Summer 2002 Volume II, Number 2
UNDERSTANDING OF THE NITROGEN CYCLE CALLED INTO QUESTION
In my writing I have discussed the importance of the nitrogen cycle and the relationship between plant growth and organic and inorganic nitrogen. As it happens, I may have been mistaken - and so might more than a century of biological science.
Nitrogen is an extremely abundant element, and it is essential for all life forms to survive. In fact, it is the primary nutrient for plant life. For the post 150 years, the dominant view in plant nutrition has been that most plants do not readily absorb organic nitrogen, that is, nitrogen which is chemically combined with carbon. Instead, it has been thought that plants need access to inorganic nitrogen, which is bound to oxygen, hydrogen, or a metal.
Certainly in agriculture, over the past decades, the extensive use of inorganic fertilizer has greatly increased crop yields. The invention of the Haber-Bosch process in the early 20th century – which transformed atmospheric nitrogen into inorganic nitrogen (ammonia) that plants can readily absorb - changed agriculture forever. We are now largely dependent on this type of nitrogen to fertilize most of our commercial crops. Worldwide, the Haber-Bosch process produces about 2 million tons of ammonia (the primary ingredient of inorganic fertilizers) every week.
Scientists have long assumed that forests work the same way as agricultural crops -that they were dependent on inorganic nitrogen for growth and that its availability was the primary factor that restricted growth in a forest. And in fact, studies of forests in the Northern Hemisphere have found that most of the dissolved nitrogen in the soil is indeed inorganic.
However, a new study published recently in the journal Nature exposes a big flaw in this conventional thinking. The problem is that our scientific understanding of forests in the Northern Hemisphere is based on studies conducted after industrialization. However, since the industrial revolution, human activities have dramatically altered both the carbon cycle, by adding vast amounts of carbon dioxide to the atmosphere, and the nitrogen cycle, by adding vast quantities of nitrogen fertilizers to the soil and nitrogen oxides to the air (by burning fossil fuels). In 200 years we have actually doubled the amount of available nitrogen in the biosphere. Has all this excess inorganic nitrogen skewed our perspective of natural systems?
It has - and perhaps profoundly. The study reported in Nature compared the type of nitrogen found in woodland streams and rivers in North America and Europe with the nitrogen found in 100relatively pristine South American streams in 26 different regions. Researchers were surprised to find that a whopping 70 percent of the dissolved nitrogen in the South American streams is organic. That's the opposite to what is found in the Northern Hemisphere, where more than 70 percent is inorganic. The researchers surmise that all that inorganic nitrogen may not be critical to forest ecosystems at all but is merely a byproduct of industrial pollution - especially atmospheric pollution from burning fossil fuels.
This finding calls into question our entire understanding of the nitrogen cycle in forests. It also questions our assumptions about how nonagricultural plants in general take up nitrogen. And because the nitrogen cycle is closely linked to the carbon cycle, these new observations have implications for computer models of changing global conditions, like those that predict a forest's ability to absorb increased carbon from the atmosphere.
It may be that inorganic nitrogen did dominate nitrogen cycling in Northern Hemisphere forests even before industrialization. However, examining that possibility could be a difficult and lengthy task. In the meantime we are left with the disturbing possibility that we have altered our world so dramatically that our most basic assumptions of what constitutes "natural" processes are tainted by human influence.