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Nitrogen: The Double-Edged Sword

By December 10, 2019September 22nd, 2022No Comments

By Christine Jones, Ph. D

Dr. Christine Jones is an internationally renowned and highly respected groundcover and soils ecologist. She has a wealth of experience working with innovative landholders to implement regenerative land management practices that enhance biodiversity, increase biological activity, sequester carbon, activate soil nutrient cycles, restore water balance, improve productivity, and create new topsoil. A native of Australia, Christine has rapidly become one of the most sought after Soil Health speakers in the world and has been wildly popular on the United States Soil Health speaking circuit. We count it a blessing to call her a mentor and a good friend of Green Cover Seed.

Nitrogen is a component of protein and DNA and as such, is essential to all living things. Prior to the Industrial Revolution, around 97% of the nitrogen supporting life on earth was fixed biologically. Over the last century, intensification of farming, coupled with a lack of understanding of soil microbial communities, has resulted in reduced levels of biological activity on agricultural land and an increased application of industrially produced forms of nitrogen.  

Impacts of Inorganic Nitrogen

Much of the nitrogen currently used in agriculture derives from the Haber-Bosch process, in which atmospheric nitrogen is catalytically combined with hydrogen to produce ammonia under conditions of high temperature and pressure. This process uses non-renewable resources and is energy intensive and expensive. Globally, over $100 billion of nitrogen fertilizers are applied to crops and pastures every year. Between 10 and 40% of the applied N is taken up by plants while the other 60-90% is leached into water, volatilized into the air or immobilized in soil. The application of high rates of inorganic nitrogen in agricultural systems has had many unintended negative consequences for soil function and environmental health. Above ground, plant growth often appears ‘normal’, hence the connection to failing soil function may not be immediately obvious. But underneath, our soils are being destroyed. 

Biological Nitrogen Fixation (BNF)

Fortunately – thanks to some ‘enzymatic magic’ – atmospheric nitrogen can be transformed to plant-available forms by a wide variety of nitrogen-fixing bacteria and archaea – for free. The ability to fix nitrogen is not limited to bacteria associated with legumes. Recent bio-molecular research has revealed a dizzying array of free-living and associative nitrogen-fixing bacteria and archaea across a wide range of environments. Their abundance is much greater in soils where diverse living groundcover is present throughout the year, compared to soils that have been monocropped or left bare. 

The Liquid Carbon Pathway

Carbon and nitrogen are essential to plant growth and integral to soil function. A massive 78% of the earth’s atmosphere is composed of dinitrogen (N2). Carbon dioxide (CO2), on the other hand, is a trace gas, currently comprising only 0.04% of the atmosphere. The incorporation of both carbon and nitrogen into stable soil organic complexes via photosynthesis and the liquid carbon pathway effectively transports these vital elements from the atmosphere to the soil. The plant’s requirement for biologically-fixed nitrogen drives this process. Liquid carbon is transferred to complex microbial communities within rhizosheaths and root-supported aggregates, where simple carbon molecules are transformed to highly stable humic polymers, composed of biologically fixed carbon, nitrogen, bacterially-solubilized phosphorus and soil minerals.

Although mycorrhizal fungi do not fix nitrogen, they play a vital role in the nitrogen nutrition of plants by transferring energy, in the form of liquid carbon (also called photosynthate), to associative and free-living nitrogen-fixing bacteria. The acquisition and transfer of both organic carbon and organic nitrogen via mycorrhizal pathways is highly energy efficient, closing the nitrogen loop, reducing nitrification, denitrification, volatilization and leaching.  

Enhancing the Liquid Carbon Pathway

We can utilize our understanding of the liquid carbon pathway to restore natural fertility to agricultural land. Enhanced carbon flow to soil – via plant root exudates – not only supports the biological fixation of atmospheric nitrogen, but also activates the vast network of microbial communities essential to the provision of minerals, trace elements, vitamins and hormones required for plant tolerance to environmental stresses such as frost and drought and resistance to insects and disease. Higher micronutrient densities in plants also translate to improved nutritional value of food. However, if nitrogen is supplied in an inorganic (fertilizer) form, it will short-circuit the liquid carbon pathway. As a result, plant mineral densities fall and immune function is reduced.

Getting the Basics Right

It is now recognized that plant root exudates make a greater contribution to the formation of stable organic complexes within the soil than does the above-ground biomass.  But here’s the rub. The microbes essential to the stabilization of carbon require living groundcover and are inhibited by high rates of inorganic N. Hence biological nitrogen fixation and humification are rare in agricultural systems where heavily N-fertilized crops are rotated with bare fallows. Further, it has been shown that up to 80lb N/acre can be volatilized and lost from bare fallows due to denitrification in warm summer months. If green plants are present, this N can be taken up and recycled, preventing irretrievable loss.  When soil is bare there is no photosynthesis and very little biological activity. Bare soils lose water, carbon and nitrogen, nutrient cycles become dysfunctional, aggregates deteriorate, structure declines and water-holding capacity is reduced. The maintenance of bare fallows – or the use of high rates of inorganic N in crops or pastures – or worse, both – results in the uncoupling of the nitrogen and carbon cycles that have functioned synergistically for thousands of years. 

Weaning Off N

The activities of both symbiotic and associative N-fixing bacteria are inhibited by high levels of inorganic N. In other words, the more nitrogen fertilizer we apply, the less N is fixed by natural processes. For this reason it is vitally important to wean your soils off high rates of inorganic N – but please do it S.L.O.W.L.Y. Microbial communities generally require around three years to adjust. Nitrogen inputs can be reduced 20% the first year, 30% the second year and a further 30% the third year. In subsequent years, the application of small amounts of inorganic N will help to prime the natural nitrogen-fixing processes. In addition to weaning off high rates of inorganic N, aim to maintain as much diverse year-round living groundcover in crops and pastures as possible. 

There is increasing recognition of the fundamental importance of soil microbial communities to plant productivity. Many biological functions are compromised by commonly used agricultural practices but fortunately redesign of farming practice is not difficult. The five basic principles for regenerative agriculture discussed earlier in this Resource Guide have proven to restore soil health and increase levels of organic carbon and nitrogen. From these, farmers and ranchers can build an integrated land management package that suits their individual property and paddock needs.

More and more farmers around the world are discovering how to restore natural topsoil fertility by moving away from bare fallows to biodiverse year-long green plant cover, coupled with appropriate livestock management and reduced applications of inorganic nitrogen. Improvements to soil function deliver benefits both on-farm and to the wider environment. 

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