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Natural Nitrogen for Crop Production
by LuAnn Rolling, District Conservationist

Dr. Christine Jones, one of the world’s preeminent microbiologists from Australia, wrote an article titled Nitrogen: the double-edged sword, in which she says that nitrogen is a component of protein and DNA and as such, is essential to all living things. She states that 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 biological activity and an increased application of industrially produced forms of nitrogen to agricultural land.

Jones says the application of high rates of inorganic nitrogen in agricultural systems has had many unintended negative consequences for soil function and environmental health. Data from North America’s longest running field experiment on the impacts of farm production methods on soil quality have revealed that high nitrogen inputs deplete soil carbon, impair soil water-holding capacity - and ironically, also deplete soil N (Khan et al. 2007, Larson 2007).

The evidence suggests that although nitrogen is essential to plant growth, the application of large amounts of N as inorganic fertilizer is detrimental to soil and to water. The USDA estimates that the cost of removing nitrate from U.S. drinking water is more than $4.8 billion per year, while nitrogen run-off from farmland is the single largest source of nutrient pollution contributing to the massive ‘dead zone’ in the Gulf of Mexico (Ceres 2014).

Cost-effective nitrogen management is the key to profitable and productive farming. It is also the key to building soil carbon. Stable forms of soil carbon (such as humus) cannot form in the presence of high levels of inorganic nitrogen, due to the inhibition of the microbes essential to sequestration.

Much of the nitrogen currently used in agriculture derives from the Haber-Bosch process, developed in the early 1900s. This process catalytically combines atmospheric nitrogen with hydrogen derived from natural gas or coal, to produce ammonia under conditions of high temperature and pressure. The Haber-Bosch process uses non-renewable resources, is energy intensive and expensive.

Jones says that atmospheric nitrogen can be transformed to ammonia by a wide variety of nitrogen-fixing bacteria and archaea - for free. “Ideally, newly fixed ammonia is rapidly incorporated into organic molecules such as amino acids and humus. These stable molecules are vital to soil fertility and cannot be volatilized or leached from the soil system. Importantly, the stabilization of nitrogen requires a steady supply of carbon - also fixed biologically.”

According to Jones it is important to recognize that the ability to fix nitrogen is not limited to bacteria associated with legumes. “Chlorophyll is part of a protein complex - hence wherever you see green plants - there will also be an association with nitrogen-fixing bacteria or archaea. Unlike rhizobial bacteria, most nitrogen-fixing microbes are not able to be cultured in the laboratory. This has presented technical challenges to assessing their ecological function.”

Although procedures for quantifying the amount of nitrogen fixed by many of these groups are lacking, what we do know is that the diversity and abundance of nitrogen fixing microbes are much greater where there is living groundcover (particularly plants in the grass family) throughout the year.  Jones says that the more nitrogen fertilizer we apply the less nitrogen is fixed by natural processes.

Jones warns that while above ground, plant growth often appears ‘normal’ the connection to failing soil function may not be immediately obvious. “Underneath, our soils are being destroyed.”