The Work of Nature by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9781597268998
Publisher: Island Press
Published: 2013-03-21T16:00:00+00:00

Rising CO2 and Nitrogen Overloads

Even without the use of plows, chainsaws, or fertilizers on lands untouched by direct human disturbances, human activity is having a profound impact on natural nutrient cycles and the interdependence between plants and the soil community. Every time people drive cars, fire up power plants or factories, or burn forests, we are releasing gases and particulates that impact the functioning of ecosystems worldwide.

The burning of carbon-rich fossil fuels, such as coal, gasoline, and oil, and the clearing and burning of forests in the tropics have helped drive carbon dioxide (CO2) levels in the atmosphere up from about 270 parts per million two centuries ago to 350 parts per million today. At current rates, CO2 concentrations could double to 700 parts per million by 2050. Even freezing carbon emissions at current levels would only slow, not halt, that rise.

Most of the worry over rising CO2 stems from the fact that it is one of the so-called greenhouse gases. These gases trap outgoing heat from the earth, raising global temperatures. Debate still rages over how much average temperatures will rise as greenhouse gases build up, and indeed, whether greenhouse-induced global warming has already begun. Without question, atmospheric CO2 levels are rising, and that trend, regardless of its larger implications for climate, has the potential to seriously disrupt plant communities.

Experiments in pots, greenhouses, and growth chambers show that high CO2 often causes plants to grow faster and larger, at least for a time. For reasons not completely understood, the effect doesn’t last. For one thing, plants still need nitrogen, phosphorus, and other nutrients; if any of these are in short supply, the potential fertilizing effect of high CO2 has little impact on plant growth.47

Another consequence of extra CO2 is its potential impact on nutrient cycling by the soil community. Plants growing in high CO2 usually pack proportionately less nitrogen-rich protein and more carbon-rich starch into their leaves and thus have less nitrogen to offer the soil microbes. It remains to be seen whether the net impact on the soil community will be negative or positive. That’s because plants raised in high CO2 often allocate much of whatever increased growth they experience to roots rather than to stems and leaves. With more roots to exploit the soil, and more sugars to nourish their mycorrhizal fungi, nitrogen-fixers, and other soil microbes, some researchers think plants just might spur greater soil fertility.48

Carbon is only half the story. Indeed, thanks to the invention of automobiles and industrial fertilizer production, humans now fix and make available to the living world more atmospheric nitrogen than all natural processes combined.

Peter Vitousek of Stanford points out that land-based ecosystems naturally fix about 100 billion kilograms of nitrogen a year; marine ecosystems add 5 to 20 billion kilograms; and lightning fixes another 10 billion. That’s a maximum of 130 billion kilograms of nitrogen drawn from the air into circulation through soils, plants, and animals by natural processes. In contrast, human beings fix some 135 billion kilograms a year. Of that amount, more than 80 billion kilograms comes from fertilizer production.

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