How ‘fixed’ nitrogen has us in a fix

There’s an oldsayingthateveryonewill have heard: “Toomuch of a good thingisbad for you.”

It’s a maximthatapplies — dramatically — to nitrogen, one of the basic elementsnecessary for the growth of all living tissue.

Nitrogenmakes up 78 per cent of the Earth’satmosphere, whereitisunreactive and unavailable to plants and mostother living things, includinghumans. To becomeavailableitneeds to beconverted or, to use the technicalword, « fixed,’’ into a biologically active compound.

However, the amount of nitrogenbeingfixedtodayis double the amountfixed by natural forces. Most of thatdoubling has happened over the past 60 years, with the largest part of the increasefalling in the past 20 years.


Such a large increase in availablenitrogenishaving far-reachingeffects, says Thomas Hutchinson, a biologist and professor of ecology at Trent University, including:
•   increasing the leaching of soils and acidification of lakes to levelsapproachingthose last seen in the nineteen seventies whenacidrainwas at itsheight;
•   helping to make air pollution in Toronto « as bad as itever has been,’’ and spreadingit far widerthanitwas in the seventies. « Within the past five years,’’ headds, « all the improvements in air quality have been reversed;’’
• increasingwinter damage to trees and other plants, because, withnitrogeninspiredgrowth, theydon’t go into a properdormancy in the fall; and
•   eventuallyreducingbiodiversity, becausesome plants willgrowfaster and crowd out others.


In the atmosphere, nitrogenisfound as twoatomsheldtogether by a triple bond, an exceptionallystronglinkthatrequires a great deal of energy to break. In nature, the linkisbrokenonly by lightening, or by the actions of microorganisms. When the linkisbroken, each of the twonitrogenatomsbecome « fixed’’ by combiningwithhydrogen to formammonia (NH3), whichisthenconvertedinto the nitrate (NO3-) that plants can use.


Humanactivitiesalsofixnitrogen. It’sdoneintentionally to create commercial fertilizerscontaining nitrate. And itoccurs as a byproductfrom the burning of fossil fuels, which fixes nitrogenintonitrogenoxides. Some of thosenitrogenoxides, emitted in car and truck exhausts, end up as a key component in the kind of smog thatisbedevilling Toronto.


In addition, the biglivestockfeed lots, especiallyhugepigfarms, release a lot of ammonia as a byproductcontained in urine.


For a time, some plants, such as ash, poplar, and elmtrees, flourishwith all the extra nitrogen, growingmuchfasterthan normal. Othersthat are naturally slow growersaccustomed to lownitrogenlevels, such as oaks, and many of the tundra and boreal plants, willlagbehind, and maybenudgedaside by more aggressive, nitrogen-enrichedspecies. So the composition of forestswill change.


Eventually, however, all plants becomesaturated, and startflushing out the extra nitrogen. Whenthathappens, there are tworesults. On the one hand, whenadditionalnitrogeniswashedintorivers and lakes, it inspires rapidgrowth in weedy plants, and they, in turn, suck more oxygen out of the water, causing eutrophication and forcing fish to change habitats.


On the other hand, some of the flushed-out nitrogenconverts in the soil to nitricacid (HNO3). Nitrogenalsoisresponsible for a muchhigher proportion thanpreviously of the acidiclevel of rainwater. Thesetwo sources of acidification leach calcium and magnesiumfrom the soil, depriving plants of theseneededelements. And theyalso dissolve aluminum, whichistoxic to plants in itdissolvedform.


« `There’sgoing to be a rumpus about all this,’’ Hutchinson predicts.


In Toronto one effectcanalreadybeseen. There are no lichens on mapletrees. In nearby Ajax, threespeciescanbefound. In the Peterborough area, where Hutchinson lives, there are up to 14 differentspecies.


It’s air pollution thatkills lichens. If it’skillingthem, what’sitdoing to you?

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