasilense are gram-negative predominately surface-colonizing rhizosphere bacteria but can also be free-living in the soil. Azotobacter chroococcum are gram-negative bacteria that are exclusively free-living in the soil.  

How do they work?  

Microbiological nitrogen fixation occurs as a result of nitrogenase activity and involves two proteins, dinitrogenase and dinitrogenase reductase. This process is regulated though, and conditions like free oxygen concentrations and levels of nitrate and ammonia in the soil can suppress or stimulate nitrogenase activity. Consequently, the soil conditions can affect the abundance and population levels of these aerobic nitrogen fixing microbes also. Though both strains are diatrophic, they contribute their greatest amount of N after they die, and the nitrogen they’ve fixed for their cell walls is mineralized in the soil. 
What effects their use? 

Like all living things, they require the ideal conditions to survive and operate efficiently. The level of free oxygen (they are aerobic), nitrate or ammonia concentrations and even soil specifics like soil pH, soil temperature and soil nutrition can affect microbiology populations and nitrogenase activity. Too acidic a pH, too high levels of salinity, extremely poor soil nutrition (especially P), rising nitrogen forms or poor oxygen concentrations negatively impact these microbes. 

It also appears that the presence of plants does play a part in their activity and population concentrations, for both A. brasilense and A. chroococcum. Trials performed in unplanted, inoculated soils showed nitrogenase activity (and therefore N fixation) remained low. However, when A. brasilense inoculated the rhizosphere of an actively growing crop, population abundance increased, N-fixation was higher, and the bacteria showed decreased sensitivity to suppression from non-ideal soil conditions like oxygen concentration and excess nitrogen concentration.  

The abundance of A. chroococcum has always been recorded much higher around the rhizosphere zone of a plant, than in surrounding soil. Though they do not exclusively host with particular plant species, they both benefit from each other’s presence in the environment. 

Timing of application is another critical aspect to be considered. Trials have noted that the highest survival rates of A. brasilense were in plants treated quickly after seedling emergence. Crops inoculated at later stages of growth showed lower values of bacterial population density in the root area and thus the N-fixing potential was lower. Subsequent applications during the crops’ development did not increase the root colonisation. It seems the colonisation process occurs during a key developmental stage of the plant and in order to receive the benefit of Azospirillum and Azotobacter presence, timing is key. 

Nitrogen-fixing, plant-growth promoting bacteria like A. brasilense and A. chroococcum  in Aspen™ arguably have the highest potential to improve agricultural productivity. Nitrogen fixation and phytohormone production by these bacteria have been considered the most important factors of their recorded plant growth promotion. While aspects must be considered with their application and use, this differs no more than the conditions you must consider with other product applications in Agriculture. Waterlogged soils, very acidic soils and very saline soils can prevent these bacteria reaching their full N-fixing potential. Under more suitable conditions research has shown excellent performance when reducing chemical N fertiliser input by two thirds, increasing yield and crop performance in numerous Agricultural crops.  

If you currently use Rhizobium spp. and are uncertain of the suitability of Aspen in your cropping system, please get in touch with us and speak to our Agronomist.

For further information on application rates and suitability for your production system visit or call your local reseller and ask for Aspen™ – part of Novum Lifesciences’ Biofilm range. 

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