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NITROGEN: THE DOUBLE-EDGED SWORD

Nitrogen is a component of protein and DNA and as such, ent throughout the year, compared to soils that have been
is essential to all living things. Prior to the Industrial Revo- monocropped or left bare.
lution, 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 Photo by Trey Archer
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 pro-
duce ammonia under conditions of high temperature and Rhizobia bacteria form N fixing colonies or factories on the roots of legume plants,
like this sunn hemp. These tiny workers split the atmospheric di-nitrogen molecule
pressure. This process uses non-renewable resources and is and transform the N into a form that plants can use!
energy intensive and expensive. Globally, over $100 billion
of nitrogen fertilizers are applied to crops and pastures The Liquid Carbon Pathway
every year. Between 10-40% of the applied N is taken up by Carbon and nitrogen are essential to plant growth and
plants while the other 60-90% is leached into water, vola- integral to soil function. A massive 78% of the earth’s at-
tilized into the air or immobilized in soil. The application mosphere is composed of dinitrogen (N2). Carbon dioxide
of high rates of inorganic nitrogen in agricultural systems (CO2), on the other hand, is a trace gas, currently com-
has had many unintended negative consequences for soil prising only 0.04% of the atmosphere. The incorporation of
function and environmental health. Above ground, plant both carbon and nitrogen into stable soil organic complex-
growth often appears “normal”, hence the connection to es via photosynthesis and the liquid carbon pathway effec-
failing soil function may not be immediately obvious. But tively transports these vital elements from the atmosphere
underneath, our soils are being destroyed. to the soil. The plant’s requirement for biologically-fixed
nitrogen drives this process. Liquid carbon is transferred to
Biological Nitrogen Fixation (BNF) complex microbial communities within rhizosheaths and
Fortunately—thanks to some “enzymatic magic”—atmo- root-supported aggregates, where simple carbon molecules
spheric nitrogen can be transformed to plant-available are transformed to highly stable humic polymers, com-
forms by a wide variety of nitrogen-fixing bacteria and posed of biologically fixed carbon, nitrogen, bacterially-sol-
archaea—for free. The ability to fix nitrogen is not limited ubilized phosphorus and soil minerals.
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 Photo by Phill Lee
wide range of environments. Their abundance is much
greater in soils where diverse living groundcover is pres-

Saprotrophic and symbiotic fungi transfer “liquid carbon” from roots to free-living
Photo by Christine Jones Although mycorrhizal fungi do not fix nitrogen, they play a
N-fixing bacteria and archaea within the rhizosheaths of plants grown without N.

vital role in the nitrogen nutrition of plants by transferring
energy, in the form of liquid carbon (also called photosyn-

N fertilizer placed below the seed inhibits the formation of rhizosheaths on the roots of thate), to associative and free-living nitrogen-fixing bac-
teria. The acquisition and transfer of both organic carbon
cereals. Note healthy rhizosheaths above the seed (away from the N).

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